Hmm.. good one. :)
1.) Genetics are what can create features or behaviors in animals or other living organisms. So this can cause a change in population because say one species of bird was born by two slightly different parents, now it has both genetics from both. This can be good or bad. Say the dad's genetics sucked but the mother's genetics were good. This bird might be better at surviving or better at being prey.
Environmental Pressures: 1.) If rabbits die because of a disease, their predators will become hungry. The predators might go down in number too and die as well. If anything in nature goes wrong, it may affect a population to either increase or decrease.