A closer look
When a massive disaster randomly wipes out the vast majority of a population, natural selection basically takes a coffee break. The lucky few who survive do not necessarily have better traits; they just happened to not be crushed by falling rocks. Rebuilding an entire population from those few survivors forces everyone to pull from a highly restricted, heavily depleted genetic pool. It is like trying to paint a massive mural when the only crayons left in the box are different shades of beige.
