so-crates

Here's a question I have about speciation with regards to the reproductive system. Two different species (say X and Y) sharing a common ancestor must have become reproductively isolated from one another by geography, mating seasons, sexual maturity, etc. In many different species, the reproductive systems must have also changed so that they can no longer <i>physically</i> mate with one another. But if an animals offspring cannot breed with other members of the species, that animal will not be able to pass on its genes and speciation would never occur. For the sake of argument, consider chimpanzees and humans, which cannot interbreed with one another even though there reproductive systems are similiar. Can anyone describe at a high level what happens to the reproductive system over time that allows speciation?

My hunch is that the reproductive system is not an all or nothing thing. Mating is not always successful in humans for instance. Perhaps the success rate among the two different populations gradually reduces until zero, so at some generation you can definitely say two different species have formed and not simply two different variations within the species. Without know the fine details of sexual reproduction, I can't confirm this idea. Can any biologist explain this? Have any specific studies been done?

[ April 07, 2002: Message edited by: so-crates ]</p>