Why Genes that Harm Health Persist

Why hasn’t natural selection eliminated genetic variations that harm health? The question is central to our ability to use new genetic technologies to improve health, but there are strong tendencies to provide simplistic single answers. The old answer has been that mutations happen and natural selection is not all-powerful. This is correct and important but only part of a full explanation. The idea that genes that cause disease also may have benefits is popular but not as widely applicable as many would wish. Many variations that cause disease are not abnormal at all; they cause problems only in modern environments. Others increase Darwinian fitness at the cost of personal health. Some benefit one sex at the expense of the other. The framework of evolutionary medicine offers a taxonomy of explanations for genetic variations that harm health.