By allowing individuals to behave adaptively given their particular phenotype, learning can permit a broader range of phenotypes and underlying genotypes to coexist in a population. For example, in one parasitoid wasp species, individuals of different size learn to use host species that they can handle most efficiently, generating a correlation between wasp size and host preference. Wasps of all sizes may be more or less equally successful, permitting a broader range of phenotypes to coexist in the population. In time, the different phenotypes may experience selection on whatever traits promote fitness in their different host niches. The end result could be greater genetic differentiation within the wasp population than would occur in the absence of learning.
Learning can also promote genetic differentiation by restricting gene flow between populations or subpopulations. In the apple maggot fly, conditioning has been suggested to facilitate the formation of host races in sym-patry on two alternative host fruit species, apple and hawthorn. Conditioning may facilitate host race formation in at least two ways: first, by biasing allocation of eggs by a female to its natal host fruit species, and second, by suppressing dispersal by adults from the natal host tree. Both effects can be considered examples of 'natal habitat preference induction' and both may reduce gene flow between the two host types. A similar process may occur in the indigobird, an avian brood parasite. In indigobirds, females exclusively lay eggs in the nests of the host bird species they were reared by as nestlings. In addition, they learn to prefer the host species' male call. Because male indigobirds mimic the songs of their host species, matings are most likely to occur between birds that were reared by, and thus adapted to, a particular host. Such assortative mating can promote sympatric differentiation and even lead to the formation of host races.
Analogous effects of learning on diversification may occur at higher taxonomic levels. For example, filial imprinting in birds has been proposed to promote specia-tion in birds. Avian learning has been linked to diversification: rates of diversification of species lineages are correlated positively with that lineages' rates of innovation, which is mediated in part by learning. Moreover, in birds, innovative behavior can spread particularly rapidly through social learning. Interspecific correlations between social learning and innovation frequency suggest that learning interacts synergistically with innovation to promote diversification.
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