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If evolution is gradual, why are there any distinct species?
Because each genetic lineage needs to fit its environment, its niche, in order to survive. There are plenty of niches which blur into one another, eg clines -- altitude up a mountainside, for instance.
But there are also plenty where a 'neighbouring' niche requires something distinct from its inhabitants. Therefore the distinctiveness becomes reinforced; sex between the two types -- hybridisation -- produces something not best adapted to either niche. So the intermediates are less successful, and further adaptation to each niche gets, in effect, encouraged.
But as has already been pointed out, there is heaps of indistinctive species. Is a hooded crow a different species from a carrion crow? Is a herring gull a separate species from a lesser black-backed gull? The crow species are separate in northern England vs Scotland, yet there is a zone of hybridisation between them. The gulls are separate, non-interbreeding species in northern Europe, yet form a continuous interbreeding ring around the north of the globe. Follow one type of gull round the top of the world in either direction, and the gull you start with gradually looks more and more like the other one, till you get back where you started and you have two species.
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I don't see why there wouldn't be as much 'intermediate' species as 'definate' ones, if evolution is how it works.
There are loads, but the tendency is towards separateness as the niches demand. This is why in nature we find everything from fully interbreeding populations, to regional variations, to ‘races’, subspecies, species that do still sometimes interbreed (especially common in plants), to fully separate species. Even then, they can interbreed, and are not always infertile. Hence lions x tigers = ligers and tigons. Is it not odd that, hybrid infertility notwithstanding, that the thing that horses
can interbreed with is the remarkably similar ass?
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I'm not saying all evolution is wrong(or that i understand it well enough to say(thats why i'm asking)),
Just as well really, but very honest and magnanimous of you to say so!
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i can definately accept natural selection,
There is no point whatever arguing with it!
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but i don't see one species changing into another.
Please define species! You can see them do just that in ring species, like the
Larus gulls mentioned, and in
Ensatina salamanders. Hence the need for you to define what it is you don’t see!
Speciation is the evolution of reproductive isolating mechanisms. These can take many forms: breeding at different times of the year, differences in mating behaviour, differences in genitalia shape or size, hybrid lower fitness, infertility and inviability... Your best bet is to have a thorough browse through
Observed instances of speciation, which explains all this and then gives a load of examples of isolating mechanism development and of the mechanisms then in action. That is, of speciation.
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All dogs have the same amount of chromosomes--- though their appearance and general behavior varies from breeds.
So? Chromosome number is not, in itself, an isolating mechanism (though the way chromosomes from two individuals work together -- and we are all hybrids, or our parents' genomes -- can be ,and so different chromosome numbers could act that way). There's plenty of different species with the same number, and there are examples of perfectly well interbreeding species with variable chromosome numbers. Chromosomes are, after all, just long chunks of DNA. Split them or fuse them, and you’ve still got all the genetic instructions, just in more or less pieces. These may or may not still work; that depends on other things, not merely the number of bits involved.
I'm curious what you think about dogs. If you wiped out all medium-sized dogs, leaving, say, only Chihuahuas and Great Danes, on what grounds could we
not consider them distinct species?
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For there to become a new species, there must be a different number of chromosomes,
Not true, as per the above.
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is there such thing as a partial chromosome(i'm pretty sure there isn't)?
I’m pretty sure there is. It’s 50/50 that you have a partial chromosome yourself, and the chances are virtually 100% if you are male. Because the partial chromosome is the Y. So, what do
you think a partial chromosome is?
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If not, how does a creature suddenly gain more or less chromosomes?
Fission and fusion. They can split in two, and can join end-to-end. Both are well documented; I suggest a trip to your library for a genetics textbook, eg Lewin’s
Genes VII or Brown’s
Genomes.
You yourself contain a good example of a chromosome fusion in every cell of your body. Your chromosome 2 is a fusion of two separate ones, the equivalent of the ape 2p and 2q. And we know this because chromosomes have very distinct endy bits, called telomeres, and a distinct central bit, called a telomere. The human chromosome 2 has extra telomeres in its middle, and spare centromeres.
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Also, just for fun--- anyone have a list of the number of chromosomes various species (plant, animal, other(don't all bacteria and protists only have one though?)?
Hell, there’s a few million species out there! Okay, so we’ve barely started looking at their genomes, but still...! Hey, let me introduce you to
Mr Google...
But you might find this pdf about the various
Karyotypes of three species of Chinese mesogastropod snails interesting...
Cheers, DT
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