[narby]

We need to define the mechanism for these changes, right?

The mechanism is survival rate. But that can mean anything from climate (dought), to hunters that like elephant tusks and elk horns. Also geology, predators, food surpluses, anything that affects survival.

[KMJames]

Your answer is well-explained, and along the lines of my understanding. Because of all the factors that make up this "mechanism of change" it seems that predictability is not currently possible. Just predicting that change will occur doesn't seem useful for anything, IMHO.