one more thing—going back to poni's presentation on cheetahs, i was wondering exactly why inbreeding is so bad for them (and for organisms in general). i know that it results in increased homozygosity of many alleles in the offspring organisms, but i wasn't sure exactly why that was such a bad thing. of course if a family carries a masked recessive gene that causes a disorder, inbreeding would increase the chances of its expression, but i'm wondering if there are other ways inbreeding compromises organism fitness.
upon further research, it looks like all of the problems associated with inbreeding are basically just iterations of that same concept. deleterious recessive alleles carried by a family can come together homozygously in offspring, leading mostly to a characteristic pool of less critical problems and less commonly to catastrophic phenotypes. the most common effects of inbreeding include reduced fertility, facial asymmetry, increased infant mortality, stunting and immune system compromise, while more serious genetic disorders crop up depending on the family. these effects serve to cull the population of inbred organisms—lower fertility and poorer physical condition make it difficult for inbred animals to survive until reproductive maturity, let alone to actually reproduce.
the concept applies at the population level as well. the term "inbreeding depression" is given to the phenomenon of reduced population fitness due to genetic similarity as a result of inbreeding. reduced immune system function and increased likelihood of particular susceptibility to a certain disease or disorder can render inbred populations less likely to survive and proliferate. if one virus or disease is particularly deadly to an individual, inbreeding makes it more likely that that same virus will have similar effects on the rest of the population. cheetahs provide a sad example of this—a 1990 study on one cheetah population recorded a 90% prevalence of feline infection peritonitis and a 60% mortality rate.
surprisingly, some species are not subject to inbreeding depression. generally, these species emerged in very small but viable populations and have been inbred since their origin. as a result, deleterious alleles are eliminated from the population extremely early on, and further inbreeding presents little risk of depressing population fitness by exposing harmful alleles.
more info here: