[PatrickHenry]

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.

Good idea. Definitely worth a paragraph.

[PatrickHenry]

Your suggestion has been included. It's one of the indented paragraphs, 3d paragraph from the end:
Micro-evolution, Macro-evolution, and Speciation.