[Matt R.]

This could lead to some difficulties, but I will try anyway.

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Matt:

A simple working definition is called for. If two individuals are potentially fertile with each other, they are of the same species. If not, they aren't.

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I don't like this definition. I think Rduke may chastise me on this one [img]/images/graemlins/blush.gif[/img].

I think a better way to think of it is that ALL individuals are *potentially* fertile with each other. The classificiation of species has nothing to do with potentially fertile. Its useful classification is in terms of probability of being fertile. For instance, if we choose a random human male and human female the probability of them being fertile with one another is pretty high. Much greater than 50% if they have access to modern technology... probably an extremely conservative estimate but you get my point anyway.

If we take two separate species, as a biologist would classically classify it, the probability of them being able to produce fertile offspring is extremely close to zero. But it is non-zero. There is always a chance that if we select one member from species A and one member from species B they can produce fertile offspring. They would have to be of extremely closely related species and probably have high deviations from the "normal" or "average" genotype of the species, but it is at least possible. I think what a biologist typically does is ignore these tiny overlaps because... well they are tiny. It isn't really necessary to consider these occurances when doing research, for instance. A species is a species due to classification purposes... I think it is a mistake to consider it an absolute. I think this may be the issue vhawk is having. But, for a biologist it isn't really necessary to consider the 10^-whatever chance that a random gorilla can mate with a random chimp.

Edit -- the gorilla/chimp example may even be stretching the possibility. But I'm sure there are "separate species" which exist that CAN mate with one another... like with the whole ring species example that was tossed about.