http://www.talkorigins.org/faqs/faq-speciation.html
That's great! I love this part:
". . . experimentally showing that A doesn't interbreed with B doesn't preclude both interbreeding with C. This gets even more complicated in groups that don't have nice, straightforward sexes."
I would add: ". . . such as people."
What I like about biology is that there are no rules without exceptions. It is much more fun than physics.
- Jed
