Interweaving Evolution & Hybrid Vigor

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(The People Who Saw Evolution)

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... It looked a lot like a fortis, but also like a scandens. Genetic analysis showed 5110 to be a cross between a fortis and a fortis-scandens hybrid. They called it the Big Bird.

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The Big Bird had a unique song and, when mature, shiny black plumage that was different from the indigenous Daphne birds. It also was extremely fit in the Darwinian sense and promiscuous, surviving another 13 years and mating with six females, producing 18 offspring. It mated with several fortis-fortis-scandens hybrids, then with fortis females, and began a new line of Big Birds that sang the song of the original immigrant.

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This Face Changes the Human Story. But How?

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Berger himself thinks the right metaphor for human evolution, instead of a tree branching from a single root, is a braided stream: a river that divides into channels, only to merge again downstream. Similarly, the various hominin types that inhabited the landscapes of Africa must at some point have diverged from a common ancestor. But then farther down the river of time they may have coalesced again, so that we, at the river’s mouth, carry in us today a bit of East Africa, a bit of South Africa, and a whole lot of history we have no notion of whatsoever. ...

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Heterosis, hybrid vigor, or outbreeding enhancement, is the improved or increased function of any biological quality in a hybrid offspring. The adjective derived from heterosis is heterotic.

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An offspring exhibits heterosis if its traits are enhanced as a result of mixing the genetic contributions of its parents. These effects can be due to Mendelian or non-Mendelian inheritance.

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Heterosis is often discussed as the opposite of inbreeding depression although differences in these two concepts can be seen in evolutionary considerations such as the role of genetic variation or the effects of genetic drift in small populations on these concepts. Inbreeding depression occurs when related parents have children with traits that negatively influence their fitness largely due to homozygosity. In such instances, outcrossing should result in heterosis.

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Not all outcrosses result in heterosis. For example, when a hybrid inherits traits from its parents that are not fully compatible, fitness can be reduced. This is a form of outbreeding depression.

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In proposing the term heterosis to replace the older term heterozygosis, G.H. Shull aimed to avoid limiting the term to the effects that can be explained by heterozygosity in Mendelian inheritance.[1]

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The physiological vigor of an organism as manifested in its rapidity of growth, its height and general robustness, is positively correlated with the degree of dissimilarity in the gametes by whose union the organism was formed The more numerous the differences between the uniting gametes at least within certain limits the greater on the whole is the amount of stimulation These differences need not be Mendelian in their inheritance To avoid the implication that all the genotypic differences which stimulate cell-division, growth and other physiological activities of an organism are Mendelian in their inheritance and also to gain brevity of expression I suggest that the word 'heterosis' be adopted.

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Punctuated equilibrium (also called punctuated equilibria) is a theory in evolutionary biology which proposes that once species appear in the fossil record they will become stable, showing little net evolutionary change for most of their geological history. This state is called stasis. When significant evolutionary change occurs, the theory proposes that it is generally restricted to rare and geologically rapid events of branching speciation called cladogenesis. Cladogenesis is the process by which a species splits into two distinct species, rather than one species gradually transforming into another.[1] Punctuated equilibrium is commonly contrasted against phyletic gradualism, the belief that evolution generally occurs uniformly and by the steady and gradual transformation of whole lineages (called anagenesis). In this view, evolution is seen as generally smooth and continuous.

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In 1972, paleontologists Niles Eldredge and Stephen Jay Gould published a landmark paper developing their theory and called it punctuated equilibria.[2] Their paper built upon Ernst Mayr's model of geographic speciation,[3] I. Michael Lerner's theories of developmental and genetic homeostasis,[4] as well as their own empirical research.[5][6] Eldredge and Gould proposed that the degree of gradualism commonly attributed to Charles Darwin is virtually nonexistent in the fossil record, and that stasis dominates the history of most fossil species.

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... Punctuated equilibrium differs from Mayr's theory mainly in that Eldredge and Gould placed considerably greater emphasis on stasis, whereas Mayr was generally concerned with explaining the morphological discontinuity (or "sudden jumps")[10] found in the fossil record.[7] Mayr later complimented Eldredge and Gould's paper, stating that evolutionary stasis had been "unexpected by most evolutionary biologists" and that punctuated equilibrium "had a major impact on paleontology and evolutionary biology". ...

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But first, what is a ring species? Ring species constitute one big and supposedly continuous population in which the attainment of biological speciation (to people like me, that means the evolution of two populations to the point that they cannot produce fertile hybrids were they to live in the same place in nature) does not require full geographic isolation of those populations. Rather, speciation in that continuous population occurs through a gradual spread of the range of the animals, coupled with selection in different places that causes their genetic divergence.

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It works like this: a species expands its range and encounters a roughly round geographic barrier like a valley, the Arctic ice cap, or an uninhabitable plateau. It divides and spreads around the edges of the barrier, so that its range becomes circular as it expands. And as the range begins to form a circle, the populations within it begin to become genetically different as they respond to local selection pressures. But the circle is never interrupted, so while each part of the expanding species becomes genetically different, it still exchanges genes with adjacent populations.

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... Because genetic studies, done by both Dick Highton at Maryland and then by Wake and his colleagues themselves (references below) also showed that in places around the ring there were sharp genetic breaks, suggesting not a process of continuous gene flow ...

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... everyone has now concluded that the formation of this ring involved sporadic and important episodes of geographic isolation between populations, so it’s not the classic continuous gene flow scenario involved in making a ring species. ...

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... gulls in the genus Larus encircling the Arctic, also fell victim to genetic studies, showing that it was very unlikely that they were ever a continuous ring that was geographically uninterrupted. ...

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That left the greenish warbler, ...

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... Now, however, genetic analysis of 95 birds and more than 2,000 sites throughout the genome (not just in the mitochondrion), has revealed four genetic clusters around the ring with a sharp intergradation between them (there’s another cluster in the Caucasus, not around the ring). ...

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Nevertheless, the results do show a ring species of a sort: isolation of two end populations of a ring that makes them look like two species, even though all through the ring you don’t see reproductive isolation of adjacent areas. And it shows that speciation can occur despite there having been some gene flow at some times. In nature, populations that form new species must often sometimes exchange genes if they’re not completely isolated by geography (i.e. the finch species that colonized the Galpago), so the dichotomy between no gene flow and pervasive gene flow may be artificial.

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1. P.t.nitidus (becomes P.nitidus?)
2. P.t.viridanus (becomes P.viridanus?)
3. P.t.trochiloides (becomes P.trochiloides?) and
4. P.t.plumbeitarsus (becomes P.plumbeitarsus?)

1. P.t.ludlowi (viridanus\trochiloides hybrids) and
2. P.t.obscuratus (trochiloides\plumbeitarsus hybrids)

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The Hardy-Weinberg ratio is the starting point for much of the theory of population genetics. It is the ratio of genotype frequencies that evolve when mating is random and neither selection nor drift are operating. For two alleles (A and a) with frequencies p and q, the Hardy-Weinberg frequencies are

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Genotype AA : Aa : aa
Frequency p^{^2} : 2pq : q^{^2}

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These frequencies are reached after a single generation of random mating from any initial genotype frequencies. The Hardy-Weinberg ratio can be understood in terms of simple combinatorial probability.

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