Why is inbreeding depression not a problem in most species?
Harmful recessive alleles occur in significant numbers in most species. (Humans have, on average, 3 to 4 lethal recessive alleles each.) When close relatives breed, the offspring are more likely to be homozygous for these harmful alleles, to the detriment of the offspring. Such inbreeding depression still shows up in cheetahs; they have about 1/6th the number of motile spermatozoa as domestic cats, and of those, almost 80% show morphological abnormalities.