Evolution and Sex

There are over 20 theories on sex⁶ including the Red Queen hypothesis, Muller's Ratchet, and the deleterious mutation hypothesis.

The deleterious mutation hypothesis and the Red Queen hypothesis dominate the debate over why sex persists³. The deleterious mutation hypothesis was briefly discussed under Asexual Disadvantages.

The Red Queen hypothesis proposes that species locally adapted are in an "arms race" with each other. Although it applies to many different interactions, the Red Queen hypothesis will be explained here in the context of host/parasite interactions. A few assumptions:

• Parasite generation times are shorter than host generation times.
• Usually focused on binding proteins.
• Host susceptibility and parasite infectivity polymorphisms are genetically correlated⁷.

Sexual reproduction introduces recombination as a method to increase variability. Both sexual and asexual populations may have locally adapted resistance alleles against a particular parasite's infective alleles, but sexual populations retain unfavored genes for future use. Parasites have "keys" for particular host "locks."

• The most common host phenotype should be disproportionately infected by parasites because the parasites are locally adapted to them.
• Heterozygotes retain binding sites that are not adapted for by the current parasite population.
• Highly inbred sexual host populations and asexual population lose heterozygosity and polymorphisms.

The mutation rate of the parasites and hosts, and the virulence of the parasites determine if the asexual advantages outweigh the sexual ones.

In this example, the asexual advantages easily overcome the effects of parasitism except in extreme situations⁸.

Asexual populations always win when disease or parasites are not a great issue. Computer models⁸ show that when disease or parasitism become more prevalent, sexual population often win.

The Red Queen hypothesis takes its name from Lewis Carrol's Through the Looking Glass. Alice runs as fast as she can after the Red Queen, but neither of them moves. This perplexes Alice, and the Red Queen responds, "Now, here, you see, it takes all the running you can do to keep in the same place"⁹. The host-parasite interaction is in the same fashion. Although they may both be perfectly, or near perfectly adapted, they cannot stop evolving. Mutation assures that the parasite will eventually be able to attack the host, so the host must continually evolve new means of resistance¹⁰.

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