It's not natural for many other species to migrate and there would be no way for them to *know* that heading south would be warmer. It certainly isn't around here, the areas slightly to our north are virtually always 3-5 degrees warmer that the hills and mountains we're in to the south, so there would be no incentive to head in that direction *looking* for warmer climates.
Many species have large ranges, and if the northern range was cut off by weather, than the southern ranges would survive to carry on the genome.
there would be no incentive to head in that direction *looking* for warmer climates.
Critters often travel great distances, even within a season, and certianly in a lifetime, but they tend to stay in a particular climate area. You don't see penguins in Panama.
If the climate area shifts over time, the range of the critters would as well. This is not an obstacle to evolution. Actually, the fact that critters do wander over wide areas would be a positive selection for their survival. You can note that Pandas, who have very small home ranges (google it) are dying. Their very lack of travel, (and lack of diversity of food) is killing them.