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Therefore, mate choice—with respect to the MHC—has probably evolved so that females choose males either based on diverse genes (heterozygote advantage and inbreeding avoidance hypotheses) or "good" genes.
The fact that females choose is naturally selected, as it would be an advantageous trait for females to be able to choose a male that provided either an indirect or direct benefit.
Depending on how parasites alter selection on MHC alleles, MHC-dependent mate-choice may increase the fitness of the offspring by enhancing its immunity, as mentioned earlier.
If this is the case, either through the heterozygote advantage hypothesis or the Red Queen hypothesis, then selection also favors mating practices that are MHC-dependent.
However, if this particular allele becomes common, selection pressure on parasites to avoid recognition by this common allele increases.
An advantageous characteristic that allows a parasite to escape recognition spreads, and causes selection against what was formerly a resistant allele.
Furthermore, one would presume that said difference in genes would impart a difference in fitness as well, which could potentially be chosen or selected for.
Pheromones function to communicate one's species, sex, and perhaps most importantly one's genetic identity.
Mating with relatives, or inbreeding, increases the amount of overall homozygosity—not just locally in the MHC.
An increase in genetic homozygosity may be accompanied not only by the expression of recessive diseases and mutations, but by the loss of any potential heterozygote advantage as well.
This enables the parasite to escape this cycle of frequency-dependent selection, and such a cycle eventually leads to a co-evolutionary arms race that may support the maintenance of MHC diversity.
The inbreeding avoidance hypothesis has less to do with host-parasite relationships than does the heterozygote advantage hypothesis or the Red Queen hypothesis.
One is that there is selection for the maintenance of a highly diverse set of MHC genes if MHC heterozygotes are more resistant to parasites than homozygotes—this is called heterozygote advantage.