Inbreeding changes no gene frequencies, yet it can doom a population
A common assumption holds that inbreeding makes harmful genes more common. It does not, at least not directly. What it changes is how often those genes pair up. Wolves on an isolated Michigan island show the result: extra bones in the spine, a flaw now fixed across the whole population.
Inbreeding means breeding between close genetic relatives. Its main effect is homozygosity, inheriting two identical copies of a gene. Most harmful recessive variants are rare, so two unrelated parents seldom both carry the same one, but relatives often do. When such copies pair up, hidden defects surface, and a population can suffer inbreeding depression, a drop in its ability to survive and reproduce. Crucially, the underlying allele frequencies stay the same; only the proportion of matched pairs rises. Because matched pairs are exposed to natural selection, the harmful variants may actually disappear faster over time.
Wild examples are real but patchily documented. The Isle Royale wolves carry vertebral malformations, and northern elephant seals show altered skull and jaw proportions. Uniform immune systems can leave a population exposed to a single disease, and once an endangered species shrinks far enough, forced mating among the survivors can push it over the edge. Yet the South American sea lion, descended mostly from just two maternal lines, still holds enough variation within them to offer some protection.
Isolation is not always a dead end. Island species are often heavily inbred, and that process can first purge many harmful genes and then let locally useful traits emerge, sometimes producing a new race or species. Some animals with haplodiploid genetics rely on mating with their own sons when no other male is around. Inbreeding also keeps a larger share of a parent's genes intact in the offspring.
Many species seem wired to avoid it. Wild and feral horses drive off young of both sexes, and most mammals, including humanity's closest primate relatives, steer clear of close relatives. Livestock breeders, by contrast, use deliberate inbreeding to fix desirable traits and to reveal harmful recessive genes so they can be bred out.
Source: Inbreeding
