Punctuated equillibria

Explanation of stasis:
1. Require very strong selection pressure , at normal (casual) condition species tends to remain unchanged because there is a high price to pay to break a already-well-adapted team of genes. But when the selection pressure is sufficiently high paying the price may be feasible.
2. animal respond to minor change in environment by migrating.
3. Different directionality of change in a population (I don't favor it).