How could a serious disease be common, if evolution is always optimizing everything? Well, if it hit you at age 85, selection wouldn't be very good at filtering it away. People have already had their children by then. The second point is that it gives us a new notion of what a disease is - anything that interferes with reproduction. How can we use this information to understand disease? Well, in the first place, if genetic problems were the only kind of problem we had, most of us would hardly get sick at all. For comparison, the most common disease of this sort in people of European ancestry is cystic fibrosis, which maybe hits one in 2500 kids, but sickle cellanemia can hitmore than 1 in 100 kids in much of Africa, in some places one in 30. They are only important in populations whose ancestors have lived with malaria for many generations. We know of a number of others. Most are also malaria defenses (G6PD deficiency, alpha- and beta-thalassemia, etc.) some protect against other things. Cystic fibrosis probably protects against typhoid. We pay a lot of attention to them, because they are much more common than other genetic diseases, but they are atypical. They are common because they are actually useful in certain situations, rather than being an expression of the noise in the system.

The most famous of these is the sickle-cell mutations, which protects carriers against the worst forms of malaria. The second category consists of mutations that are surprisingly common - because, it turns out that they offer carriers some advantage, usually protection against some really unpleasant infectious disease.

The first is simply the consequence of the fact that mutations happen:you might call it the noise in the system, the inevitable price of imperfect gene-Xeroxing. This means single-gene Mendelian disorders like classic dwarfism or muscular dystrophy, and I suppose I would include chromosomal disorders like Down's syndrome. Maybe 1% of the population (In Europeans say) has some Mendelian disorder, mostly mild ones, but no single disorder is at all common. They do cause some genetic diseases which fall into two categories. So you don't expect defective genes to be the main cause of human disease, and they aren't. When we say that a mutation, agene change, helps or hurts the organism, what we really mean is that it helps or hurts the organism reproduce.Reproduction is the currency of natural selection: a gene that made you long-lived, happy, and sterile would never become common. 'Good' means good at being passed into the next generation. Good genes are a recipe for all kinds of complex functions, and for good health - but from an evolutionary point of view, strength and health are just means, while babies are the end product. It magnifies the rare changes that improve things. You could think of it as a filter, constantly removing changes that don't work. If it causes trouble, as most changes do that make any difference at all, the change decreases over the generations, becomes rare. So although changes (mutations) happen, they don't happen very often, and natural selection tends to keep them rare. If there is a change in a gene that helps the organism, that change increases over the generations, becomes more common. The first key idea is that evolution optimizes function. An Evolutionary Look at Human Homosexuality An Evolutionary Look at Human Homosexuality

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