Sexual imprinting promotes assortative mating, and may thereby restrict gene flow between populations. It may thus be instrumental in the establishment and maintenance of premating isolation between species or populations, which may result in speciation. If two populations diverge in some mate recognition trait, so will the populations' preferences for that trait. The tight link between the appearance of the trait and the preference for the trait may ensure premating isolation if the trait values of the two populations do not overlap.
It has also been suggested that sexual imprinting may be a driving force in the evolution ofexaggerated traits by sexual selection. However, a preference in offspring for individuals that look exactly like their parents will not lead to directional selection for exaggeration of ornaments. Furthermore, frequency-dependent selection resulting from assortative mating will select against novel mates. How, then, may imprinted mate preferences drive sexual selection? To balance the costs and benefits along the inbreeding-outbreeding continuum, it may be advantageous to choose a mate who looks rather similar, but not identical, to one's own parents. In addition to facilitating inbreeding avoidance, this asymmetric mate preference may drive sexual selection, since deviations from the population mean are favored. Sexual imprinting may hence produce preferences for novel modifications of secondary sexual characters, and this process may in turn drive speciation.
Preferences for traits that deviate substantially from the population mean have been shown experimentally in several species. Moreover, theoretical models confirm that sexual imprinting and an asymmetric mate preference can lead to sexual selection. The exact mechanism for the development of an asymmetric mate preference has not been firmly established, but it could be a product of sexual imprinting itself, or sexual imprinting combined with a perceptual bias. Recent empirical evidence suggests that skewed mating preferences may be a result of a by-product of the learning process, called peak shift. If two stimuli are similar in appearance, and are differently reinforced in the responding individual, this process shifts the peak in response to increase the contrast between the stimuli. Zebra finches show natural sexual dimorphism in beak coloration, and it has been shown that males prefer females with a beak of a more extreme color than that of their social mothers. The preference is in the opposite direction of the paternal beak color. It thus seems like sexual imprinting through peak shift can generate skewed sexual preferences for exaggerated phenotypes that have not been present at the time of learning.
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