Blackwell Publishing

Multi-locus population genetics - Why does recombination exist?


Why doesn't selection reduce the recombination rate to zero?

The vague answer is probably that recombination is favored because of environmental change.

Recombination, we saw, is only disadvantageous when it disrupts coadapted sets of genes in linkage disequilibrium. Its disadvantage will depend on how much of this sort of linkage disequilibrium exists in nature; if the amount is low, the disadvantage will be small. As we have seen the data suggests quite low amounts of linkage disequilibrium. Coadapted gene complexes, in the strong sense of haplotypes in greater than random frequencies, may be exceptional in nature. To this extent, the problem of recombination is reduced.

But what positive advantage might recombination provide?

Recombination could be favored if disadvantageous gene associations should exist in disproportionately high frequency. Recombination is selected against when it breaks up favorable gene combinations: it is selected for when it breaks up unfavorable gene combinations.

Hitch-hiking is a case in point. When a selected polymorphic locus interferes with selection at a linked locus, recombination between the loci is advantageous. The argument is fine in itself, but it may be a special circumstance. The process would be commoner in a rapidly changing environment, because the fitnesses of different alleles would then be changing and selection attempting to adjust the frequencies of genes at many, potentially interfering, loci.

The existence of recombination is closely related to the problem of sex. Here the main point is that recombination is a puzzle: it can have clear disadvantages, and it does not exist simply because natural selection cannot remove it.

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