I'm curious as to why sexual reproduction generates genetic variation. For me, the term genetic variation is a little ambiguous. The way I understand it, it's the number of alleles at a locus in a population or "group" of organisms.

But sexual reproduction only "reshuffles" alleles, and doesn't generate new ones (most of the time, at least). So why does it create genetic variation?

Thank you for reading.

There are two questions here. 1) What is genetic variance? It is actually clearly defined. 2) How does sexual reproduction generate more genetic variation?

What is genetic variance?

Assuming bi-allelic loci, just assign a value of 0 to one allele and 1 to the other allele. Then compute the variance! Average this value over all loci. Done! Have a look at any intro book in quantitative genetic for more info (see here)

How does sexual reproduction generate more genetic variation?

You are right "sexual reproduction only "reshuffles" alleles, and doesn't generate new ones". It is a common misconception that sexual reproduction generate genetic variation. The details of whether it does or it does not depends upon the detailed distribution of dominance coefficient and distribution of epistatic effects.

There has been a lot of work on the subject and it is not an easy work to summarize it. Sally Otto does a great job to summarize this misconception in her speech On the Evolution of Sex and the Advantages of Recombination. You should definitely have a look at it.

Note however, in some species that preforms both asexual and sexual selection, it happens that sexual selection is coupled with higher mutation rate than asexual selection. In such cases, sexual selection does indeed increase genetic variance but only so because of tis correlated effect on mutation rate.

Short answer it doesn't. Sexual reproduction generates new combinations of traits and increases the difference between individuals in the same populations.

New combinations means beneficial traits have a better chance of finding complimentary traits while the opposite are also true detrimental traits have a greater chance to find combinations that make them worse. It increases variance in the sense that individual genes can disperse easier in the population, and genes can form more unique combinations, but not in the sense of actually increasing the overall variability.

Likewise it also keeps individuals from being identical which makes a population more vulnerable to parasites and disease.

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