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On this article, first column, eighth line of the introduction:

By bringing together favourable alleles from different chromosomes, sex and recombination increase the additive genetic variance for fitness [..]

Can you please help me understanding why does recombination increase the additive genetic variance for fitness?

Note: In reaction @mgkrebbs comment, I think that given the subject of this article, when talking about different chromosomes, the authors actually referred exclusively to homologues chromosomes. My question concerns the second part of this sentence anyway: "Why does sex and recombination increase the additive genetic variance for fitness"?


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This is a little tricky.

First of all lets be clear about 'bringing together favorable alleles' (or any alleles) represented by site mutations on 2 chromosomes:

--------A------------------
             X     
------------------B--------

If the two dashed lines are two copies of the same chromosome, then a recombination event at X may produce:

--------A---------B--------

Allowing both A and B to be passed along to offspring.

Any given case of recombination might actually harm resulting offspring. But in cases where A and B together are beneficial, over the course of generations you will find AB genotype combinations show up more frequently in the population because of selection and competition.

Firstly each individual variant experiences some selection for fitness and many are lost. Because of this, recombination will typically put two or more such variants together onto a single chromosome. Recombination allows for individual mutations of benefit to pool together and collect into a single place. This multiplies the chance that stronger combinations of mutations will show in the gene pool. This increases the fitness variance - the most powerful combinations are more beneficial than any single variant alone. On the other end detrimental combinations will also make some individuals exceptionally weak.

So its not just recombination, but recombination and selection which create favorable combinations of alleles in populations. The link you give discusses how when selection is not strong, recombination is not so great, but we never seem to lose recombination where these conditions don't hold sway.

By comparison, consider a simple case where recombination does not occur where there is an A genotype and then one waits for B to show up on the same chromosome by random mutation. That would take perhaps thousands or even millions of generations before such a thing happens at random. Recombination is a powerful accelerator of evolution.

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thank you. As I understand your answer, you explain the advantage of recombination just as it has been considered by Muller. But if I am not mistaken you don't address the question of why the additive genetic variance (you actually don't use the words "variance") increases thanks to recombination (or thanks to the combined effects of recombination and selection). Can you try to further elaborate your answer so that it explains why recombination causes an increase in additive genetic variance. –  Remi.b Feb 21 at 19:48
    
I don't think this response addresses the quote since this concerns crossing over. The quote says "bringing together favourable alleles from different chromosomes", which doesn't involve crossing over. –  mgkrebbs Feb 21 at 20:38
    
"In meiosis and mitosis, recombination occurs between similar molecules (homologs) of DNA. In meiosis, non-sister homologous chromosomes pair with each other so that recombination characteristically occurs between non-sister homologues. In both meiotic and mitotic cells, recombination between homologous chromosomes is a common mechanism used in DNA repair." en.wikipedia.org/wiki/Genetic_recombination –  shigeta Feb 21 at 21:41
    
@sigeta I agree with you shigeta. But I don't think your answer answers my question. I understand that if a highly beneficial mutation occurs before another beneficial allele on the same chromosome had time to reach fixation, then these two mutations can only be found in the same individual if recombination occurs. This concept is also conveyed by Gould when saying that "selection creates the fit". But I still don't understand why the additive genetic variance for fitness increases as a result of recombination. –  Remi.b Feb 23 at 10:38
    
Its tough to write answers here - remember that responders want to explain terms to other readers as well as the poster or they get complaints. I thought I'd answered your question though. Beneficial recombinations are among offspring; the vast majority of recombinations are probably less fit or have no substantial effect on fitness. Over time selection brings those combinations into common occurrence in the gene pool. I didn't explain how selection works... maybe i need to define that in detail too? –  shigeta Feb 23 at 13:44
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