Why isnt everyone beautiful, smart and healthy? Or, in a less-polite formulation, why havent ugly, stupid, unhealthy people been bred out of the populationugly people because no one will have them as mates, meaning they dont get the chance to pass their ugliness to the next generation; stupid people because theyre 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 traitsor, 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 youll see that that has not happenednot 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 dont 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 Scotlands Isle of Rum. They carefully noted each animals 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, Edinburghs Katharina Foerster and her colleagues conclude in tomorrows 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, prestomore 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. This discovery reminds me of other seminal studies that contribute to our understanding of why bad genes persist. The best know is the gene for sickle-cell disease, which is prevalent through the Mediterranean region and much of Africa. Why wouldnt natural selection get rid of it? Because, it turns out, carrying one copy of the gene increases your resistance to malaria (this is explained well here), which is prevalentsurprise!in the exact same regions. Much of the theorizing about fitness in human biology has been undermined by empirical studies (see, for instance, Adapting Minds by David Buller). The study of red deer provides one more cautionary tale for those who would be tempted to weed out undesirable traits in the human population. On a less lofty note, it should make us think twice before we reject as a mate someone who does not conform to the fitness stereotype promulgated by evolutionary psychologists (such as a waist-to-hip ratio of 0.7 for women, and alpha-male behavior for men). You may think, subconsciously, that youre choosing someone who will transmit good genes to your kids, but just ask yourself how that perky little nose will look on your son or those rippling pecs on your daughter.