[PatrickHenry]

I added an essay to my homepage. It's a bit long, but I'm hoping that it may be a useful link:
Micro-evolution, Macro-evolution, and Speciation.

[VadeRetro]

Very good. Maybe add a bit on what the model ISN'T. "So one day a snake gives birth to a bird, but where is there another little bird for it to mate with?; When have you ever seen a rat turn into a human?" etc. One strawman has macro changes in one generation, the other in less than a single generation. That kind of thing.

[Coyoteman]

Good writeup.

One suggestion: near the end, deal with the splitting of the herd. One part of the herd goes on the north side of the mountain (river, continent, etc.) while the other part goes to the south side. One part of the herd stays in the forest while the other part carefully explores the spreading grasslands.

This gives changed (and changing) conditions which can in turn speed up adaptation.