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Glossary of Terms for the Creation/Evolution Debate

(From Evolution, Third Edition by Monroe W. Strickberger. Jones and Bartlett, Sudbury, MA, Copyright © 2000)

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Abduction Movement of an appendage or body structure in a direction away from the midline (median sagittal) plane (for example, extending an arm laterally).
Abiotic Substances that are of nonbiological origin, or environments characterized by the absence of organisms.
Acidic A compound that produces an excess of hydrogen ions (H+) when dissolved in water. Using quantitative hydrogen ion measurements, such solutions have a pH value less than 7.0. (See ph scale.)
Acoelomate An animal that lacks a coelom (internal body cavity).
Acritarchs Single-celled eukaryote-type microfossils of Precambrian age whose biological relationships are uncertain.
AcrocentricChromosomes whose centromeres are near one end (between metacentric and telocentric locations).
Active sites Specific regions of an enzyme that bind substrates on which the enzyme acts.
Active transport Biochemical transport that requires the input of energy (for example, hydrolysis of ATP).
Adaptation This notion derived from the typical relationship between structure and function: that an organism's structures seem suitable ("adapted") for their tasks. Until Darwin, the cause for adaptation was commonly ascribed to intelligent (divine) guidance. Darwinians replaced this view by proposing that an adaptation is any trait that replaces other variants because of selection for greater reproductive success (See Fitness). An adaptation is a trait whose presence enhances survival or fertility. It is selection rather than intelligent design that produces and/or maintains the correlation between structure and function. The complexities of evolution, however, shroud the Darwinian concept with many qualifications. For example, should selection cease or reverse its direction, as occurs for traits that become vestigial, then the trait is no longer an adaptation, although it may have been in the past. Traits that are not maintained by selection (that is, not related to reproductive success) are generally considered "nonadaptive." Such traits may be introduced or persist in a population through mutation, random genetic drift, the accidental extinction of adaptive varieties, developmental constraints that now impede their elimination, close linkage with genes selected for other functions (See Hitchhiking), or as one of the multiple phenotypic effects of a selected gene (See Pleiotropy). Also, not all selected traits are necessarily beneficial to a population, since some may increase the reproductive success of genes or individuals but not benefit (or even decrease) population fitness (See Segregation distortion, Sexual selection). Even when selected traits are unquestionably adaptive, they often involve "trade-offs" in other traits that can lose adaptive advantages. (For example, trees that grow competitively taller put more resources into wood production than seed production.) In addition, earlier selected stages of an adaptation may have been for a function different from that of a later stage (See Preadaptation). In general, since it is quite difficult to examine historical circumstances leading to a particular trait, it can be difficult to determine how or to what extent it is an adaptation. Mostly, such determinations depend on evaluating functional utility ("optimality") for reproductive success, based on the reasonable assumption that a useful trait generally replaces or has replaced less useful variants. Unfortunately, since it can also be challenging to establish functional utility—to uncover a trait's many possible variations, and to compare their relative reproductive success—identifying adaptations can be controversial. Although selection may not be obvious, it is difficult to accept that any prominent nonadaptive trait can long persist without being affected by selection in some way and to some degree. The term is also frequently used for the process that produces adaptations (natural selection). However adaptations are defined, it is the genetic transmission of traits whose structure and function let their carriers interact successfully with the environment that drives evolution and makes biology unique and historical.
Adaptive landscape A model originally devised by Sewall Wright that describes a topography in which high fitnesses correspond to peaks and low fitnesses to valleys; each position potentially occupied by a population bearing a unique and frequent genotype.
Adaptive radiation The diversification of a single species or group of related species into new ecological or geographical zones to produce a large variety of species and groups. Such events may include the following: 1. Survivors of a catastrophe (for example, mammals) invade the adaptive zones that were abandoned by extinct species (for example, dinosaurs). 2. One or a few colonizers enter a new habitat in which competing species are absent (for example, the Hawaiian Drosophilidae). 3. One group of species (for example, pollinating insects) evolves in step with the adaptive radiation of another group (for example, angiosperms), or parasites (for example, viruses and bacteria) evolve new strains in concert with proliferation of their hosts (for example, humans). 4. A preadaptive feature (for example, the shelled reptilian amniotic egg) allows invasion into a previously inaccessible ecological zone (for example, terrestrial habitats). 5. A new morphological or physiological character (for example, pharyngeal jaw innovations in cichlid fishes) causes divergent evolution by partitioning the environment into different niches. These various historical contingencies promote genetic diversity by letting selection—the primary biological force changing organisms through time—channel evolution in new directions.
Adaptive value The relative reproductive success (relative fitness) of an allele or genotype as compared to other alleles or genotypes. (See also Fitness.)
Adduction Movement of an appendage or body part toward the midline (median sagittal) plane, for example, bringing a laterally extended arm to the side of the body.
Adenosine monophosphate (AMP) An organic compound commonly involved in the transfer of phosphate bond energy, composed of adenosine (an adenine base + a D-ribose sugar) and one phosphate group.
Adenosine diphosphate (ADP) An organic compound commonly involved in the transfer of phosphate bond energy, composed of adenosine (an adenine base + a D-ribose sugar) and two phosphate groups.
Adenosine triphosphate (ATP) An organic compound commonly involved in the transfer of phosphate bond energy, composed of adenosine (an adenine base + a D-ribose sugar) and three phosphate groups.
Aerobic The use of molecular oxygen for reactions that provide growth energy from the oxidative breakdown of food molecules.
Aerobic respiration An electron transport system in which oxygen serves as the terminal electron acceptor.
Algae Photosynthetic members of the eukaryotic kingdom of Protista.
Allele One of the alternative forms of a single gene (that is, a particular nucleotide sequence occurring at a given locus on a chromosome).
Alien's rule The generalization that warm-blooded animals (mammals) tend to have shorter extremities (for example, ears and tail) in colder climates than they have in warmer climates.
Allometry Differential growth rates of different body parts; during development one feature may change at a rate different from that of another feature, resulting in achange of shape. (For a change in developmental timing, see Heterochrony.)
AllopatricSpecies or populations whose geographical distributions do not contact each other.
Allopatric speciationSpeciation between populations that are geographically separated.
Allopolyploid An organism or species that has more than two sets (2n) of chromosomes (that is, 3n, 4n, and so on) that derive from two or more different ancestral groups.
Allozyme The particular form (amino acid sequence) of an enzyme produced by a particular allele at a gene locus when there are different possible forms of the enzyme (different possible amino acid sequences), each produced by a different allele.
Alternation of generationsLife cycles in which a multicellular haploid stage (1n) alternates with a multicellular diploid stage (2n).
Altruism Behavior that benefits the reproductive success of other individuals because of an actual or potential sacrifice of reproductive success by the altruist.
Amino acidsOrganic molecules of the general formula R-CH(NH,)COOH, possessing both basic (NH2) and acidic (COOH) groups, as well as a side group (R) specific for each type of amino acid. Normally 20 different types of amino acids are used in cellularly synthesized proteins.
Amino group An -NH2 group.
Amniotic egg The type of egg produced by reptiles, birds, and mammals (Amniota), in which the embryo is enveloped in a series of membranes (amnion, allantois, chorion) that help sustain its development.
Anaerobic Growth (energy obtained from the oxidative breakdown of food molecules) in the absence of molecular oxygen,
Anaerobic respiration An electron transport system in which substances other than oxygen serve as the terminal electron acceptor (for example, sulfates, nitrates, methane).
Anagenesis The evolution of new species that takes place progressively over time within a single lineage (branch), as opposed to cladogenesis where a group diverges into two or more branches. (See also Phyletic evolution.)
Analogy The possession of a similar character by two or more quite different species or groups that arises from a developmental pathway unique to each group; that is, the similarity is caused by factors other than their distant common genetic ancestry. (See also Convergence.)
Aneuploidy The gain or loss of chromosomes leading to a number that is not an exact multiple of the basic haploid chromosome set (n) (for example, n+1, 2n+1, 2n-1, 2n-2, 2n+3, and so on).
Angiosperms The flowering plants, an advanced group of vascular plants with floral reproductive structures and encapsulated seeds.
Angstrom (Å) A length one-ten billionth (10-10) of a meter.
Antibody A protein produced by the immune system that binds to a substance (antigen) typically foreign to the organism.
Anticodon A sequence of three nucleotides (a triplet) on transfer RNA that is complementary to the codon on messenger RNA that specifies placement of a particular amino acid in a polypeptide during translation.
Antigen A substance, typically foreign to an organism, that initiates antibody formation and is bound by the activated antibody.
Apomixis (apomictic) Reproduction without fertilization; offspring produced from unfertilized eggs in which meiosis has been partially or completely suppressed. (See also Parthenogenesis.)
Apomorphy A character that has been derived from, yet differs from, the ancestral condition. (See also Synapomorphy.)
Aposematic Conspicuous warning coloration in potential prey species that advertises their toxicity or distastefulness to predators. Aposematic patterns usually contain bright colors or shades such as those found among wasps, monarch butterflies, coral snakes, skunks, and poisonous salamanders.
Arboreal Living predominantly in trees.
ArchaebacteriaProkaryotes that, unlike eubacteria, do not incorporate muramic acid into their cell walls and possess other distinguishing characteristics. They are considered to represent one of the early cell forms.
Archetype The concept of an ideal primitive plan ("Bauplan") on which organisms, such as vertebrates, are presumably based. Called by Richard Owen the "primal pattern" and "divine idea."
Artificial selectionSelection process in which humans are the selective agents.
Asexual reproduction Offspring produced by one parent in the absence of sexual fertilization or in the absence of gamete formation.
Assortative mating Mating among individuals on the basis of their phenotypic or genotypic similarities (positive assortative) or differences (negative assortative) rather than mating among all individuals on a random basis.
Autocatalytic reaction Instances in which the agent that promotes (catalyzes) a reaction is formed as a product of
Autopolyploid A species or organism that has more than two sets of chromosomes (polyploid) derived from one or more duplications in a single ancestral source.
Autosome A chromosome whose presence or absence is ordinarily not associated with determining the difference in sex (that is, a chromosome other than a sex chromosome).
Autotroph An organism capable of synthesizing complex organic compounds needed for growth from simple inorganic environmental substrates: photoautotroph, an organism that can use light as an energy source and carbon dioxide as a carbon source; chemoautotroph (chemolithotroph), an organism that obtains energy for growth by oxidizing inorganic compounds such as hydrogen sulfide.

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Bacteriophage A virus (phage) that parasitizes bacteria.
Balanced genetic load The decrease in overall fitness of a population caused by defective genotypes (for example, homozygotes for deleterious recessives) whose alleles persist in the population because they confer selective advantages in other genotypic combinations (for example, heterozygote advantage).
Balanced polymorphism The persistence of two or more different genetic forms through selection (for example, heterozygote advantage) rather than because of mutation or other evolutionary forces.
Banded iron formation An iron-containing laminated sedimentary rock, often composed of layers of tiny quarts crystals (chert).
Basalt A fine-grained igneous rock found in oceanic crust and produced in lava flows.
Base (nucleotide) The nitrogenous component of the nucleotide unit in nucleic acids, consisting of either a purine (adenine. A, or guanine, G) or pyrimidine (thymine, T, or cytosine, C, in DNA; uracil, U, or cytosine, C, in RNA). (See also Purine, Pyrimidine.)
Base pairs See Complementary base pairs.
Basic (alkaline) A compound that produces an excess of hydroxyl (OH ) ions when dissolved in water. Using quantitative hydrogen ion measurements, such solutions have a pH value greater than 7.0. (See pH scale.)
Batesian mimicry The similarity in appearance of a harmless species (the mimic) to a species that is harmful or distasteful to predators (the model), maintained because of selective advantage to the relatively rare mimic.
Bauplan Structural body plan that characterizes a group of organisms. (See also Archetype.)
Benthic Refers to the floor of a body of water (for example, ocean bottom, riverbed, lake bottom) and to organisms that live in it, on it, or near it.
Bergmann's rule The generalization that animals living in colder climates tend to be larger than those of the same group living in warmer climates.
Bible Holy book of Christianity. Its canon has been fixed for over 1700 years, although the rise of Protestantism and the Protestant Bible has led to minor variations. Comprised of two smaller books called the Old Testament and the New Testament.
Big bang theory The concept that the universe was born in a gigantic explosion about 10 to 20 billion years ago.
Bilateral symmetry Instances in which the left and right sides of a longitudinal (sagittal) plane that runs through an organism's midline are approximately mirror images of each other.
Binary fission Replication of an organism by its division into two mostly equal parts; the common form of asexual reproduction in prokaryotes and protistan eukaryotes.
Binomial expansion The binomial (a + b) raised to a power n [(a + b)n] where a and b represent alternative states whose sum equals the probability of 1.
Binomial nomenclature The Linnaean principle of designating a species by two names: the name of a genus followed by the name of a species.
Biogenetic law (Haeckel) The concept that stages in the development of an individual (ontogeny) recapitulate the evolutionary history (phylogeny) of the species. (See also Heterochrony.)
Biogeography The study of the geographical distributions of organisms. A biogeographical realm is a region characterized by a distinctive biota.
Biological species concept The view that the primary criterion for separating one species from another is their reproductive isolation.
Biosphere That part of the earth containing all living organisms.
Biota All organisms, including animals (fauna) and plants (flora) of a given region or time period.
Biotic Relating to or produced by biological organisms.
Bipedal A term used mostly to describe terrestrial tetrapod locomotion that is restricted to the hind limbs when these two limbs move alternately (for example, human walking) rather than together (for example, kangaroo jumping).
Blastocoele The cavity of a blastula.
Blastopore The opening formed by the invagination of cells in the embryonic gastrula, connecting its cavity (archenteron) to the outside. In protostome phyla the blastopore is the site of the future mouth, whereas in deuterostomes the blastopore becomes the anus and the mouth is formed elsewhere.
Blastula A hollow sphere enclosed within a single layer of cells, occurring at an early stage of development in various multicellular animals.
Blending inheritance The abandoned concept that offspring inherit a dilution, or blend of parental traits, rather than the particles (genes) that determine those traits.
Bottleneck effect A form of genetic drift that occurs when a population is reduced in size (population crash) and later expands in numbers (population flush). The enlarged population that results may have gene frequencies that are distinctly different from those before the bottleneck. (See also Founder effect.)
Brachiation Apelike locomotion through trees: hanging from branches and swinging alternate arms (left, right, left,...) from branch to branch, accompanied by a rotation of the body during each swing.
Brackish Water whose salt content (salinity) is intermediate between fresh water and sea water; usually at the mouths of rivers that empty into the ocean (estuaries).
Bradytelic A relatively slow evolutionary rate.
Bryophyte Mosses and liverworts, small "primitive" land plants.
Buccal Pertaining to the inside of the mouth; side of a tooth closest to the cheek.
Burrowing animal In aquatic forms, a bottom-dweller that moves through soft benthic sediments.

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Calorie The amount of heat necessary to raise the temperature of 1 gram of water by 1 degree centigrade at a pressure of 1 atmosphere.
Calvin cycle A cyclic series of light-independent reactions that accompany photosynthesis and that reduce carbon dioxide to carbohydrate.
Cambrian period The interval between about 545 and 505 million years before the present, marking the plentiful appearance of fossilized organisms with hardened skeletons. It is considered the beginning of the Phanerozoic time scale (eon) and is the first period in the Paleozoic era.
Carbohydrate A compound in which the hydrogen and oxygen atoms bonded to carbons are commonly in a ratio of 2/1 (for example, glucose (C6H12O6), starch (C6H12O6)n, and cellulose, (C6H10O5)n).
Carbonaceous Possessing organic (carbon) compounds.
Carbonaceous chondrites Meteorites containing carbon compounds.
Carboxylic acid An organic compound that has an acidic group consisting of a carbon with a double-bond attachment to an oxygen atom and a single-bond attachment to a hydroxyl group (0=C-OH).
Carnivores Flesh eaters; organisms (almost entirely animal, rarely plant) that feed on animals.
Carrying capacity The theoretical maximum number of organisms in a population, usually designated by K, that can be sustained in a given environment.
Catalyst A substance that lowers the energy necessary to activate a reaction but is not itself consumed or altered in the reaction.
Catastrophism The eighteenth- and nineteenth-century concept that fossilized organisms and changes in geological strata were produced by periodic, violent, and widespread catastrophic events (presumably caused by capricious supernatural forces) rather than by naturally explainable events based on laws that act uniformly through time. (See also Uniformitarianism.)
Cell wall The rigid or semirigid extracellular envelope (outside the plasma membrane) that gives shape to plant, algal, fungal, and bacterial cells.
Cenozoic era The period from 65 million years ago to the present, marked by the absence of dinosaurs and the radiation of mammals. This is the third and most recent era of the Phanerozoic eon and is divided into two major periods, the Tertiary and Quaternary.
Centigrade scale (oC) A scale of temperature in which the melting point of ice is taken as 0o and the boiling point of water as 100o, measured at 1 atmosphere of pressure.
Centrifugal selection See Disruptive selection.
Centripetal selection See Stabilizing selection.
Centromere The chromosome region in eukaryotes to which spindle fibers attach during cell division.
Character A feature, trait, or property of an organism or population. If possible, the description of a character should include the conditions under which it is observed.
Character displacement Divergence in the appearance or measurement of a character between two species when their distributions overlap in the same geographical zone, compared to the similarity of the character in the two species when they are geographically separated. When common resources are limited, it is presumed that competition between overlapping species leads to divergent specializations and therefore to divergence in characters that were formerly similar.
Cheek teeth Mammalian premolar and molar teeth.
Chemiosmosis Linkage between a chemical process (electron transport chain) and a proton pump that causes protons (H+) to be transferred across a membrane. This creates a proton gradient that drives a membrane-bound enzyme, ATP synthetase, to catalyze the reaction ADP + Pi -> ATP.
Chemoautotroph (chemolithotroph) See Autotroph.
Chert A sedimentary rock composed largely of tiny quartz crystals (SiO,) precipitated from aqueous solutions.
Chloroplast A chlorophyll-containing, membrane-bound organelle that is the site of photosynthesis in the cells of plants and some protistans. These organelles contain their own genetic material (circular DNA without histones) and are believed to be descendants of cyanobacteria that entered eukaryotic cells via endosymbiosis.
Chromatid One of the two sister products of a eukaryotic chromosome replication, marked by an attachment between the sister chromatids at the centromere region. When this attachment is broken during the mitotic anaphase stage, each sister chromatid becomes an independent chromosome.
Chromosome A length of nucleic acid comprising a linear sequence of genes that is unconnected to other chromosomes. In eukaryotes, histone proteins are bound to nuclear chromosomes, and this protein-nucleic acid complex can be made microscopically visible as deeply staining filaments.
Chromosome aberration A change in the gene sequence of a chromosome caused by deletion, duplication, inversion, or translocation.
Citric acid cycle See Krebs cycle.
Clade A cluster of taxa derived from a single common ancestor.
Cladistics A mode of classification based principally on grouping taxa by their shared possession of similar ("derived") characters that differ from the ancestral condition.
Cladogenesis "Branching" evolution involving the splitting and divergence of a lineage into two or more lineages.
Cladogram A tree diagram representing phylogenetic relationships among taxa,
Class A taxonomic rank that stands between phylum and order; a phylum may include one or more classes, and a class may include one or more orders.
Classification The grouping of organisms into a hierarchy of categories commonly ranging from species to genera, families, orders, classes, phyla, and kingdoms, each category reflecting one or more significant features. In practice, the decision as to the species in which to place an organism, or the genus in which to place a species, and so forth, is most often based on phenotypic similarity to other members of the group: organisms in a species are more similar to each other than they are to organisms in other species of the same genus, species in a genus are more similar to each other than they are to species in other genera of the same family, and so forth.
Cline A gradient of phenotypic or genotypic change in a population or species correlated with the direction or orientation of some environmental feature, such as a river, mountain range, north-south transect, or altitude.
Clone A group of organisms derived by asexual reproduction from a single ancestral individual,
Cloning (gene) Techniques for producing identical copies of a section of genetic material by inserting a DNA sequence into a cell, such as a bacterium, where it can be replicated.
Coacervate An aggregation of colloidal particles in liquid phase that persists for a period of time as suspended membranous droplets.
Coadaptation The action of selection in producing adaptive combinations of alleles at two or more different gene loci.
Coalescence A statistical term used to describe relationships among different gene sequences that are all descended from a common ancestral sequence (the "coalescent"). Instead of a genealogical tree expanding each generation from its apex toward its base, the tree is conceived as collapsing (coalescing) by proceeding in reverse, from base to apex.
Coarse-grained environment A heterogeneous environment in which individuals in a population are exposed to conditions different from other individuals.
Codominance The independent phenotypic expression of two different alleles in a heterozygote (for example, genotypes carrying both Mand N alleles of the MN blood group show the MN blood type).
Codon The triplet of adjacent nucleotides in messenger RNA that codes for a specific amino acid carried by a specific transfer RNA or that codes for termination of translation ( stop codons). Placement of the amino acid is based on complementary pairing between the anticodon on tRNA and the codon on mRNA. (See also Anticodon.)
Coelom An internal body cavity, lined in eucoelomates (true coelomates) with mesoderrnal tissue that may contain organs such as testes and ovaries.
Coenzymes Nonprotein enzyme-associated organic molecules (for example, NAD, FAD, and coenzyme A), that participate in enzymatic reactions by acting as intermediate carriers of electrons, atoms, or groups of atoms.
Coevolution Evolutionary changes in one or more species in response to changes in other species in the same community.
Cofactor A small molecule, which may be organic (that is, a coenzyme) or inorganic (that is, a metal ion), required by an enzyme in order to function.
Cohort Individuals of a population that are all the same age.
Commensalism An association between organisms of different species in which one species is benefited by the relationship but the other species is not significantly affected.
Competition Relationship between organismic units (for example, individuals, groups, species) attempting to exploit a limited common resource in which each unit inhibits, to varying degrees, the survival or proliferation of another unit by means other than predation.
Complementary base pairsNucleotides on one strand of a nucleic acid that hydrogen bond with nucleotides on another strand according to the rule that pairing between purine and pyrimidine bases is restricted to certain combinations: A pairs with T in DNA, A pairs with U in RNA, and G pairs with C in both DNA and RNA.
Complexity A state of intricate organization caused by arrangement or interaction among different component parts or processes: presumably, the greater the number of interacting parts, the greater the complexity. The term levels of complexity describes gradations in which complex organizations are included (nested) within others. (See also Hierarchy.) Attempts to compare the degrees of complexity among organisms have used numbers of their different kinds of structures, organs, tissues, cells, genes, and proteins. However, such numbers do not always change in a consistent fashion, and MeShea points out, "Something maybe increasing [in evolution]. But is it complexity?"
Concerted evolution The process by which a series of nucleotide sequences or different members of a gene family remain similar or identical through time.
Condensation (by dehydration) The formation of a covalent bond between two molecules by removal of H2O.
Condylarths A mammalian order that became extinct during the Miocene period of the Cenozoic era but whose first occurrences are in the late Cretaceous period of the Mesozoic. It includes a diversity of early herbivorous placental mammals and the ancestors of all later herbivores.
Constraint Constraint has been used in biology to describe factors that limit character variation or evolutionary direction. According to some authors, the term applies to traits molded primarily by physical agents and laws, such as crystallization, friction, gravity, and surface tension, but unaffected by historical contingencies, such as selection. Others argue that excluding biological factors does not help explain distinctions among phenotypic variants, nor the causes for differences among lineages. A biological concept of constraint therefore seems more useful to evolutionists - phenotypic channeling and evolutionary trends caused mostly by processes involving adaptation. That is, because adaptation depends on the availability of appropriate genes, adaptive constraints are mostly tied to organismic histories: constraints are affected by genes that evolved and were selected previously. Among such constraining forces are directional selection, stabilizing selection, canalization, and factors that may limit or direct genetic mutability, as well as developmental innovation. Some constraints may focus phenotypic trends in adaptive directions, but others (limited genetic variation and restricted developmental ability) may also limit response to new environmental challenges, leading even to extinction. Although constraints can provide reasonable explanations for trends and attributes, postulating which particular constraints were in force seems highly conjectural in the absence of detailed historical-phylogenetic information. For a term that can apply to so many different phiiiomena, constraint's usage in specific instances needs to be defined.
Continental drift The movement, over time, of large landmasses—tectonic plates—on the earth's surface relative to each other. (See also Paleomagnetism, Sea floor spreading.)
Continuous variationCharacter variations (such as height in humans) whose distribution follows a series of small nondiscrete quantitative steps from one extreme to the other. (See also Quantitative character.)
Convergence (also called Homoplasy) The evolution of similar characters in genetically unrelated or distantly related species, mostly because they have been subjected to similar environmental selective pressures. (See also Analogy.)
Cope's rule The generalization (not always confirmed) that body size tends to increase in an animal lineage during its evolution.
Correlation The degree to which two measured characters tend to vary in the same quantitative direction (positive correlation) or in opposite directions (negative correlation).
Cosmology Study of the structure and evolution of the universe.
Covalent bond A strong chemical bond that results from the sharing of electrons between two atoms.
Creation Creationist term for the beginning when God created the earth and the universe. For YECs creation was only some thousands of years ago and is literally described in Genesis. For OECs creation was billions of years ago and Genesis is interpreted allegorically.
Creationism The belief that each different kind of organism was individually created by one or more supernatural beings whose activities are not controlled by known physical, chemical, or biological laws. (See also Young Earth Creationism (YEC), Old Earth Creationism (OEC))
Creodonts An extinct order of early Cenozoic placental mammals that were the dominant carnivores until replaced by the modern order Carnivora during the Oligocene period.
Crepuscular A lifestyle characterized by activity mostly during the hours around dawn and dusk.
Crossovers (chromosome) Results of a process (crossing over) in which the chromatids of two homologous chromosomes exchange genetic material. (See also Recombination. )
Cryptic A feature that is normally not visible.
Culture (social) The learned behaviors and practices common to a social group.
Cursorial Adapted for running on land.
Cusps (teeth) Elevations on the crowns of premolars and molars. The number, shapes, and positions of cusps are inherited characters that can provide useful phylogenetic information.
Cyanobacteria Photosynthetic prokaryotes possessing chlorophyll a but not chlorophyll b. Many are photosynthetic aerobes (oxygen producing) and some are anaerobes (not oxygen producing). Formerly called blue- green algae, their color caused by a bluish pigment masking the chlorophyll.
CytochromesProteins containing iron-porphyrin (heme) complexes that function as hydrogen or electron carriers in respiration and photosynthesis.
Cytology The study of cells—their structures, functions, components, and life histories.
Cytoplasm All cellular material within the plasma membrane, excluding the nucleus.

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Darwinism The concept, proposed by Charles Darwin, that biological evolution has led to the many different highly adapted species through natural selection acting on hereditary variations in populations.
Deciduous (teeth) Teeth that are replaced during development by permanent teeth.
Deficiency See Deletion.
Degenerate (redundant) code The type of genetic code used by existing terrestrial organisms, for which there is more than one triplet codon for a particular amino acid but a specific codon cannot code for more than one amino acid. Thus, the 20 different amino acids translated in protein synthesis are coded by 61 of the 64 possible different triplet codons, some by as many as six different "synonymous" codons. (See also Genetic code.)
Dehydrogenase An enzyme that catalyzes the removal of hydrogen from a molecule (oxidation).
Deleterious allele An allele whose effect reduces the adaptive value of its carrier when present in homozygous condition (recessive allele) or in heterozygous condition (dominant or partially dominant allele).
Deletion An aberration in which a section of DNA or chromosome has been lost.
Deme A local population of a species (in sexual forms, a local interbreeding group).
Density dependent The dependence of population growth and size on factors directly related to the numbers of individuals in a particular locality (for example, competition for food, accumulation of waste products).
Density independent The dependence of population growth on factors (climatic changes, meteorite impacts, and so on) unrelated to the numbers of individuals in a particular locality.
Dentin The hard inner layer of a tooth that surrounds the tooth pulp. It is covered by even harder enamel at the crown, and by softer cement at the root.
Derived character A character whose structure or form differs (apomorphic) from that of the ancestral stock.
Deuterium An isotope of hydrogen containing one proton and one neutron, giving it twice the mass of an ordinary hydrogen atom.
Deuterostomes Coelomate phyla in which the embryonic blastopore becomes the anus.
Development The recurrent sequence of progressive changes in organisms from inception to maturity.
Dicotyledons Flowering plants (angiosperms) in which the embryo bears two seed leaves (cotyledons).
Differentiation Changes that occur in the structure and function of cells and tissues as the development of the organism proceeds. Generally, the change from an immature embryo to a more complex mature organism.
Dimorphism Presence in a population or species of two morphologically distinctive types of individuals (for example, differences between males and females, pigmented and nonpigmented forms).
DioeciousOrganisms in which the male and female sex are in separate individuals.
Diploblastic An animal that produces only two major types of cell layers during development, ectoderm and endoderm (for example, Cnidaria).
Diploid An organism whose somatic cell nuclei possess two sets of chromosomes (2n), providing two different (heterozygous) or similar (homozygous) alleles for each gene.
Directional selectionSelection that causes the phenotype of a character to shift toward one of its phenotypic extremes.
Discontinuous variationCharacter variations that are sufficiently different from each other that they fall into nonoverlapping classes.
Disruptive selectionSelection that tends to favor the survival of organisms in a population that are at opposite phenotype extremes for a particular character and eliminates individuals with intermediate values (centrifugal selection).
Diurnal A lifestyle characterized by activity during the dayrather than at night (nocturnal).
Divergent evolution Change leading to differences between lineages.
DNA (deoxyribonucleic acid) A nucleic acid that serves as the genetic material of all cells and many viruses; composed of nucleotides that are usually polymerized into long chains, each nucleotide characterized by the presence of a deoxyribose sugar.
DNA ligase An enzyme that joins sections of DNA together.
Domain In molecular biology, an amino acid sequence within a polypeptide chain that performs a particular subfunction in the protein. The term has also been used in systematics to provide a tripartite division of organisms - Archaea, Bacteria, Eucarya - as a substitute for the rank of superkingdom, which commonly designates prokaryotes and eukaryotes.
Dominance (allele) Instances in which the phenotypic effect of a particular allele (for example, A, the dominant) is expressed in both the heterozygote (Aa) and hornozygote (AA), but the phenotypic effect of the other allele (for example, a, the recessive) is not expressed in heterozygotes but only in homozygotes (aa).
Dominance (social) Relations within a group in which one or more individuals, sustained by aggression or other behaviors, rank higher than others in controlling the conduct of group members.
Doppler effect The shift in wavelength of light or sound that is perceived as the emitting body moves toward us (shorter wavelengths, for example, blue-shifted) or away from us (longer wavelengths, for example, red-shifted).
Dorsal The back side or upper surface of an animal; opposite of ventral, (In vertebrates, the surface closest to the spinal column.)
Dosage compensation A mechanism that compensates for the difference in number of X chromosomes (or Z chromosomes) between males and females so the metabolic activities (gene expression) of their X-linked genes are equalized. Although dosage compensation is widespread among animals, the mechanism by which it is accomplished varies. In species with XY males and XX females such as Drosophila, male X-linked genes show increased gene expression, whereas in mammals only one X chromosome in each sex is metabolically active and any additional X chromosomes are inactivated.
Double fertilization A distinctive feature of angiosperm plants in which two nuclei from a male pollen tube fertilize the female gametophyte, one producing a diploid embryo and the other producing polyploid (usually triploid) nutritional endosperm.
Duplication Instances in which a particular section of DNA or visible chromosome segment occurs more than once.

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[ E ]
Ecogeographical rules Generalizations that correlate adaptational tendencies of species with environmental factors such as climate. (See Alien's rule, Bergmann's rule, Gloger's rule.)
Ecological niche The environmental habitat of a population or species, including the resources it uses and its interactions with other organisms. Since resources and interactions are rarely constant, populations remain continually subject to selective pressures for adaptational change. A particular organism's ecological niche is commonly reflected in its adaptations when these can be specified. (See Adaptation.)
Ecology The study of the relations between organisms and their environment, in terms of their numbers, distributions, and life cycles.
Ecotype A phenotypic and genotypic variant of a species associated with a particular environmental habitat. (See also Race.)
Ectoderm The outermost layer of cells that covers the early animal embryo, from which nerve tissues and outermost epidermal tissues are derived.
Ectothermic "Cold-blooded": a body temperature primarily determined by the ambient (environmental) temperature.
Ediacaran strata Geological formations containing soft- bodied invertebrate fossils found in South Australia and other places, dating to a Precambrian period lasting about 60 or more million years.
Electron carrier In oxidation reactions, a molecule that acts alternatively as an electron donor (becomes oxidized) and as an electron acceptor (becomes reduced).
Electrophoresis A technique that separates dissolved particles subjected to an electrical field according to their mobility. Given a particular medium through which a particle moves, electrophoretic mobility depends on the size of the particle, its geometry, and electrical charge.
Endemic A species or population that is specific (indigenous) to a particular geographic region.
Endocytosis Cellular engulfment of outside material, followed by its transfer into the cellular interior encapsulated in a membrane.
Endoderm The layer of cells that lines the primitive gut (archenteron) during the early stages of development in animals, and later forms the epithelial lining of the intestinal tract and internal organs such as the liver, lung, and urinary bladder.
EndonucleasesEnzymes that fragment DNA chains. (See also Restriction enzymes.)
Endosymbiosis A relationship between two different organisms in which one (the endosymbiont) lives within the tissues or cell of the other, benefiting one or both. It is now generally thought that some eukaryotic organelles, such as mitochondria and chloroplasts, had an endosymbiotic prokaryotic origin.
Endothermic "Warm-blooded": a body temperature maintained by internal physiological mechanisms at a level independent of the ambient (environmental) temperature.
Enhancer A nucleotide sequence that allows gene transcription to increase even though the gene may be quite distant. It does this by changing the configuration of the intervening nucleotide sequence, making the gene's promoter sequence more available for transcription.
Entropy The measure of disorder of a physical system. In a closed system, to which energy is not added, the second law of thermodynamics essentially states that entropy, or energy unavailable for work, will remain constant or increase but never decrease. Living systems, however, are open systems, to which energy is added from sunlight and other sources, and order can therefore arise from disorder in such systems, that is, energy available for work can increase and entropy can decrease.
Environment The complex of external conditions, abiotic and biotic, that affects organisms or populations. It provides the facilities and resources that enable hereditary data (genotypes) to produce organismic features (phenotypes).
Enzyme A protein that catalyzes chemical reactions.
Eon A major division of the geological time scale, often divided into two eons beginning from the origin of the earth 4.5 billion years ago: the Precambrian or Cryptozoic (rarity of life forms) and the Phanerozoic (abundance of life forms).
Epigamic selectionSelection for mating success based on appearance or behavior during courtship.
Epigenesis The concept that tissues and organs are formed by interaction between cells and substances that appear during development, rather than being initially present in the zygote (preformed). (See also Preformationism. )
Epistasis Interactions between two or more gene loci that produce phenotypes different from those expected if each locus were considered individually. In statistical population studies that evaluate the causes for phenotypic differences, the term epistasis is commonly used for all phenotypic variation caused by interaction between nonallelic genes.
Epoch One of the categories into which geological time is divided; a subdivision of a geological period. For periods divided into three epochs, they are often named Early, Middle, and Late; for example, Early Cambrian period, . . .
Equilibrium (genetic) The persistence of the same allelic frequencies over a series of generations. Equilibria may be stable or unstable. In a stable equilibrium (for example, when the heterozygote is superior in fitness to the homozygotes), the population returns to a particular equilibrium value when the allelic frequencies have been disturbed. In an unstable equilibrium (for example, when the heterozygote is inferior in fitness to the homozygotes), such disturbances are not followed by a return to equilibrium frequencies.
Era A division of geological time that stands between the eon and the period: the Phanerozoic eon is divided into Paleozoic, Mesozoic, and Cenozoic eras; and each era is divided into two or more periods.
Estrus The interval during which female mammals exhibit maximum sexual receptivity, usually coinciding with the release of eggs from the ovary.
EubacteriaProkaryotes, other than archaebacteria, marked by sensitivity to particular antibiotics and by the incorporation of muramic acid into their cell walls.
Euchromatin Normally staining chromosomal regions that possesses most of the active genes. (See also Heterochromatin.)
Eucoelomates See Coelom.
Eugenics The concept that humanity can be improved by altering human genotypes or their frequencies.
EukaryotesOrganisms whose cells contain nuclear membranes, mitochondrial organelles, and other characteristics that distinguish them from prokaryotes. Eukaryotes may be unicellular or multicellular and include protistans, fungi, plants, and animals.
EuploidyVariations that involve changes in the number of entire chromosome sets (n) (for example, 3n, 4n, 5n).
Eutelegenesis The use of artificial insemination to improve genetic endowment.
Eutely Constancy in the numbers of cells or nuclei from the larval stage to the adult stage.
Eutheria See Placentals.
Eutrophication The process in which an aquatic system becomes overloaded with nutrients, thereby increasing its organic productivity and causing an accumulation of debris.
Evolution Genetic changes in populations of organisms through time that lead to differences among them.
Evolutionary clock (Molecular clock) The concept that the rate at which mutational changes accumulate is constant over time. To which genes or genomes this clock may apply, and whether it is really constant, are disputed.
Exon A nucleotide sequence in a gene that is transcribed into messenger RNA and spliced together with the transcribed sequences of other exons from the same gene. The continuous RNA molecule formed is then transferred to the ribosome and forms the template used in polypeptide synthesis. Exons ("expressed sequences") are separated from other exons in the same gene by intervening nontranslated sequences (See Intron) that are removed from the rnRNA. Such intron-exon split genes are commonly found in eukaryotes but are almost entirely absent in prokaryotes,
Extant Currently in existence.
Extension Movement of an appendage so that the angle of the joint increases.
Extinction The disappearance of a species or higher taxon.

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F (inbreeding coefficient) See Inbreeding coefficient.
Family A taxonomic category that stands between order and genus; an order may comprise a number of families, each of which contains a number of genera.
Fauna All animals of a particular region or time period.
Fecundity A measure of potential fertility, often calculated in terms of the quantity of gametes produced while sexually mature.
Feedback When the products of a process affect its own function.
Fermentation The anaerobic degradation of glucose (glycolysis) or related molecules, yielding energy and organic end products.
Fertility A trait measured by the number of viable offspring produced.
Filter feeder An animal that obtains its food by filtering suspended food particles from water.
Fine-grained environment A heterogeneous environment whose varied conditions can normally be experienced by a single individual during its lifetime.
Fitness Central to evolutionary concepts evaluating genotypes and populations, fitness has had many definitions, ranging from comparing growth rates to comparing long-term survival rates. The basic fitness concept that population geneticists commonly use is relative reproductive success, as governed by selection in a particular environment; that is, the ability of an organism (genotype) to transmit its genes to the next reproductively fertile generation, relative to this ability in other genotypes in the same environment ("relative fitness"). Since there are forces other than selection that influence genotype frequencies (for example, mutation, random genetic drift, migration), fitness is not the only way of characterizing short-term populational genetic changes. Nevertheless, because reproductive success, sooner or later, affects most variation, fitness and selection enter into practically all enduring organismic-environmental interactions, with adaptations their phenotypic manifestations.
Fixation Achievement of a frequency of 100 percent (monomorphism) by an allele or genotype that begins in a population at a lesser frequency (polymorphism).
Fixity of species A concept held by Linnaeus and others that members of a species could only produce progeny like themselves, and therefore each species was fixed in its particular form(s) at the time of its creation.
Flexion Movement of an appendage so that the angle of the joint decreases.
Flora All plants of a particular region or time period.
Fossils The geological remains, impressions, or traces of organisms that existed in the past.
Founder effect The effect caused by a sampling accident in which only a few "founders" derived from a large population begin a new colony. Since these founders carry only a small fraction of the parental population's genetic variability, radically different gene frequencies can become established in the new colony. (See also Bottleneck effect.)
Frequency-dependent selection Instances where the effect of selection on a phenotype or genotype depends on its frequency (for example, a genotype that is rare may have a higher adaptive value than when it is common).
Frozen accident The concept that an accidental event in the distant past was responsible for the presence of a universal feature in living organisms. Such events may include an accident in which the present genetic code was used by a group of early organisms that managed to survive some populational bottleneck, thereby conferring this particular code on later organisms.
Fundamentalism (religious) The belief that creation stories and the many events and rules given in religious documents (for example, the Judeo-Christian Bible, the Moslem Koran) are to be taken literally.

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Galaxy A system of numerous stars such as the Milky Way (150 billion stars, 100,000 light-years across) held together by mutual gravitational effects. Galaxies, in turn, are grouped into clusters and superclusters. Our own supercluster, centered on Virgo, contains many thousands of galaxies and is more than 100 million light-years across.
Gamete A germ cell (usually haploid) that fuses with a germ cell of the opposite sex to form a zygote (usually diploid) in a process called fertilization.
Gametophyte The haploid gamete-producing stage of plants that have alternating generations (haploid gametophyte and diploid sporophyte). The gametophyte is produced by meiosis in the sporophyte, and its gametes are produced by mitosis.
Gamma ray A high-frequency, highly penetrating radiation emitted in nuclear reactions.
Gastnila A cuplike embryonic stage in multicellular animals that follows the blastula stage. Its hollow cavity (archenteron) is lined with endoderm and opens to the outside through a blastopore. (See also Haeckel's gastrula hypothesis.)
Gene A unit of genetic material composed of a sequence of nucleotides that provides a specific function to an organism, either by: The position a gene occupies on a chromosome is called a locus, and each different nucleotide sequence of a gene is called an allele.
Genealogy A record of familial ties and ancestral connections among members of a group.
Gene family Two or more gene loci in an organism whose similarities in nucleotide sequences indicate they have been derived by duplication from a common ancestral gene (for example, the p-globin gene family, which includes beta, gamma, delta and epsilon genes).
Gene flow The migration of genes into a population from other populations by interbreeding.
Gene frequency The proportion of a particular allele among all alleles at a gene locus. (Also called alkle or alklic frequency.)
Gene locus The chromosornal position (nucleotide sequence) occupied by a particular gene.
Gene pair The two alleles present in a diploid organism at a specific gene locus on two chromosomes.
Gene pool All the genes present in a population during a given generation or period.
Gene therapy Human-directed repair or replacement of genes that cause inherited diseases. When confined to somatic (body) cells rather than to sex cells (sperm or eggs), such gene repairs are not passed on to future generations.
Genetic code The sequences of nucleotide triplets (codons) on messenger RNA that specify each of the different kinds of amino acids positioned on polypeptides during the translation process. With few exceptions, the genetic code used by all organisms is identical: the 20 amino acids are each specified by the same codons (total: 61 codons), and the same three triplet codons are used to terminate polypeptide synthesis. (See also Degenerate redundant code, Universal genetic code.)
Genetic crossing over See Crossovers (chromosome), Recombination.
Genetic death The inability of a genotype to reproduce itself because of selection.
Genetic distance A measure of the divergence among populations based on their differences in frequencies of given alleles.
Genetic drift See Random genetic drift.
Genetic engineering Manipulation of genetic material from different sources to produce new combinations that are then introduced into organisms in which such genetic material does not normally occur.
Genetic load The loss in average fitness of individuals in a population because the population carries deleterious alleles or genotypes. (See also Balanced Genetic Load, Mutational Load.)
Genetic polymorphism The presence of two or more alleles at a gene locus over a succession of generations. (Called balanced polymorphism when the persistence of the different alleles cannot be accounted for by mutation alone.)
Genome The complete genetic constitution of a cell or an individual.
Genotype The genetic constitution of cells or individuals, often referring to alleles of one or more specified genes. Provides the hereditary information necessary for phenotypic development.
Genus (plural, genera) A taxonomic category that stands between family and species: a family may comprise a number of genera, each of which contains a number of species that are presumably related to each other by descent from a common ancestor. In taxonomic binomial nomenclature, the genus is used as the first of two words in naming a species; for example. Homo (genus) sapiens (species).
Geographic isolation The separation between populations caused by geographic distance or geographic barriers.
Geographic speciation See Allopatric speciation.
Geological strata A series of layers of sedimentary rock.
Geological time scale The correlation between rocks (or the fossils contained in them) and time periods of the past.
Germinal choice See Eutelegenesis.
Germ plasm Cells or tissues in a multicellular organism that are exclusively devoted to transmitting hereditary information to offspring, either asexually or by means of gametes (sex cells). These "germ-line" cells are in contrast to the somatic cells that produce the nongerm-line body tissues.
Gloger's rule The generalization that warm-blooded (endothermic) animals tend to have more pigmentation in warm, humid areas than in cool, dry areas.
Glycolysis The energy-producing conversion of glucose to pyruvate under anaerobic conditions (fermentation). Subsequent steps may yield lactic acid or ethanol.
Gondwana The supercontinent in the Southern Hemisphere formed from the breakup of the larger Pangaea landmass about 180 million years ago. Gondwana was composed of what is now South America, Africa, Antarctica, Australia, and India.
Grade A level of phenotypic organization or adaptation reached by one or more species. Distantly related or unrelated species that reach the same grade are considered to have undergone parallel or convergent evolution.
Gradient Changes in the amount of a substance as it is displaced from its source.
Granite A coarse-grained igneous rock commonly intruded into continental crust.
Great Chain of Being The eighteenth-century concept that instead of a static universe, there is a continuous progression of stages leading to a superior supernatural being; the transformation of the "Ladder of Nature" into a succession of moving platforms.
Grooming Body surface cleaning by use of mouth, fingers, or claws.
Group selectionSelection acting on the attributes of a group of related individuals in competition with other groups rather than only on the attributes of an individual in competition with other individuals. For example, altruism may not be beneficial to the individual altruist but can be quite beneficial to a group containing altruists. The fitness of an individual in such a group is thus, at least partially, associated with the properties of the group.
Gymnosperms A group of vascular plants with seeds unenclosed in an ovary (naked); mainly cone-bearing trees.

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Habitat The place and conditions in which an organism normally lives. (See also Environment.)
Haeckel's gastrula hypothesis The concept that metazoans developed from swimming hollow-balled colonies of flagellated protozoans that evolved an anterior-posterior orientation in searching for food. The anterior cells, specialized for digestion, invaginated through a circular blastopore to form a digestive archenteron, and this bilayered cup, called a gastrula or gastraea, was, according to Haeckel, the progenitor of the gastrula developmental stage found in some present-day metazoans.
Half-life (radioactivity) The time required for the decay of one-half the original amount of a radioactive isotope: a period of one half-life reduces the isotope amount by one half so that a length of two half-lives leaves a remainder of one-quarter, three half-lives, a remainder of one-eighth, and so on. Each radioactive isotope has a distinctive half-life period, which remains constant over time.
Haplodiploidy A reproductive system found in some animals, such as bees and wasps, in which males develop from unfertilized eggs and are haploid, while females develop from fertilized eggs and are diploid.
Haploid Cells or organisms that have only one set (1n) of chromosomes, meaning the presence of only a single allele for each gene.
Haplotype A sequence of nucleotides, restriction sites, or marker genes inherited as a linked unit from one parent. Since more than a single genetic locus may be involved, a haplotype may be composed of a string of alleles.
Hardy-Weinberg principle The conservation of gene (allelic) and genotype frequencies in large populations under conditions of random mating and in the absence of evolutionary forces, such as selection, migration, and genetic drift, which act to change gene frequencies.
HemizygousGenes, such as those on the X chromosome in a male mammal (hemizygote), which are unpaired in a diploid cell.
Herbivores Animals that feed mainly on plants.
Heritability In a general sense, the degree to which — - variations in the phenotype of a character are caused by genetic differences; traits with high heritabilities can be more easily modified by selection than traits with low heritabilities. (One measure of the heritability of a trait is the ratio of its genetic variance to its phenotypic variance—"broad sense heritability.") Obtaining a trustworthy heritability estimate demands considerable experimental control and is often valid for extremely limited conditions; that is, for specific genotypes in specific environments.
Hermaphrodite An individual possessing both male and female sexual reproductive systems. (See also Monoecious.)
Heterocercal Fish tail in which the vertebral axis is curved (usually upward)
Heterochromatin A region of the eukaryotic chromosome that stains differently from normal-staining "euchromatin" because of its tightly compacted structure. Compared to euchromatin, it is also characterized by possessing very few active genes and many more repetitive DNA sequences. It constitutes about 15 percent of the human genome and about 30 percent of the Drosophila genome, much of it located on either side of chromosome centromeres. In Drosophila, it also constitutes almost the entire Y chromosome.
Heterochrony A term Haeckel originally proposed to describe changes in timing of an organ's development during evolution. Such changes were used to explain departures from the "recapitulation" of phylogeny expected during ontogeny of descendant species. (See Biogenetic law.) Its present usage varies but still hinges on a phylogenetic change in developmental timing, whether of one organ relative to other organs, or of one organ relative to the same ancestral organ. (For organs whose growth rate changes relative to other organs, see Allometry.) Among the consequences of heterochrony are shifts in relative development of reproductive and nonreproductive tissue. (See also Paedomorphosis.) Such changes can cause an organism: (a) to appear more juvenile because its nonreproductive tissues develop more slowly (neoteny), or (b) reach sexual maturity earlier because its reproductive tissues develop more rapidly (progenesis). Some authors add other terms to describe degrees of developmental contraction or extension.
Heterodont An organism with structural and functional differences among its teeth.
Heterogametic The sex that produces two kinds of gametes for sex determination in offspring, one kind for males and the other for females. The heterogametic sex is the male in mammals and the female in birds. (See also Sex chromosomes.)
Heterosis (hybrid vigor) The increase in vigor and performance that can result when two different, often inbred strains are crossed. Since each inbred parental strain may be homozygous for different deleterious recessive alleles (for example, a1a1×a2a2), the cause for has been ascribed by some authors to thesuperiority of heterozygotes (for example, a1a2). (See Heterozygote advantage.)
Heterotroph An organism that cannot use inorganic materials to synthesize the organic compounds needed for growth but obtains them by feeding on other organisms or their products, such as a carnivore, herbivore, parasite, scavenger, or saprophyte.
Heterozygote A genotype or individual that possesses different alleles at a particular gene locus on homologous chromosomes (for example, Aa in a diploid).
Heterozygote advantage (superiority) The superior fitness of some heterozygotes (for example, A1A2) relative to homozygotes (for example, A1A1, A2A2). (See also Heterosis, Overdominance.)
Hierarchy A term used in some evolutionary and developmental studies to designate increasing levels of complexity or organization.
Histories A family of small acid-soluble (basic) proteins that are tightly bound to eukaryotic nuclear DNA molecules and help fold DNA into thick chromosome filaments.
Hitchhiking When a gene persists in a population, not because of selection, but because of close linkage to one or more selected genes. (See also Linkage disequilibrium.)
Homeostasis The term was classically defined by W. B. Cannon to denote the tendency of a (physiological) system to react to an external disturbance so that the system is not displaced from normal values. Probably its most common use applies to traits that measure or perform at constant values in the face of disturbing forces. An example is the persistence of a specific phenotype although confronted with genetic or environmental differences (for example, canalization, p. 358).
Homeotic mutations (homeoboxes and homeodomains) Homeosis was originally defined by William Bateson as "something [that] has been changed into the likeness of something else." In modern genetic usage, homeotic mutations cause the development of tissue in an inappropriate position; for example, the bithorax mutations in Drosophila that produce an extra set of wings. In Drosophila, homeotic genes are clustered into two chromosomally separate groups, the antennapedia and bithorax complexes, in which each cluster contains several independently functioning homeotic genes. Each of these homeotic genes contains a nucleotide sequence (homeobox) coding for a DNA-binding polypeptide (homeodomain) involved in embryonic development along the animal's anterior-posterior axis. Homologous homeobox sequences are found throughout rnetazoan phyla, and the chromosomal organization of their horneobox-containing genes often follows the linkage order noted in Drosophila. The overall conservation of homeobox sequences, of the genes containing them, and of their linkage orders, indicate common developmental functions in different phyla preserved for many hundreds of millions of years, extending back to Precambrian times.
Hominid A member of the family Hominidae, which includes humans, whose earliest fossils can now be dated to about 4 million years ago (genus Australopithecus). Only a single hominid species (Homo sapiens) presently exists.
Hominoids A group (superfamily Hominoidea) that includes hominids (Hominidae), gibbons (Hylobatidae), and anes CPongidae).
Homogametic The sex that produces only one kind of gamete for sex determination in offspring, thus causing sex differences among offspring to depend on the kind of gamete contributed by the heterogametic sex. The homogametic sex is the female in mammals and the male in birds. (See also Sex chromosomes.)
Homologous chromosomesChromosomes that pair during meiosis, each pair usually possessing a similar sequence of genes.
Homology A common use of this term is to characterize the similarity of biological features in different species or groups because of their descent from a common ancestor. Homologous features may include those found in development, structure, and morphology, although similarity on the genetic level probably provides a more reliable estimate of common descent. Since such conservative features can sometimes be quantified, especially for amino acid sequences in protein or base sequences in nucleic acids, homology has also been defined as the extent to which two species share an ancestral character (that is, homology = degree of ancestral similarity), and the value obtained can then be used to help establish phylogenetic relationships among species. (See also Coalescence.) Thus, if species A and B are homologous for 70 percent of a particular ancestral protein whereas they share only 40 percent homology with species C, the assumption can be made that species A and B are more closely related to each other for this protein (have a more recent common ancestor) than to species C. We should keep in mind that homologous genes do not necessarily produce the same features, since an ancestral gene may be recruited for different functions in different lineages. Also, as is obvious from convergence phenomena, functional, morphological, and developmental similarities can be produced by nonhomologous genetic elements of independent evolutionary origin. Emphasis on genetic homology can lead to a different, yet more realistic, phylogeny than the use of other features. Among other definitions are those that consider homology strictly qualitatively—for example, two structures in different species are or are not homologous (derived from a common ancestor)—and omit any quantitative comparative considerations as to the degree of homology.
HomoplasyCharacter similarity that arose independently in different groups whether through parallelism or convergence.
Homozygote A genotype or individual that possesses the same alleles at a particular gene locus on chromosomes.
Horotelic Evolving at a comparatively average rate.
Hubble constant A ratio [(speed of galactic recession)/(distance from earth)] that indicates the rate at which the universe is expanding. Although many astronomers agree on the speed of galactic recession as determined by the red shift (See Doppler effect), they still debate galactic distance from earth, which is based on the brightnesses of celestial bodies. Estimates of the Hubble constant have ranged from about 50, signifying an age for the universe of about 15 billion years, to as much as 100, indicating a more rapid expansion, and therefore a younger age of about 7 or 8 billion years. Some newer distance measurements, using the brightnesses of supernovae, indicate a Hubble constant of 55 to 65, or a universe about 12 billion-years old—a value more in accord with age estimates of the oldest stars.
Hybrid breakdown, inviability, sterility Hybrids that suffer from loss of fitness and reproductive failure.
Hybrids (hybridization) Offspring of a cross between genetically different parents or groups.
Hybrid vigor See Heterosis.
Hydrogen bond A weak, noncovalent bond between a hydrogen atom and an electronegative atom such as oxygen.
Hydrogen ion A proton (H+) that in aqueous solution exists only in hydrated form (H3O+, hydronium ion).
Hydrolysis Splitting of a molecule by the addition of the three atoms from a water molecule (H2O).
Hydrophilic A compound (for example, charged molecule) or part of a compound (for example, polar group) that has an affinity for water molecules.
Hydrophobic Compounds such as lipids that do not readily interact with water but tend to dissolve in organic solvents.
Hydrostatic pressure The pressure exerted by a liquid. When the liquid is in an elastic, muscularly controlled container (for example, the coelom of a worm), changes in shape of the container can be effected by muscularly generated hydrostatic pressure.

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Idealism The philosophy that the universe is constituted of nonmaterial ideas.
Igneous rock A rock such as basalt (fine-grained) and granite (coarse-grained), formed by the cooling ofmolti material from the earth's interior.
Implantation (mammals) The attachment of the embryo to the uterine wall.
Inbreeding Mating between genetically related individuals often resulting in increased homozygosity in their offspring.
Inbreeding coefficient (F) The probability that the two alleles of a gene in a diploid organism are identical because they originated from a single allele in a common ancestor.
Inbreeding depression Decrease in the average value of a character, or in growth, vigor, fertility, and survival, as a result of inbreeding.
Inclusive fitness The fitness of an allele or genotype measured not only by its effect on an individual but also by its effect on related individuals that also possess it (kin selection).
Independent assortment A basic principle of Mendelian genetics—that a gamete will contain a random assortment of alleles from different chromosomes because chromosome pairs orient randomly toward opposite poles during rneiosis.
Industrial melanism The effect of soot and pollution in industrial areas in increasing the frequency of darkly pigmented (melanic) forms perhaps because of selection by predators against nonpigmented or lightly pigmented forms.
Inheritance of acquired characters The concept used by Lamarck to explain evolutionary adaptations—that phenotypic characters acquired by interaction with the environment during the lifetime of an individual are transmitted to its offspring.
Insectivore An animal that feeds primarily on insects.
Instinct An inherited (innate), relatively inflexible behavior pattern that is often activated by one or several environmental factors (releasers).
Intrinsic rate of natural increase The potential rate at which a population can increase in an environment free of limiting factors.
Introgressive hybridization The incorporation of genes from one species into the gene pool of another because some fertile hybrids are produced from crosses between the two species.
Intron A nucleotide sequence in a split gene that intervenes between two exons. The intron sequences are removed from messenger RNA, and only the exon sequences translate into polypeptides.
IntrusionIgneous rock that is inserted within or between geological strata rather than on the earth's surface.
Inversion An aberration in which a section of DNA or chromosome has been inverted 180 degrees, so that the sequence of nucleotides or genes within the inversion is now reversed with respect to its original order in the DNA or chromosome.
Ion An atom or molecule carrying a positive or negative electrostatic charge.
Isolating mechanisms Biological mechanisms that act as barriers to gene exchange between populations. These are generally divided into two groups: premating isolating mechanisms that inhibit cross-fertilization (for example, behavioral differences in courtship) and postmating isolating mechanisms that interfere with the success of the gamete or zygote even when cross-fertilization has occurred (for example, hybrid inviability or sterility).
Isomerase An enzyme that catalyzes the rearrangement of atoms within a molecule.
Isotope One of several forms of an element, with a distinctive mass based on the number of neutrons in the atomic nucleus. (The number of protons and electrons is the same in different isotopes of an element.) Radioactive isotopes decay at a rate that is constant for each isotope and release ionizing radiation as they decay. (See Half-life, Radioactive dating.)

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Karyotype The characteristic chromosome complement of a cell, individual, or species.
Kelvin scale (oK) A scale of temperature in which absolute zero (the point at which molecules oscillate at their lowest possible frequency, -273oC) is designated as 0oK, and the boiling point of water as 373oK.
Kilocalories (kcal) Units of 1,000 calories. (See Calories.)
Kingdom The highest inclusive category of taxonomic classification. Each kingdom includes phyla or subkingdoms. The most common presently used classification system proposes five kingdoms: Monera (prokaryotes). Protista, Fungi, Animalia, and Plantae, although some authors emphasize a tripartite division of organisms—Archaea, Bacteria, Eucarya (See Domain).
Kin selectionSelection effects (for example, altruism) that influence the survival and reproductive success of genetically related individuals (kin). This contrasts with selection confined solely to an individual and its own offspring. (See also Inclusive fitness.)
Knuckle-walkingQuadrupedal gait of chimpanzees and gorillas, performed by curling the fingers toward the palm of the hand and using the backs (dorsal surfaces) of the knuckles to support the weight of the front part of the body.
Krebs cycle The cyclic series of reactions in the mitochondrion in which pyruvate is degraded to carbon dioxide and hydrogen protons and electrons. The latter are then passed into the oxidative phosphorylation pathway to generate ATP.
K-selectionSelection based on a population being maintained at or near the limit of its carrying capacity; selection is theoretically for improved competitive ability rather than for rapid numerical increase. rvalue (See Carrying capacity.)

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Lactation Formation and secretion of milk in maternal mammary glands for nursing offspring, a distinctive characteristic of mammals.
Ladder of Nature A concept based on Aristotle's view (the Scale of Nature) that nature can be represented as a succession of stages or ranks that leads from inanimate matter through plants, lower animals, higher animals, and finally to the level of humans. (See also Great Chain of Being.)
Lamarckian inheritance The concept that the phenotype of an organism is itself hereditary: that characters acquired or lost during the life experience of an organism, as well as characters that organisms attempt to acquire in order to meet environmental needs, can be transmitted to offspring. Lamarck proposed that it is through such means that changes in organisms (evolution) takes place. (See also Inheritance of acquired characters, Use and disuse.)
Language A structured system of communication among individuals using vocal, visual, or tactile signs to describe thoughts, feelings, concepts, and observations. Rather than communication, some writers emphasize the representational nature of language, defining it as a symbolic system used to store and retrieve information about experiences and concepts.
Larva A sexually immature stage in various animal groups, often with a form and diet distinct from those of the adult.
Laurasia The supercontinent in the Northern Hemisphere (comprising what is now North America, Greenland, Europe, and parts of Asia) formed from the breakup of Pangaea about 180 million years ago.
Learning Acquisition of a behavior through experience.
Lethal allele An allele whose effect prevents its carrier from reaching sexual maturity when present in homozygous condition for a recessive lethal or in either heterozygous or homozygous condition for a dominant lethal.
Life The capability of performing various organismic functions such as metabolism, growth, and reproduction of genetic material.
Life cycle The series of stages that takes place between the formation of zygotes in one generation of a species and the formation of zygotes in the next generation. (Also life history: the series of stages experienced by an individual of a species, from birth to death.)
Light-year The distance traveled by light, moving at 186,282 miles a second, in a solar year; approximately 6 X 1012 miles or 9.5 X 1012 kilometers.
Lineage An evolutionary sequence, arranged in linear order from an ancestral group or species to a descendant group or species (or vice versa).
Lingual Side of a tooth closest to the tongue.
Linkage (gene) The occurrence of two or more gene loci on the same chromosome.
Linkage disequilibrium The absence of linkage equilibrium (that is, the presence of nonrandom associations between alleles at different loci). (See also Hitchhiking.)
Linkage equilibrium The attainment of genotypic frequencies in a population that indicates that recombination between two or more gene loci has reached the point at which their alleles are now found in random genotypic combinations. For example, when an allele at one locus (for example, A1) and an allele at another locus (for example, B1) are found in combination at a frequency (fA1B1) equal to the product of their individual frequencies (fA1 X fB1).
Linkage map The linear sequence of known genes on a chromosome obtained from recombination data.
LipidsOrganic compounds such as fats, waxes, and steroids that tend to be more soluble in organic solvents of low polarity (for example, ether, chloroform) than in more polar solvents (for example, water).
Living fossil An existing species whose similarity to ancient ancestral species indicates that very few morphological changes have occurred over a long period of geological time.
Locus (plural loci) Strictly defined, it is the site (nucleotide sequence) on a chromosome occupied by a specific gene, Some researchers use it more broadly as a synonym of gene.
Logistic growth curvePopulation growth that follows a sigmoid (S-shaped) curve in which numbers increase slowly at first, then rapidly, and finally level off as the population reaches its maximum size or carrying capacity for a particular environment.
Longevity The average life span of individuals in a population.

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MacroevolutionEvolution of taxa higher than the species level (for example, genera, families, orders, classes), commonly entailing major morphological changes. This concept is often associated with the school of thought proposing that evolutionary events different from those responsible for changes in populations or the origin of species have caused the origin of higher taxa. (See Punctuated equilibrium.)
Macromolecules Very large polymeric molecules such as proteins, nucleic acids, and polysaccharides.
Macromutation A concept that attempts to explain the origin of a new species or an even higher taxonomic category by a single large mutation rather than by selection acting on many mutations. Although most geneticists agree that mutations can produce major as well as minor developmental changes, no single mutation is yet known that can cause an instantaneous speciation event, probably because such a sudden large radical change would dislocate normal genetic and developmental processes.
Malthusian parameter See Intrinsic rate of natural increase.
Mammary glands One or more pairs of ventrally placed glands used by mammalian females for nursing offspring. (See also Lactation.).
Marsupials Mammals of the infraclass Metatheria possessing, among other characters, a reproductive process in which tiny live young are born, and then nursed in a female pouch (marsupium).
Meiosis The eukaryotic cell division process used in producing haploid gametes (animals) or spores (plants) from a diploid cell. Meiosis is characterized by a reduction division that ensures that each gamete or spore contains one representative of each pair of chromosomes in the parental cell.
Meiotic drive See Segregation distortion.
Mendel's laws See Independent assortment, Segregation.
Mesoderm The embryonic tissue layer between ectoderm and endoderm in triploblastic animals that gives rise tomuscle tissue, kidneys, blood, internal cavity linings, and so on.
Mesozoic era The middle era of the Phanerozoic eon, covering the approximately 180-million-year interval between the Paleozoic (ending about 245 million years ago) and Cenozoic (beginning about 65 million years ago). It is marked by the origin of mammals in the earliest period of the era (Triassic), the dominance of dinosaurs throughout the last two periods of the era (Jurassic and Cretaceous), and the origin of angiosperms.
Messenger RNA (mRNA) An RNA molecule produced by transcription from a DNA template, bearing a sequence of triplet codons used to specify the sequence of amino acids in a polypeptide.
Metabolic pathway A sequence of enzyme-catalyzed reactions that convert a precursor substance to one or more end products.
Metabolism A network of enzyme-catalyzed reactions used by living organisms to maintain themselves.
Metacentric A chromosome whose centromere is at or near the center.
Metamerism Division of the body, or a major portion of the body, into a series of similar segments along the anterior-posterior axis. (See also Segmentation.)
Metamorphic rock Rock that has been subjected to high but nonmelting temperatures and pressures, causing chemical and physical changes.
Metamorphosis The transition from one form into another during development (for example, a larva into a different adult form).
Metatheria See Marsupials.
Metazoa Multicellular animals.
Microevolution Evolutionary changes of the kinds usually responsible for causing differences between populations of a species (for example, gene frequency changes and chromosornal variations). Many evolutionists suggest that accumulations of such changes over time are sufficient to explain the origin of most or all taxa.
Microsatellites Tandem repeats of short di-, tri-, and tetra- nucleotide sequences such as cytosine-adenine-cytosine-adenine-cytosine-adenine, and so on. Such loci are abundant (humans are estimated to possess at least 35,000 loci for repeats of the C-A sequence) and mutate (change in sequence number) at a relatively high rate.
Microspheres Microscopic membrane-bound spheres formed when proteinoids are boiled in water and allowed to cool. Some cell-like properties, such as osmosis, growth in size, and selective absorption of chemicals, have been ascribed to them.
Migration The transfer of genes from one population into another by interbreeding (gene flow). (Also used to indicate movement of a population to a different geographical area or its periodic passage from one region to another.)
Mimicry Resemblance of individuals in one species (mimics) to individuals in another (models) because of selection. (See also Batesian mimicry, Müllerian mimicry.)
Mitochondrion An organelle in eukaryotic cells that uses an oxygen-requiring electron transport system to transfer chemical energy derived from the breakdown of food molecules to ATP. Mitochondria have their own genetic material (circular DNA without histones) and generate some mitochondrial proteins by using their own protein-synthesizing apparatus. (Most of the mitochondrial proteins are coded by nuclear DNA and produced on cytoplasmic ribosomes)
Mitosis The mode of eukaryotic cell division that produces two daughter cells possessing the same chromosome complement as the parent cell.
Modern synthesis (evolution theory) See Neo-Darwinism.
Modifier (gene) A gene whose effect alters the phenotypic expression of one or more genes at loci other than its own.
Molecular clock See Evolutionary clock.
Monocotyledons Flowering plants (angiosperms) in which the embryo bears one seed leaf (cotyledon).
Monoecious An individual bearing both male and female organs. (See also Hermaphrodite.)
Monomers The subunits linked together to form a polymer (for example, nucleotides in nucleic acids, amino acids in proteins, sugars in polysaccharides).
Monomorphic A population or species that shows no genetic or phenotypic variation for a particular gene or character.
Monophyletic Derivation of a taxonomic group from a single ancestral lineage.
Monotremes Egg-laying mammals, presently restricted to Australasia; the platypus (Ornithorhyncus) and echidna spiny anteater (Tachyglossus, Zaglossus).
Morphology Study of the anatomical form and structure of organisms.
Müllerian mimicry Sharing of a common warning coloration or pattern among a number of species that are all dangerous or toxic to predators; resemblances maintained because of common selective advantage.
Multigene family See Gene family.
Mutation A change in the nucleotide sequence of genetic material whether by substitution, duplication, insertion, deletion, or inversion.
Mutational load That portion of the genetic load caused by production of deleterious genes through recurrent mutation.
Mutualism A relationship among different species in which the participants benefit.

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Natural selection Differential reproduction or survival of replicating organisms caused by agencies that are not directed by humans (See Artificial selection). Since such differential selective effects are widely prevalent, and often act on hereditary (genetic) variations, natural selection is a common major cause for a change in the gene frequencies of a population that leads to a new distinctive genetic constitution (evolution). (See also Adaptation, Fitness, Selection.)
Negative eugenics Proposals to eliminate deleterious genes from the human gene pool by identifying their carriers and restraining or discouraging their reproduction.
Neo-Darwinism The theory (also called the Modern synthesis) that regards evolution as a change in the frequencies of genes introduced by mutation, with natural selection considered as the most important, although not the only, cause for such changes.
Neoteny The retention of juvenile morphological traits in the sexually mature adult. (See also Heterochrony, Paedomorphosis.)
Neutral mutation A mutation that does not affect the fitness of an organism in a particular environment.
Neutral theory of molecular evolution The concept that most mutations that contribute to genetic variability (genetic polymorphism on the molecular level) consist of alleles that are neutral in respect to the fitness of the organism and that their frequencies can be explained in terms of mutation rate and random genetic drift.
Niche See Ecological niche.
Nocturnal A lifestyle characterized by nighttime activity.
Non-Darwinian evolution See Neutral theory of molecular evolution.
Nondisjunction The failure of homologous chromosomes (or sister chromatids) to separate ("disjoin") from each other during one of the two meiotic anaphase stages and go to opposite poles. Because of nondisjunction, one daughter cell will receive both homologues (or sister chromatids) and the other daughter cell will receive none, leading to an increase or decrease, respectively, in chromosome number.
Nonrandom mating See Assortative mating.
Nonsense mutation A mutation that produces a codon that terminates the translation of a polypeptide prematurely. Such codons were previously called "nonsense" codons but are now generally called stop codons or chain termination codons.
Nucleic acid An organic acid polymer, such as DNA or RNA, composed of a sequence of nucleotides.
Nucleotide A molecular unit consisting of a purine or pyrimidine base, a ribose (RNA) or deoxyribose (DNA) sugar, and one or more phosphate groups.
Nucleus A membrane-enclosed eukaryotic organelle that contains all the histone-bound DNA in the cell (that is, practically all the cellular genetic material).
Numerical (phenetic) taxonomy A statistical method for classifying organisms by comparing them on the basis of measurable phenotypic characters and giving each character equal weight. The degree of overall similarity between individuals or groups is then calculated, and a decision is made as to their classification. (See Phenetic.)

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Old Earth Creationism (OEC) The belief that the earth and universe are ancient, but that God played an active role in their history. Often associated with ID. (See also Creationism.)
Old Earth Creationist (OEC) A believer in OEC. (See also Creationism, Old Earth Creationism (OEC).)
Olfactory Referring to the sense of smell.
Omnivores Animals that feed on both plants and animals.
Ontogeny The development of an individual from zygote to maturity,
Operon A cluster of coordinately regulated structural genes. In prokaryotes and in a few eukaryotes, this cluster is transcribed as a unit into a single long ("polycistronic") messenger RNA molecule which is then translated into a sequence of individual gene products that often function together.
Order A taxonomic category between class and family: a class may contain a number of orders, each of which contains a number of families.
Organelles Functional intracellular membrane-enclosed bodies, such as nuclei, mitochondria, and chloroplasts.
Organic Carbon-containing compounds. Also refers to features or products characteristic of biological organisms.
Organism A living entity. (See Life.)
Orthogenesis The concept that evolution of a group of related species proceeds in a particular direction (for example, an increase in size) because of unknown internal or vitalistic causes rather than because of nonmystical factors such as selection.
Orthologous genesGene loci in different species that are sufficiently similar in their nucleotide sequences (or amino acid sequences of their protein products) to suggest they originated from a common ancestral gene.
Overdominance Instances when the phenotypic expression of a heterozygote (for example, A1A2) is more extreme than that of either hornozygote (for example, A1A1 or A2A2) Overdominance has been considered a cause for hybrid vigor. (See Heterosis.)
Oviparous Females that lay eggs that develop outside the body.
Oxidation-reduction Reactions in which electrons are transferred from one atom or molecule (the reducing agent that is oxidized by the loss of electrons) to another (the oxidizing agent that is reduced by the gain of electrons). For example, we can diagram an atom or molecule as A or B, each carrying a proton (p+) and an electron (e-), as follows:

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Paedomorphosis The incorporation of adult sexual features into immature developmental stages. Causes for such effects are changes in developmental speed of sexual tissues relative to nonsexual tissues (See Heterochrony), In progenesis, sexual development is so rapid that the sexually mature form may remain quite small in size. In neoteny, sexual maturation proceeds normally but somatic development slows down, producing a full-sized sexual adult with juvenile appearance. A classic neotenous form is the Mexican axoloti, an amphibian salamander that can retain its gills in tadpole form even when sexually mature.
Paleomagnetism The magnetic fields of ferrous (iron- containing) materials in ancient rocks. Among other applications, paleomagnetism provides information on the position oflandmasses and continents relative to the earth's magnetic poles at the time that the rocks were formed and thus can be used to describe the historical movement of continents relative to each other (continental drift).
Paleontology The study of extinct fossil organisms.
Paleozoic The first era of the Phanerozoic eon, extending from 545 to about 250 million years ago.
Pangaea A very large supercontinent formed about 250 million years ago comprising most or all of the present continental landmasses. (See Gondwana, Laurasia.)
Pangenesis The concept of heredity, held by Darwin and others, that small, particulate "gemmules," or "pangenes," are produced by each of the various tissues of an organism and sent to the gonads where they are incorporated into gametes. The increase or decrease of specific gemmules during the use or disuse of organs was proposed in order to explain the Lamarckian concept of inheritance of acquired characters.
Panmixis (panmictic) See Random mating.
Panspermia The concept that life was introduced on earth from elsewhere in the universe.
Parallel evolution The evolution of similar characters in related lineages whose common ancestor was phenotypically different. (See also Convergence.)
Paralogous genes Two or more different gene loci in the same organism that are sufficiently similar in their nucleotide sequences (or in the amino acid sequences of their protein products) to indicate they originated from one or more duplications of a common ancestral gene. (See also Gene family.)
Parapatric Geographically adjacent species or populations whose distributions do not overlap but are in contact at one or more of their mutual boundaries.
Paraphyletic A taxonomic grouping which includes some descendants of a single common ancestor, but not all.
Parasitism An association between species in which individuals of one species (the parasite) obtain their nutrients by living on or in the tissues of another species (the host), often with harmful effects to the host.
Parental investment Parental provision of resources to offspring that increase the offspring's reproductive success at a cost of further reproductive success of the parents.
Parsimony method Choice of a phylogenetic tree that minimizes the number of evolutionary changes necessary to explain species divergence.
ParthenogenesisDevelopment of an individual from an egg that has not been fertilized by a male gamete. Diploid eggs may arise during the meiotic process— called by some, automictic parthenogenesis—when the number of chromosomes in the maternal oocyte doubles, or when two of the four haploid meiotic products unite. By combining identical homologous chromosomes in a single egg nucleus, automixis can produce homozygotes for all chromosomal loci. Parthenogenesis may also occur in the absence of meiosis, when eggs are produced rnitotically—called by some, apomictic parthenogenesis. In apomixis, a parent—whether homozygous or heterozygous—transmits its own genotype to its offspring.
Partial (incomplete) dominance Instances where two different alleles of a gene in a heterozygote (for example, A1A2) produce a phenotypic effect intermediate between the effects produced by the two homozygotes (for example, A1A1, A2A2).
Pelagic Refers to an entire body of water and the organisms within it, excluding the bottom (benthic) zone.
Peptide An organic molecule composed of a sequence of amino acids covalently linked by peptide bonds (a bond formed between the amino group of one amino acid and the carboxyl group of another through the elimination of a water molecule).
Period (geological) A major subdivision of an era of geological time distinguished by a particular system of rocks and associated fossils. The Cretaceous period is named after the abundance of chalk (Latin, creta), and the Carboniferous period for the abundance of coal (Latin, carbo), in their component rock systems.
PeripatricPopulations that bud off from the geographic periphery of a parental population and become genetically isolated.
Phagocytic Cellular engulfment of external material. (See also Endocytosis.)
Phanerozoic eon A major division of the geological time scale marked by the relatively abundant appearance of fossilized skeletons of multicellular organisms, dating from about 545 million years ago to the present.
Phenetic Referring to phenotypic characters that can be described or measured. Also, a system of classification that groups taxa by their degree of similarity for measured or numerically evaluated characters. (See also Numerical taxonomy.)
Phenotype The characters that constitute the structural and functional properties of an organism. Phenotypic features result from interaction between the genotype, which provides developmental information, and the environment, which provides developmental facilities.
Phosphorylation The addition of one or more phosphate groups (HPO4-) to a compound (for example, the phosphorylation of ADP to ATP),
Photoautotroph See Autotroph.
Photosynthesis The synthesis of organic compounds from carbon dioxide and water through a process that begins with the capture of light energy by chlorophyll.
pH scale [The negative logarithm of the hydrogen ion (H+) concentration in an aqueous solution.] A scale used for measuring acidity (pH less than 7) and alkalinity (pH greater than 7), given that pure water has a neutral pH of 7.
Phyletic evolution Evolutionary changes within a single nonbranching lineage. Although new species are produced by this lineage over time (chronospecies) there is no increase in the number of species existing at any one time. (See also Anagenesis.)
Phylogenetic evolution Evolutionary changes that produce two or more lineages that diverge from a single ancestral lineage. (Also called "branching evolution," see Cladogenesis.)
Phylogeny The evolutionary history of a species or group of species in terms of their derivations and connections. A phylogenetic tree is a schematic diagram designed to represent that evolution—ideally, a portrait of genetic relationships.
Phylotype (phylotypic) A proposed stage in embryonic development that characterizes some basic features in the body plan of a phylum (for example, see Fig. 3-10). Further evolved taxa in a lineage can then use this stage as a foundation for developing their own unique derived features.
Phylum (plural, phyla) The major taxonomic category below the level of kingdom, used to include classes of organisms that may be phenotypically quite different but share some general features or body plan.
Placenta A mammalian organ formed by union between the female uterine lining and embryonic membranes that provides nutrition to the embryo, allows exchange of gases, and aids elimination of embryonic waste products.
Placentals Mammals of the infraclass Eutheria, possessing, among other features, a reproductive process that uses a placenta to nourish their young until a relatively advanced stage of development compared to other mammalian groups (monotremes and marsupials).
Planula hypothesis The concept that metazoans evolved from small primitive organisms that consisted of solid balls of cells (planulae) similar to embryonic stages of sponges and Cnidaria.
Plasma membrane The boundary membrane consisting of phospholipids and proteins surrounding the cytoplasm of a cell.
Plasmid A self-replicating cellular DNA element that can exist outside the host chromosome. There are various kinds, some maintaining more than one copy per cell.
Plate tectonics The concept that the earth's crust is divided into a number of fairly rigid plates, whose movements (tectonics) relative to each other are responsible for continental drift and many crustal features. (See also Tectonic plates.)
Pleiotropy Instances when a single gene produces phenotypic effects on more than one character.
Plesiomorphy Instances when a species character is similar to that character in an ancestral species.
Polygene A gene that interacts with other polygenes to produce a quantitative phenotypic effect on a character,
Polymer A molecule composed of many repeating subunits (monomers) linked together by covalent bonds.
Polymerase An enzyme that catalyzes the synthesis of a polymer by linking together its component monomers.
Polymerase chain reaction (PCR) A laboratory technique that can replicate a sequence of DNA nucleotides into millions of copies in a very short time.
Polymorphism The presence of two or more genetic or phenotypic variants in a population. Usually refers to genetic variations where the frequency of the rarest type is not maintained by mutation alone. (See also Balanced polymorphism.)
Polypeptide (peptide) See Peptide.
Polyphyletic The presumed derivation of a single taxonomic group from two or more different ancestral lineages through convergent or parallel evolution.
PolyploidyVariations in which the number of chromosome sets (n) is greater than the diploid number (2n). For example, triploidy (3n) and tetraploidy (4n),
Population A geographically localized group of individuals in a species that, in sexual forms, share a common gene pool. (See also Deme.)
Preadaptation A character that was adaptive under a prior set of conditions and later provides the initial stage (is "co-opted") for the evolution of a new adaptation under a different set of conditions.
Precambrian eon A major division of the geological time scale that includes all eras from the origin of the earth about 4.5 billion years ago to the beginning of the Phanerozoic eon, about 545 million years ago. The Precambrian (also known as the Cryptozoic) is marked biologically by the appearance of prokaryotes about 3.5 billion years ago and small, nonskeletonized multicellular organisms in the Ediacarian period about 50 or 60 million years before the Phanerozoic.
Predation The killing and consumption of one living organism (prey) by another (predator).
Preformationism The concept that an organism is preformed at conception in the form of a miniature adult and development consists of enlargement of the already preformed structures.
Progenote A hypothetical ancestral cellular form that gave rise to prokaryotes (archaebacteria, eubacteria) and eukaryotes.
ProkaryotesOrganisms such as bacteria and cyanobacteria that lack histone-bound DNA, endoplasmic reticulum, a membrane-enclosed nucleus, and other cellular organelles found in eukaryotes.
Promoter A DNA nucleotide sequence that enables transcription (RNA synthesis) by binding the enzyme RNA polymerase. (See also Enhancer.)
Protein A macromolecule composed of one or more polypeptide chains of arnino acids, coiled and folded into specific shapes based on its amino acid sequences.
Proteinoids Synthetic polymers produced by heating a mixture of amino acids. Some show proteinlike properties in respect to enzyme activity, color test reactions, hormonal activity, and so on.
Protista One of the four eukaryotic kingdoms; includes protozoa, algae, slime molds, and some other groups. (Called "Protoctista" by some authors.)
Protoplasm Cellular material within the plasma membrane.
Protostomes Coelomate phyla in which the embryonic blastopore becomes the mouth.
Prototheria See Monotremes.
PseudocoelomatesOrganisms that have a coelom derived from a persistent embryonic blastocoele, largely unlined with mesodermal tissue.
Punctuated equilibrium The view that evolution of a lineage follows a pattern of long intervals in which there is relatively little change (stasis, or equilibrium), punctuated by short bursts of speciation and macroevolutionary events during which new taxa arise.
Purine A nitrogenous base composed of two joined ring structures, one five-membered and one six-membered, commonly present in nucleotides as adenine (A) or guanine (G).
Pyrimidine A nitrogenous base composed of a single six-membered ring, commonly present in nucleotides as thvmine (T), cvtosine (C). or uracil (U).

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QuadrupedalTetrapod locomotion using all four limbs.
Quantitative character A character whose phenotype can be numerically measured or evaluated; a character displaying continuous variation.
Quantum evolution A rapid increase in the rate of evolution over a relatively short period of time.

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Race A population or group of populations in a species that share a geographically and/or ecologically identifiable origin and have unique gene frequencies and phenotypic characters that distinguish them from other races. Because of the large amount of genetic and phenotypic variability in most species, the number of racial distinctions that can be made is often arbitrary. (See also Subspecies.)
Racemic mixture A mixture of two kinds of molecules whose structures are similar but differ in that they are mirror images of each other (one kind cannot be superimposed on the other). Each of the two molecular forms rotates the plane of polarized light in a particular direction, but the racemic mixture is optically inactive.
Radiation (phylogenetic) See Adaptive radiation.
Radioactive dating The dating of rocks by measuring the proportions of a radioactive element in an igneous intrusion and the isotopes produced by its radioactive decay. Since the rate at which a particular radioactive element decays is constant, these proportions provide an estimate of the age of the rock, which can often be confirmed by dating with other radioactive elements.
Radioactivity Emission of radiation by certain elements as their atomic nuclei undergo changes.
Random genetic drift The random change in frequency of alleles in a population. These can be caused by sampling errors that are of greater magnitude in small populations or by bottlenecks (founder effects), when population size is suddenly reduced to a few individuals. As population size increases, random drift becomes less important and selection more important in causing gene frequency changes.
Random mating Mating within a population regardless of the phenotype or genotype of the sexual partner (panmixis).
Range The geographical limits of the region habitually traversed by an individual or occupied by a population or species.
Recessive allele An allele (for example, a), which has no obvious phenotypic effect in a heterozygote (for example, Aa), producing its phenotypic effect only when homozygous (for example, aa).
Recessive lethal An allele whose presence in homozygous condition causes lethality. (See also Lethal allele.)
Reciprocal altruism A mutually beneficial exchange of altruistic behavioral acts between individuals. (See also Altruism.)
Recombinant DNA A DNA molecule composed of nucleotide sequences from different sources.
Recombination A chromosomal exchange process (See Crossovers) that produces offspring that have gene combinations different from those of their parents. (Also used by some authors to describe the results of independent assortment.)
Red Queen hypothesis The view that adaptive evolution in one species of a community causes a deterioration of the environment of other species. As a consequence, each species must evolve as fast as it can in order "to stay in the same place" (to survive).
Reductionism The concept that explanations for events at one level of complexity can or should be reduced to explanations at a more basic level. For example, that all biological events should be explained in the form of chemical reactions.
Regulator gene A gene that controls the rate at which other genes, adjacent or distant, will synthesize their products.
Repetitive DNADNA nucleotide sequences that are repeated many times in the genome.
Represser protein A regulator gene product that binds to a particular nucleotide sequence and prevents transcription.
Reproductive isolation The absence of gene exchange between populations. (See Isolating mechanisms.)
Reproductive success The proportion of reproductively fertile offspring produced by a genotype relative to other genotypes. (See also Fitness.)
Restriction enzymesEnzymes that recognize particular nucleotide sequences and cut DNA molecules at or near those sequences. (See also Endonucleases.)
Restriction fragment length polymorphisms (RFLPs) Differences between individuals in the size of DNA fragments for a particular DNA section cut by restriction enzymes. These are inherited in mendelian fashion, and furnish a basis for estimating genetic variation. They also provide linkage markers used to track mutant genes between generations.
Ribosomal RNA (rRNA)RNA sequences that are incorporated into the structure of ribosomes.
Ribosomes Intracellular particles composed of ribosomal RNA and proteins that furnish the site at which messenger RNA molecules are translated into polypeptides.
Ribozymes Sequences of RNA nucleotides that can perform catalytic roles.
RNA (ribonucleic acid) A typically single-strand nucleic acid, characterized by the presence of a ribose sugar in each nucleotide, whose sequences serve either as messenger RNA, ribosomal RNA, or transfer RNA in cells, or as genetic material in some viruses. In contrast to the base composition of DNA, RNA usually bears uracil instead of thymine.
RNA editing Information changes in RNA molecules by the addition, deletion, or transformation of ribonucleotide bases after these molecules have been transcribed from their DNA templates.
RNA splicing The Joining of exons by the excision of introns.
RNA world The concept that RNA nucleotide sequences possessing catalytic and self-replicating capabilities predated catalytic protein systems in prebiological times.
r-selectionSelection in populations subject to rapidly changing environments with highly fluctuating food resources. Theoretically, selection in such populations emphasizes adaptations for rapid population growth rather than for the competitive ability experienced in X-selected populations.

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[ S ]
Saltation The concept that new species or higher taxa originate abruptly because of macromutations, or because of sudden unknown causes.
Sampling error (gene frequencies) Variability in gene frequencies caused by the fact that not all samples taken from a population have exactly the same gene frequency as the population itself.
Saprophyte An organism that feeds on decomposing organic material.
Scavenger An organism that habitually feeds on animals who died naturally or accidentally or were killed by another carnivore.
Sea floor spreading Expansion of oceanic crust through the deposition of mantle material along oceanic ridges. (See also Continental drift, Tectonic plates.)
Sedimentary rock Rock formed by the hardening of accumulated particles (sediments) that had been transported by agents such as wind and water. The prime source of fossils. (See also Geological strata.)
Segmentation The repetition of body structures along an animal's anterior-posterior axis, as found generally in annelids, arthropods, and chordates. (See also Metamerism.)
Segregation The mendelian principle that the two different alleles of a gene pair in a heterozygote segregate from each other during meiosis to produce two kinds of gametes in equal ratios, each bearing a different allele.
Segregational genetic load See Balanced genetic load.
Segregation distortion (meiotic drive) Aberrant segregation ratios among the gametes produced by heterozygotes because of the presence of certain alleles (segregation distorters).
Selection A composite of all the forces that cause differential survival and differential reproduction among genetic variants. When the selective agencies are primarily those of human choice, the process is called artificial selection; when the selective agencies are not those of human choice, it is called natural selection. (See also Adaptive value, Fitness.) Although evolutionary biologists recognize other factors that contribute to genetic change, and therefore to evolution (for example, Mutation, Random genetic drift), selection remains the most commonly accepted cause to account for organismic adaptive features. However, selection does not have the foresight nor can development supply the means to enable a single population to face every eventuality. That is, although selection is a cause for evolutionary change, the amount and direction of change is limited by an organism's past history. The regularity of extinction, embracing many lineages and practically all fossil species, indicates such limitations. That evolution proceeds continuously in the face of successive environmental contingencies is because selection is exercised in different populations, some of which possess adaptations that replace other populations which lack them. Among other "adaptive" hypotheses that have generally been rejected are unknown or mystical causes believed to guide evolutionary changes in non-selective yet adaptive directions, such as directed mutation, directed responses to environmental needs (Lamarckianism), orthogenesis, and saltation.
Selection coefficient (symbol s) A relative measure of the effect of selection, usually in terms of the loss of fitness endured by a genotype, given that the genotype with greatest fitness has a value of 1.
Self-assembly The spontaneous aggregation of macromolecules into biological configurations that can have functional value.
Selfish DNA The concept that the persistence of DNA sequences with no discernible cellular function (for example, various DNA sequences) arises from the likelihood that, once present in the genome, they are impossible to remove without the death of the organism—that is, they act as "selfish," or "junk" DNA, which the cell has no choice but to replicate along with functional DNA.
Semipermeable membrane A membrane that selectively permits transmission of certain molecules but not others.
Senescence The process of aging.
Septum (plural, septa) A dividing wall or partition between sections of an organism.
Serial ("iterative") homology Similarities between parts of the same organism, such as the vertebrae of a vertebrate or the different kinds of hemoglobin molecules produced by a mammal. The genetic basis for such homology can often be ascribed to gene duplications that have diverged over time but still produce somewhat similar effects. (See Gene family, Paralogous genes.)
Sessile Attached to a substrate. An organism whose behavior is mostly nonmotile.
Sex chromosomesChromosomes associated with determining the difference in sex. These chromosomes are alike in the homogametic sex (for example, XX) but differ in the heterogametic sex (for example, XY).
Sex linkageGenes linked on a sex chromosome. The results of such linkage may be differences between the sexes in the appearances of certain traits. For example, a recessive allele on the X chromosome is not expressed in the XX homogametic sex if that individual has a dominant allele on its second X chromosome but can be expressed in the XY heterogametic sex, which has only one X chromosome.
Sex ratio The relative proportions of males and females in a population.
Sexual reproductionZygotes produced by the union of genetic material from different sexes through gametic fertilization.
Sexual selectionSelection that acts directly on mating success through direct competition between members of one sex for mates (intrasexual selection), or through choices made between them by the opposite sex (epigamic selection), or through a combination of both selective modes. In any of these cases, sexual selection may cause exaggerated phenotypes to appear in the sex on which it is acting (large antlers, striking colors, and so on).
Sibling speciesSpecies so similar to each other morphologically that they are difficult to distinguish but that are nevertheless reproductively isolated. (Sometimes called "cryptic species.")
Sister group A term used commonly in cladistic systematics to designate the most closely related group to a particular taxon. It derives from the concept that each significant evolutionary step marks a dichotomous split that produces two sister taxa equal to each other in rank.
Social Darwinism The concept that social and cultural differences in human societies (political, economic, military, religious, and so on) arise through processes of natural selection, similar to those that account for biological differences among populations and species.
Sociobiology The study of the biological basis of social behavior.
Somatic cells (or tissues) Body cells other than the germ-line tissues that produce sperm or eggs (See Germ plasm).
Speciation The splitting of one species into two or more new species (See Cladogenesis, Phylogenetic evolution) or the transformation of one species into a new species over time (See Anagenesis, Phyletic evolution.)
Species A basic taxonomic category for which there are various definitions. Among these are an interbreeding or potentially interbreeding group of populations reproductively isolated from other groups (the biological species concept) and a lineage evolving separately from others with its own unitary evolutionary role and tendencies (Simpson's evolutionary species concept). Employing the terms of population genetics, some definitions can be combined into the concept that a species is a population of individuals bearing distinctive genes and gene frequencies, separated from other species by biological barriers preventing gene exchange.
Split gene A gene whose nucleotide sequence is divided into exons and introns.
Spontaneous generation An early concept that complex organisms can appear spontaneously from inert materials without biological parentage.
Sporophyte The diploid spore-producing stage of plants that have alternating generations (haploid gametophyte and diploid sporophyte). The sporophyte arises from the union of gametophyte gametes and produces its haploid spores (which become gametophytes) by meiosis.
Stabilizing selectionSelection that favors the survival of organisms in a population that are at an intermediate phenotypic value for a particular character and eliminates the extreme phenotypes. (Also called centripetal, or normalizing, selection.)
Stasis A period of equilibrium during which change appears to be absent, for example, in the concept of punctuated equilibrium. (See also Stabilizing selection.)
Stop codon One of the three messenger codons (UAA, UAG, UGA) that terminates the translation of a polypeptide. (Also called cllain-termination codon or nonsense codon.)
Strata See Geological strata.
Stromatolites Laminated rocks produced by layered accretions of benthic microorganisms (mainly filamentous cyanobacteria) that trap or precipitate sediments.
Structural gene A DNA nucleotide sequence that codes for RNA or protein. Some definitions restrict this term to a protein-coding gene.
Subspecies A taxonomic subdivision of a species often distinguished by special phenotypic characters and by its origin or localization in a given geographical region. Like other species subdivisions (See Race), a subspecies can still interbreed successfully with the remainder of the species. However, in some cases, interbreeding capabilities are unknown, and subspecies designations (for example, Homo sapiens neanderthalensis and Homo sapiens sapiens) are based entirely on phenotype.
Survivorship The proportion of individuals born at a given time (cohort) who survive to a given age.
Symbiont A participant in the interactive association (symbiosis) between two individuals or two species. This term is often restricted to mutually beneficial associations (mutualism).
SympatricSpecies or populations whose geographical distributions coincide or overlap.
Sympatric speciationSpeciation that occurs between populations occupying the same geographic range.
Synapomorphy The possession by two or more related lineages of the same phenotypic character derived from a different but homologous character in the ancestral lineage.
Synteny The retention of homologous genes on the same chromosome in different species, irrespective of their linkage order.
Synthetic theory of evolution See Neo-Darwinism.
Systematics Although defined by Simpson as the study of the diversity of organisms and all their comparative and evolutionary relationships, it is often used interchangeably with the terms classification and taxonomy.

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[ T ]
Tachytelic A relatively rapid evolutionary rate.
Tautomer A nucleic acid base in which a hydrogen atom has moved from one position to another leading to a change in its pairing relationships.
Taxon (plural taxa) A named taxonomic unit consisting of a distinctive group of organisms placed in a taxonomic category, whether the unit is that of a species, genus, family, order, and so on.
Taxonomy The principles and procedures used in classifying organisms.
Tectonic plates The fairly rigid plates composing the earth's crust whose boundaries are marked by earthquake belts and volcanic chains. In oceanic regions, accretions to these plates occur at midoceanic ridges (sea floor spreading), and they are subducted under other plates at the deep oceanic trenches. Continental masses ride on some of these plates, accounting for continental drift and such processes as the mountain building that occurs when these plates collide.
Teleology The concept that natural processes such as development or evolution are guided by their final stage (telos) or for some particular purpose. Various philosophers separate "external" from "internal" (or "immanent") teleology. The former term is used to indicate guidance of a process toward some specified end decided by an external mystical or sagacious source; for example, "the ultimate purpose why the world was created [by God] was for the benefit of man." Thus, in contrast to science, which proposes that cause precedes effect, external teleological cause (for example, supernatural "design") arises after an effect's function or purpose has been foreseen or visualized. In contrast, internal teleology is generally used to indicate the end point of a process that has an understandable materialistic basis that develops from the process itself; for example, "the reason plants engage in photosynthesis and animals seek food is for survival, and the ultimate purpose of survival is for reproductive success." Among other factors, the absence of external teleology in biological evolution differentiates it from cultural evolution, into which humans have introduced external teleology by consciously designing many of its features.
TelocentricChromosomes whose centromeres are located at one end.
Terminal electron acceptor The molecule that is the final acceptor of electrons in a metabolic pathway (for example, in aerobic respiration, oxygen is the terminal electron acceptor).
Terrestrial On the ground; also on or of the planet Earth.
Tetrapod Literal meaning, "four-footed." Commonly used to specify a member of the land-evolved vertebrate classes: amphibia, reptiles, and mammals.
Therapsids An order of synapsid mammal-like reptiles, composed mainly of fairly large herbivorous and carnivorous forms, which were dominant reptilian stocks during the Permian and Triassic periods. During the Triassic, some or many of these stocks were probably already endothermal, and one or more groups (cynodonts - "dog teeth") made the transition to smaller mammalian forms such as the morganucodontids.
Theria The viviparous mammalian subclass, consisting of marsupials and placentals.
Tissue A group of cells, all performing a similar function in a multicellular organism.
Trait See Character.
Transcription The process by which the synthesis of an RNA molecule (for example, messenger RNA) is initiated and completed on a DNA template by RNA polymerase enzyme.
Transfer RNA (tRNA) Relatively small RNA molecules (about 80 nucleotides long) that carry specific amino acids to the ribosome for polypeptide synthesis. Each kind of tRNA has a unique anticodon complementary to messenger RNA codons that specify the placement of particular amino acids in the polypeptide chain.
Transformation The process by which a cell takes up introduced DNA.
Translation The protein-synthesizing process that takes place on the ribosome, linking together a particular sequence of amino acids (polypeptide) on the basis of information received from a particular sequence of codons on messenger RNA.
Translocation An aberration in which a sequence of nucleotides is moved to a different position in the genome.
Transposons (transposable elements)Nucleotide sequences that produce enzymes to promote their own movement from one chromosomal site to another and may carry additional genes such as those for antibiotic resistance,
Triploblastic An animal that produces all three major types of cell layers during development - ectoderm, endoderm, and mesoderm.
Tritium An isotope of hydrogen that has three times the mass of an ordinary hydrogen atom.
Typology The study of organic diversity based on the principle that all members of a taxonomic group conform to a basic plan, and variation among them is of little or no significance. (See also Archetype.)

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[ U ]
Ultraviolet radiation Electromagnetic radiation at wavelengths between about 4 and 400 nanometers, shorter than visible light but longer than X-rays. It is absorbed by purine and pyrimidine ring structures and is therefore quite damaging to nucleic acid genetic material.
Unequal crossing over The result of improper pairing between chrornatids, causing their crossover products to differ from each other in the amounts of genetic material.
Ungulate A hoofed mammal.
Uniformitarianism A concept, popularized by Lyell in geology, that none of the forces active in past Earth history were different from those active today.
Universal genetic code The use of the same genetic code in all living organisms. (A few codons differ from the universal code in mitochondria, mycoplasmas, and some ciliated protozoa.)
Use and disuse A concept used by Lamarck to explain evolution as resulting from the transmission of characters that became enhanced or diminished because of their use or disuse, respectively, during the life experience of individuals. (See also Lamarckian inheritance.)

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Variation A term commonly used to indicate differences in the qualitative or quantitative values of a character among individual members of a population, whether molecules, cells, or organisms.
Vascular plants Plants that have special water- and food- conducting vessels and tissues (xylem and phloem).
Vector A vehicle (for example, plasmid or virus carry a genetically engineered DNA sequence into a cell.
Ventral The belly or downward surface of an animal, (In vertebrates, the surface opposite the spinal column.
Vestigial organs Organs or structures that appear to be small and functionless but can be shown to be homologous with ancestral organs and structures that were larger and functional.
Virus A small intracellular parasite, often composed of little more than nucleic acid and a few proteins, that depends on the host cell to replicate its genetic material and to synthesize its proteins.
Vitalism The concept that the activities of living organisms cannot be explained by any underlying physical or chemical principles but arise from mystical or supernatural causes.
Viviparous Mode of reproduction in which eggs develop into live young before leaving the maternal body.

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[ W ]
Wild type The most commonly observed phenotype or genotype for a particular character. Variations from wild type are considered mutants.

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X chromosome The name given in various groups to a sex chromosome usually present twice in the homogametic sex (XX) and only once in the heterogametic sex (XY or XO).
X-linked genesGenes present on the X chromosome. (See Sex linkage.)

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Y chromosome A sex chromosome present only in the heterogametic sex (XY).
Young Earth Creationism (YEC) The belief that the earth is only some thousands of years old and that the geology and geography of the modern earth can be explained in terms of a world-wide flood. The belief is based upon a literal interpretation of Genesis. (See also Creationism.)
Young Earth Creationist (YEC)A believer in Young Earth Creationism. (See also Creationism, Young Earth Creationism (YEC).)

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Zootype A proposed stage in development characterized by the expression of a particular set of genes, that governs spatial development in multicellular animals.
Zygote The cell formed by the union of male and female gametes.


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