Human Intercourse (Illustrated)

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In this way, well-adapted genotypes are not broken apart by recombination, but poorly adapted genotypes can be recombined to create new combinations in offspring. Models that account for the fact that population sizes are finite have found that sex and recombination evolve much more readily. With a limited number of individuals in a population, selection erodes easily accessible variation, leaving only hidden variation Figure 2.

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Recombination can then reveal this hidden variation, improving the response to selection. By improving the response to selection, genes that increase the frequency of sex become associated with fitter genotypes, which rise in frequency alongside them. Interestingly, the requirement that fitness surfaces exhibit weak and negative curvature is relaxed in populations of finite size; here, fitness surfaces may be uncurved or positively curved and still favor sex.

This diagram depicts a population consisting of 14 haploid individuals who carry plus or minus alleles at each of four sites in their genome left panel. In a new environment favoring the plus alleles, selection will, over time, increase the frequency of the plus alleles throughout the genome right panel.

For example, in a hotter climate, alleles conferring tolerance to higher temperatures would rise in frequency. Selection favors the good gene combinations here, the ones containing two plus alleles and eliminates the bad gene combinations. In the absence of sex, the only variation that remains after several rounds of selection is hidden in the sense that plus alleles at the first site are found with minus alleles at the second site or vice versa.

This problem is irrelevant in an infinitely large population, because mutation will immediately create beneficial combinations e.

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Two populations are represented as black circles with fourteen line segments, each composed of four black plusses or minuses. The population at left, representing the Initial population, contains two line segments with two plus signs, seven line segments with one plus sign, and five line segments with zero plus signs.

Arrows point to another population at right.. This resulting population also contains fourteen line segments, each containing two plus signs and two minus signs. Eight of the line segments contain a minus sign, two plus signs, then one minus sign, whereas six of the line segments contain alternating plus and minus signs.

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This last result is particularly interesting, because it suggests that August Weismann might have been right all along in arguing that sex evolved to generate variation. Modeling Weismann's hypothesis with infinitely large populations failed because variation is too easily generated by mutation and too easily maintained by selection within these populations.

Altering this size-related assumption by modeling selection among a finite number of individuals reveals just how important sex and recombination are as processes that allow genes residing in different individuals to be brought together, thereby producing new genotypic combinations upon which selection can act.

De Visser, J. The evolution of sex: Empirical insights into the roles of epistasis and drift. Nature Reviews Genetics 8 , — doi Felsenstein, J. The evolutionary advantage of recombination. Genetics 78 , — Otto, S. Resolving the paradox of sex and recombination.

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Nature Reviews Genetics 3 , — link to article. Origins of New Genes and Pseudogenes. Evolutionary Adaptation in the Human Lineage. Genetic Mutation.

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Negative Selection. Sexual Reproduction and the Evolution of Sex. Haldane's Rule: the Heterogametic Sex. Hybrid Incompatibility and Speciation. Hybridization and Gene Flow. Why Should We Care about Species? What, then, are the true costs and benefits of sex? Aa Aa Aa. The Importance of Sexual Reproduction. Indeed, theoretical models developed in the s and s demonstrate that genes promoting sex and recombination increase in frequency only when all of the following conditions hold true: The population is under directional selection.

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This means that increased variation can improve the response to selection. Fitness surfaces are negatively curved. This means that sex and recombination can restore variation eliminated by past selection. The surface curvature is not too strong. If too strong, the recombination load is severe. Genetics 78 , — Otto, S.

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