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.
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.
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.
From your question: "... only "reshuffles" alleles, and doesn't generate new ones (...). So why does it create genetic variation?"
There are no trick questions, still my answer goes:
Because genetic variation is also created by independent assortment.
There are two ways of crossing-over in meiosis.
First, there is Chromosomal crossover, or crossing over, which "is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes." (Wikipedia)
Second, however, there is, much lesser known Independent assortment of chromosomes:
"The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent chromosome along the metaphase plate with respect to the other bivalent chromosomes. Along with crossing over, independent assortment increases genetic diversity by producing novel genetic combinations."
Without alleles being changed (if exchanged, as a result), chromosomes from on parent are exchanged with (homologous) chromosomes of the other parent. This, too, results in genetic variation, and - as such does not involve so called crossing over of allelic genes. Exchange of alleles as a result of independent assortment must be distinguished from creating new alleles by crossing over,i.e. exchanging genes or parts of genes.
Independent assortment can be differentiated from "allelic variation". Hermaphrodite reproduction as sexual reproduction shows that in one single individual there are a million combinations of alleles. By definition, however, not all alleles existent in a population. Interestingly, this rule is not valid in respect of the rather unknown mechanism of mingling of chromosomes (not alleles). Any indivual has got the potential to find any - among all - combination of chromosomes. From this finding results that intercourse of different individuals in contrast to hermaphrodite interchanging of chromosomes and alleles can be distinguished by potenitally combining all alleles, whereas all chromosomes can - potentially - combined by one hermaphrodite individual.
Surprising about this answer is that, in the words of your question, "new genetic variation" that is not considered allelic variation as it does not involve the changing of genes is thus produced by sexual reproduction.
