People often ask ill-conceived questions about whether, by using things like modern medicine, we obstruct natural selection. My own thought is that this thinking is facile, because it presumes that either medicine or caring for sick people are somehow "unnatural" interventions for our species. Animals like chimpanzees are known to use tools, and we wouldn't say that is unnatural for them--isn't medicine just a human tool? But then I wonder if conceiving of natural selection in this way makes it impossible to say that we ever obstruct natural selection, which seems like an logically odd consequence. I was wondering if there is a more discriminating formulation of natural selection according to which we obstruct natural selection in some cases and not others.

You're right to be suspicious of the idea that we somehow "obstruct natural selection." That way of putting things assumes a dubious notion of "natural" and suggests that if we influence evolution (which we do) this is somehow a bad thing. As you point out, medicine and other human inventions are natural in a perfectly good sense: we are part of nature and we create these things. Of course, there are also perfectly good uses of the word "natural" according to which medicines, computers, cars and so on aren't natural. But whether something is "natural" in that sense and whether it's good or bad are two very different questions. So is the question of whether these "unnatural" things have a place in evolution.

Some people think that because we can manipulate our environment, evolution doesn't apply to humans. This is confusion. Natural selection is a matter of differences in how likely an organism is to reproduce because of features of the environment it finds itself in. Even if we shape the environment, it's still true that in the environment we've partly shaped, people with some traits are more likely to reproduce and pass on their traits than others. And it isn't an all-or-nothing affair; it's a matter of more or less likely.

It's true that some traits are no longer important for selection. For example: in much of the world, being near-sighted is far less of a threat to reproductive success than it was centuries ago. This is because we invented glasses. On the other hand, we've created a world in which people who are better at multi-tasking have advantages that they may not have had two hundred years ago. This doesn't mean that bad multi-taskers will eventually die out, but it could still have an effect on differential rates of reproduction. We've also introduced toxins into the environment that humans once didn't have to contend with. This creates a selection pressure. If some people are less susceptible to those toxins, that could make them more likely to reproduce and pass their resistance along.

Principles used in evolutionary science such as the Hardy-Weinberg principle ( don't distinguish between "natural" and "unnatural," and that's the key point. Evolutionary science isn't about a special set of environmental pressures that count as "natural." It's about how the distribution of heritable traits changes in a population because of how likely creatures with those traits are to reproduce, given the environment they find themselves in. I think you'd be hard-pressed to find an evolutionary scientist who took the idea of "obstructing natural selection" seriously.

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