become extinct. The organisms can survive if their numbers increase so the relationship can be sustained. Humans also have mutualistic relationships. The flora in the gut is necessary for efficient digestion of nutrients. Head lice in humans reduces the threat of lethal diseases from body lice. Domesticated animals and plants rely on humans for propagation and humans use these organisms for food. Bean plants grow on corn stalks and provide nitrogen for the soil plants. Mutualism is not a static event and can shift by evolutionary processes. The relationship can switch to parasitism; one partner can become autonomous; one partner may become extinct, which can lead to the extinction of the other; and one partner will switch to have a different mutualistic species partner.
HOST-PARASITE COEVOLUTION Host-parasite relationships, as we discussed, also involve coevolution. There is reciprocal selective pressure that helps to maintain the relationship and allows for evolution to continue. There are several reasons for these relationships. One thing you need to know is that increased virulence of the parasite can be detrimental to the parasite because the parasite might kill off the host too quickly, practically ensuring that the host cannot help to transmit the organism to others. Because of reciprocal selective pressures, there can be rapid adaptation of both species. Based on the Red Queen Hypothesis, each species must evolve and adapt in response to the adaptations of the opposite species. With the host-parasite relationship, there are always changes in allele frequencies in the environment. The parasite must adapt to the most common genotype of the host so that it can affect the largest number of hosts. Rare host genotypes are then selected for so it becomes more common. When this happens, the parasite adapts to this new, more common genotype. This can happen quickly over just a few generations and favors uncommon alleles. Overdominance can happen if the heterozygote is more advantageous than the homozygotes. This happens with sickle cell anemia. The homozygotes for this disease have an increased risk of death, while the homozygous normal person has an increased risk of getting malaria. This favors the dominance of the heterozygote, called heterozygote advantage, in areas where there is malaria.
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