In hostparasite associations, there is often a geneforgene mechanism of infection. What th

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In host-parasite associations, there is often a gene-for-gene mechanism of infection. What this means is that certain parasite genes must "match" certain host genes for the parasite to be able to infect: Gene 1 Gene 2 parasite A B host A b ^---match ^---No match. No infection parasite a B host A b ^-No match ^-No match. No infection parasite A b host A b ^---match ^---match. Infection Essentially, the fittest parasites in a population are those which can match the genotype which is at the highest frequency in the population. The fittest hosts, however, are those which are matched by the lowest- frequency parasite genotype. Since parasites generally have shorter generation times than their hosts, parasite genotype frequencies tend to "track" host genotype frequencies; they are able to evolve so that their most common genotype matches the host's most common genotype. This has the seemingly-paradoxical result that THE HOST GENOTYPE WITH THE HIGHEST FITNESS IS AT THE LOWEST FREQUENCY IN THE POPULATION. Of course, that genotype therefore increases in frequency until it is at a higher frequency. However, since the parasites are evolving faster, almost as soon as that host genotype has reached high frequency, THAT GENOTYPE BECOMES THE LOWEST-FITNESS GENOTYPE. This leads to really neat genotypic cycling, where host genotypes bounce from common/low fitness to rare/high fitness, and parasite genotypes follow them. This cycling is the basis for the Red Queen theory for the maintenance of sexual reproduction.

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