Comment by nwatson

Yeah ... animal (and plant, etc.) populations in a stable environment have a lot of random variations that confer no outright distinct advantages over their peers, they are distributed along the normal plane to the current evolutionary gradient. When environments suddenly change, it's not like animal populations rapidly create new random mutations that confer advantages in subsequent generations. Rather, the new conditions make previously existing traits that some animals within the population had and others didn't have more significant to survival, and so those advantageous traits are emphasized in subsequent generations.

After this "evolutionary bottleneck", other new random mutations / variations will occur generations later that might have no seeming immediate advantage / disadvantage to the "new normal", but at some subsequent change in the environment, the cycle of weeding out "bad" traits will happen again.