Quote:
Originally Posted by duga
I'm a supporter of the reproductive isolation bit. Unless someone finds an organism (say, an amphibian) that can breed with a totally different organism (say, a mammal) and produce viable offspring, I'll stick with that.
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In theory it works well for many organisms, it would seem, but it sure is hard to check since all organisms in nature don't occupy the same geographical areas and don't all have the same courtship behaviour and so on.
You get further troubles when you get to plants. They hybridize a lot and such evolutionary events are thought to have given rise to a multitude of the species out there. Another problem is this; imagine that species 1 can hybridize with species 2 and 3, but 2 and 3 can't hybridize with eachother. The sexual isolation definition would have to refuse and accept 2 and 3 as the same species at the same time!
This problem does happen in nature, sometimes involving a lot more species than 3. Brassicas are an example. Although I don't know for sure, I assume this must happen in some bird species as well.
Quote:
Originally Posted by noise
not necessarily...
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I agree and if you flip the problem over on it's head, many species that should obviously be different look almost exactly the same. A good example is the phylum Nematoda (nematodes), a huge group of wormlike creatures that have differing ecology. Some are parasites while others for example live in the soil. However, they all follow the same general body plan and look similar to eachother. Two seemingly identical nematode worms can be further apart genetically than humans are to any other mammal.
So I would argue that both sexual isolation and similarity/dissimilarity are not good species criterias.