Doctors treat patients one by one, but public health is about the whole population, or at least subsets of it treated as aggregates. This is something we've touched on before, including here this week. Generally, the latter is a higher-level abstraction of risk relative to individuals. But doctors are not particularly trained in how to apply aggregate data to individuals, or at least there are important, often subtle differences between the two perspectives. An article at the Huffington Post nicely discusses this from a physician's point of view.
But the same issues may apply to evolution. A given trait, such as presence of some condition, like brilliance of feather color, or some level of a quantitative variable like blood pressure or stature, may have a net or average reproductive success, and such success rates may vary by the value of the trait. The success rate can be a matter of chance or may be due to systematic functional effects of the trait value on reproductive success; the latter case is what we mean by 'natural selection'. We view species today as aggregates, but each individual has its own trait value, so the distinction between population and individual is important on both contemporary and evolutionary time scales, and in similar ways.
In evolution, our models generally ascribe the relative fitness of a given trait, compared to the variation in the rest of the population. This is because the frequency of trait values in the future depends on what is transferred from this generation to the next. But in public health, things are somewhat different. Values (judgments, treatment decisions, and so on) are made on individuals and for individuals, without regard to their effect on the future or on the whole population. Your doctor treats your tuberculosis, not the population's.
However, in both cases there are risks, or probabilities, involved. In evolution, what is the probability of having a particular number of children for someone with a given trait? Note that we refer to traits, not genes -- the effect on contributing genes is an indirect result of what happens to individuals who bear them. In medicine it is the risk of getting a particular disease for someone with a given level of exposure to some risk factor, or of a given response to therapy.
But while we need individual predictions, and evolution selects on individuals, risks are estimated from populations. So things are a bit circular, or at least not straightforward. This provides much to think about. Risks that are large are easy and behave just as what you were taught in Statistics 101 said they would. But risks that are small are not different from chance, or from other small risks, and that is not so easy to deal with. Unfortunately, small risks are often what we most have to deal with both in medicine and evolution.
What about risks that are hard to detect, or perhaps even impossible, such as the risk of cancer from dental x-rays? Should you avoid such exposures because radiation is clearly proven, at the cell level, to cause mutations and mutations can cause cancer? Who decides what an acceptable risk is, or the statistical criteria for saying that there is, in fact, a risk? We usually use significance tests for this, but they're subjective judgments.
In evolutionary terms, change due to selection accumulates over generations, but so do chance changes. If selective differences between contributing genotypes are very small, chance (genetic drift) can be a major force for change.
And what about even large risks? We've mused on this before. If the risk that someone with your cholesterol level will have a stroke is, say 15%, does that mean a die is going to be rolled with 6 sides, and if it comes up 6 you're a goner? Or does it mean that 15% of people like you are certain to have a stroke, and the others just as certain not to (but that we don't know how to tell who's who)? Public health, or aggregate perspectives in evolutionary biology, don't concern themselves about this, even though of course they know that everything happens to individuals.
But doctors and those who are arguing that some particular genetic variation is important in evolution have to think about the individual level.