Why isn’t everyone beautiful, smart and healthy? Or, in a less-polite formulation, why haven’t ugly, stupid, unhealthy people been bred out of the population—ugly people because no one will have them as mates, meaning they don’t get the chance to pass their ugliness to the next generation; stupid people because they’re outgunned in the race to financial success (that is, acquiring resources needed to survive and reproduce); unhealthy people because they die before they get a chance to reproduce?

Evolutionary theory predicts that the unfeeling hand of natural selection would lead to a culling of disadvantageous traits—or, as biologists more delicately phrase it, “depletion of genetic variation in natural populations as a result of the effects of selection.”

But look around, and you’ll see that that has not happened—not in people, and not in wild animals where homely and infirm offspring are born all the time.

Evolutionary geneticists try to explain this paradox by positing that mutations for disadvantageous traits keep popping up no matter how hard natural selection attempts to wipe them out, but in their more honest moments the scientists admit that in real life undesirable traits are way more common than this mechanism would account for; “ugly” mutations just don’t occur that often. In a groundbreaking study, biologists at the University of Edinburgh in Scotland have figured out why, at least in one species: genes that are good for males are bad for females and, perhaps, vice versa.

The scientists studied red deer, 3,559 of them from eight generations, living on Scotland’s Isle of Rum. They carefully noted each animal’s fitness, who mated with whom, how many offspring survived, which offspring mated and with what results. Bottom line: “male red deer with relatively high fitness fathered, on average, daughters with relatively low fitness,” Edinburgh’s Katharina Foerster and her colleagues conclude in tomorrow’s issue of the journal Nature. “Male red deer with a relatively high lifetime [fitness, which includes their reproductive success, the only thing evolution cares about] sired, on average, daughters with a relatively low [fitness].” The reverse also holds. Males that were relatively less successful in their reproductive success and fitness had daughters that were extra successful.

The reason is that any particular gene-based trait may have very different effects on males than in females. Extrapolating to humans (and oversimplifying, sorry) you might imagine that a particular shape of the nose or turn of the chin would look drop-dead hunky on a male, but horsey on a woman; dad got to mate because his looks attracted a female, but the result of their togetherness produced daughters whose pulchritude was less than obvious. Traits that evolutionary psychologists tell us make women unfit for mating (having the “wrong” shape) remain abundant in the human race because the DNA for the traits, when inherited by sons, confers a selective advantage; when those sons have daughters, presto—more females with less-than-hourglass shapes. Or as the Edinburgh biologists put it, “optimal genotypes differ between male and female red deer, because a genotype that produces a male phenotype with relatively high fitness will, on average, produce a phenotype with lower fitness when expressed in a female.”