A question about natural selection as an explanation for everything

[Riemann]

@Colibri

To clarify terminology, what you’re describing is not genetic drift, which by definition is due to random events rather than selection. The phenomenon you describe is termed a selective sweep or selective hitchhiking, in which genes in the vicinity of a strongly selected gene are dragged along with it even though they are not under direct selection themselves.

A clever alternative name for hitchhiking is genetic draft. It was obviously coined because of the similarity to the word drift, but I don’t like it because it’s so similar that I have in the past misread it for drift wondered wtf the writer was on about.

[Riemann]

@Smeghead

To oversimplify one argument for illustration, consider this: every positive selection event that occurs to one gene carries along with it a whole slew of nearby genes despite their having nothing at all to do with the selection. As far as those genes are concerned, they lucked out - in other words, drift. Thus, the total number of selection events at the gene level is far smaller than the total number of drift events.

As Colibri has noted, a selective sweep or genetic hitchhiking is the correct term here, rather than drift. Furthermore, it’s a more complex (and interesting) picture than your last sentence [my bold] suggests.

Meiotic recombination “reshuffles” the linked genes on a chromosomes, so that linked genes are not always inherited together. Recombination is a rather slow process, with only one or two recombination events (“crossovers”) per chromosome per meiosis. However, selection can be slow, too. The key, therefore, is the rate of selection vs the rate of recombination. A selective sweep will occur when selection is rapid compared to the rate of recombination.

This leads to an understanding of why sex is important. In a bacterial population, if we assume for the sake of argument that bacteria never have sex (not quite true), when a beneficial mutation arises it one bacterium, if that mutation is to proliferate and go to fixation in the population (completely replace the prior version of the gene) that means that every other bacterium dies off with no descendants and the future population is entirely descended from that one bacterium that acquired the mutation. That means that many other beneficial mutations that might have been present in other bacterium are lost - i.e. in an asexual species, every positive selection event is a 100% selective sweep. So mutations arise and go to fixation “one at a time”, a slow process. This can work for bacteria, because their generation time is measured in minutes rather than decades.

Sex means that genes are constantly reshuffled in the population and not always inherited together, so different mutations that arise in different individuals can come together. Selection can work simultaneously on many traits, and diversity is not necessarily lost to a sweep unless the selection pressure is strong and recombination doesn’t have time to operate.