Coevolution is reciprocal evolutionary change in two or more species resulting from the interaction among them. Species also display many adaptations to interspecific interactions that appear one-sided rather than reciprocal.
The phylogenies of certain symbionts and parasites are congruent with the phylogenies of their hosts. This may imply that they di-verged in parallel because the symbionts did not disperse between different host lineages. It does not necessarily imply that they co-evolved, in the sense of reciprocal adaptation to each other.
The Red Queen hypothesis states that species may continue to evolve indefinitely because of changes in interacting species. For ex-ample, coevolution in predator-prey and parasite-host interactions can theoretically result in a stable genetic equilibrium under some conditions, but often involves an ongoing evolutionary arms race, indefinite fluctuations in genetic composition, or even extinction. Among the many interesting adaptations in predator-prey interactions are aposematism (warning coloration) and mimicry.
Parasites (including pathogenic microorganisms) may evolve to be more or less virulent depending on the correlation between viru-lence and the parasite’s reproductive rate, the parasite’s mode of transmission between hosts (vertical versus horizontal), infection of hosts by single versus multiple parasite genotypes, and group selection. Parasites do not necessarily evolve to be benign. New pathogens sometimes emerge by evolutionary change that enables them to infect new hosts (e.g., humans).
In mutualism, each species obtains some benefit from the other. This does not entail altruism, and it often involves some conflict. Se-lection favors genotypes that provide benefits to another species if this action yields benefits to the individual in return. Thus the conditions that favor low virulence in parasites, such as vertical transmission, can also favor the evolution of mutualisms. Mutual-isms may be unstable if “cheating” is advantageous, or stable if it is individually advantageous for each partner to provide a benefit to the other.
Evolutionary responses to competition among species may lead to divergence in resource use and sometimes in morphology (character displacement). Thus, competition is a cause of ecological diversification. However, selection for greater ability to compete can also result in greater aggression, and competitive exclusion of less competitive species.
Both ongoing evolution and phylogenetic legacies can influence which species coexist in local ecological communities. Phylogenet-ically conservative characters may be subject to environmental filtering, so that the species in a habitat are phylogenetically clus-tered; conversely, very closely related species tend to be spatially separated. Because evolutionary history determines the features of species that affect their interactions, it helps explain the networks of interactions among species in a community.