"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."
This is true