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Originally Posted by Diablo
I've been taught, and always thought, that a species is a group of individuals that do not successfully interbreed with other groups in nature, not in captivity or anything along those lines. If that is what you meant, then think about this: do you think that every kind of dog can physically breed with every other dog? Pick some very small breed, and a very large breed, and think about whether or not it is physically possible for them to interbreed naturally. Sure, the genetics may be there, but is it physically possible? For some combinations, it isn't.
Also, do the finches interbreed naturally? If they do, then I agree with you, but I've always thought that they do not interbreed naturally.
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The problem with adding "in nature" to the mix is that some animals separated geographically may have compatible chromosomes and be therefore biologically the same species, but the don't interbreed simply because they cannot reach each other. The biological argument deals not with physical limitations, but with the chromosomes which is the true test of species (if the chromosomes are the same, then they are the same species). Consider that very few Kalahari Desert people naturally interbreed with New York socialites (probably none), but their genes are compatible and are considered the same species despite many very obviuos physical (adaptive) differences.
It is true that a Great Dane male and a Chihuahua female cannot physically interbreed due to the obvious size problems, but a Great Dane female can interbreed with a Chihuahua male, or for that matter, a Great Dane male can interbreed with a Mastiff who can then breed with a German Shepherd, etc.
As to the finches, yes, they do interbreed naturally. Here's a
page to read, but speicifcally the last paragraphs are important (emphasis on the last sentence is mine):
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Moreover, the Grants' observations undermine another myth about Darwin's finches - that individual species are 'confined to certain islands'. In order for different species to mate, they clearly have to occupy the same territory. Other visitors to the Galapagos have confirmed that this is this case. Television documentary filmmaker Gillian Brown spent a year working at the Darwin Research Station on the islands. It is common, says Brown, to find the different species all over the archipelago, rather than obeying the colored territorial maps drawn up by Darwinist ornithologists.
In almost all respects, the finches of the Galapagos are so similar that it is difficult to tell them apart. Indeed, Weiner himself remarks that, 'Some of them look so much alike that during the mating season they find it hard to tell themselves apart.' This mirrors David Lack's observation that 'In no other birds are the differences between species so ill-defined.' The finches all have dull plumage, which varies from light brown to dark brown, all have short tails, all build nests with roofs, and lay white eggs spotted with pink, four to a clutch.
It is very difficult for an objective observer to see how a group of finches who 'find it hard to tell themselves apart', and who do in fact interbreed, can legitimately be called different species.
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-- Jeff