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Rare Genotype Advantage

hypothesis genotypes common variation

The rare genotype advantage hypothesis asserts that genotypes (the set of genes (alleles) carried by an organism) that have been rare in the recent past should have particular advantages over common genotypes under certain new and challenging environmental conditions.

Rare genotype advantage can be best illustrated by a host-parasite interaction. Successful parasites are those carrying genotypes that allow them to infect the most common host genotype in a population. Thus, hosts with rare genotypes, those that do not allow for infection by the pathogen, have an advantage because they are less likely to become infected by the common-host pathogen genotypes. This advantage is often temporary (transient) and lasts for only a few generations as the once rare genotype increases in the population along with the numbers of pathogens that infect the initially rare host genotype. The pattern then repeats.

The rare genotype advantage hypothesis is similar to the so-called Red Queen hypothesis first suggested in 1982 by evolutionary biologist Graham Bell (1949–) (so named after the Red Queen's remark to Alice in Lewis Carroll's Through the Looking Glass: "Now here, you see, you have to run as fast as you can to stay in the same place."). In other words, genetic variation represents an opportunity for hosts to produce offspring to which pathogens are not adapted. Then, sex, mutation, and genetic recombination provide a moving target for the evolution of virulence by pathogens. Thus, hosts continually change to stay one step ahead of their pathogens.

Bell's hypothesis and subsequent mathematical and experimental refinements now allow scientists to better identify and characterize rare genotypes. By 2002, the rare genotype advantage hypothesis and other theories of variation and diversity became essential to concepts involving a Darwinism of disease (a developing hypothesis that explains many facets of the disease process in evolutionary terms).

Another example of rare genotype advantage can be observed in the increasing use of antibiotics on bacterial populations. Bacterial genomes harbor genes giving resistance to particular antibiotics. Bacterial populations tend to maintain a high level of variation of these genes, even when they seem to offer no particular advantage. The variation becomes critical, however, when the bacteria are first exposed to an antibiotic. Under those conditions, the high amount of variation increases the likelihood that there will be one rare genotype that will confer resistance to the new antibiotic. That rare genotype then offers a great advantage to those individuals. As a result, the bacteria with the rare genotype will survive and reproduce, and their genotype will become more common and, perhaps, ultimately the most common genotype.

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