Every time an animal mates, its offspring receives a mixed bag of genes, half from its mother and half from its father. If the father and the mother share many of the same genes, their offspring will receive doubles of some of them. The doubles don’t help, and they mean that some other gene that could have been passed on wasn’t. Think of the M&Ms. At the point where you failed to pull a red out of the bag, you have lost one gene. If your mate doesn’t have that gene either, then your descendants will never have it. Low populations of animals means less genetic variation, which means genetic drift is more likely. If the gene that gets lost was critical to the survival of the species, they might begin dying off altogether.