Dissertation Research: Mate Choice and Stable Polymorphism in the White-throated Sparrow

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Most plants and animals reproduce sexually. Therefore, understanding the causes and consequences of mate choice will increase our general understanding of nature. White-throated sparrows (Zonotrichia albicollis) offer a rare opportunity to study the interaction of mate choice, natural selection, and a population's genetic diversity. White-throated sparrows of both sexes occur in two distinct forms, white-striped and tan-striped. White-striped birds carry a normal and an "inverted" second chromosome; tan-striped birds have two copies of the normal chromosome. tan-striped males are more aggressive than tan- striped, and so may mate more often. Most individuals pair with the opposite form during the breeding season; over 90 percent of territorial pairs are composed of one white and one tan individual. The project asks how natural selection has influenced these preferences. Matings between two white-striped birds can produce offspring with two "inverted" second chromosomes; this is probably lethal to the offspring. Matings between two tan-striped individuals cannot produce any white-striped offspring, and so lose the potential reproductive advantage obtained through white-striped sons. Essentially, one copy of the "inverted" chromosome is apparently beneficial, but two copies is lethal. Given this hypothesis, selection should favor mating with the opposite form. Hence genetic, morphological and behavioral diversity in this species my be maintained as a consequence of selection on mate choice.