is characterized by reciprocal genetic change and thus changes in allele frequencies within populations. These changes may be determined by three main types of selection dynamics. Negative frequency dependent selection An allele is subject to negative frequency dependent selection if a rare allelic variant has a selective advantage. For example, the parasite should adapt to the most common host genotype, because it can then infect a large number of hosts. In turn, a rare host genotype may then be favored by selection, its frequency will increase and eventually it becomes common. Subsequently, the parasite should adapt to the former infrequent genotype. Coevolution determined by negative frequency dependent selection is rapid, potentially occurring across few generations. It maintains high genetic diversity by favoring uncommon alleles. This selection mode is expected for multicellular hosts, because adaptations can occur without the need for novel advantageous mutations, which are less likely to be frequent in these hosts because of relatively small population sizes and relatively long generation times. Overdominant selection Overdominance occurs if the heterozygote phenotype has a fitness advantage over both homozygotes (heterozygote advantage, causing heterosis). One example is sickle cell anemia. It is due to a mutation in the hemoglobin gene leading to sickle shape formation of red blood cells, causing clotting in blood vessels, restricted blood flow, and reduced oxygen transport. At the same time, the mutation confers resistance to malaria, caused by Plasmodium parasites, which are passed off in red blood cells after transmission to humans by mosquitoes. Hence, homozygote and heterozygote genotypes for the sickle-cell disease allele show malaria resistance, while the homozygote suffers from severe disease phenotype. The alternative homozygote, which does not carry the sickle cell disease allele, is susceptible to infection by Plasmodium. As a consequence, the heterozygote genotype is selectively favored in areas with a high incidence of malaria. Directional selection If an allele provides a fitness benefit, its frequency increases within a population – selection is directional or positive. A selective sweep is a form of directional selection where the increase in frequency eventually leads to the fixation of the advantageous allele and other alleles near it. The process is considered to be slower than negative frequency dependent selection. It may produce an "arms race", consisting of the repeated origin and fixation of new virulence traits in the parasite and new defence traits in the host. This mode of selection is likely to occur