Polymorphisms are a frequent and fascinating form of biodiversity. They occur when discrete types of individuals (“morphs”) persist in a population at frequencies above the mutation rate. Girardinus metallicus, a member of the live-bearing fish family Poeciliidae, exhibits a male-specific color polymorphism, in which males occur in multiple distinct morphs: a common “plain morph” with drab coloration, a rare “black morph” with black ventral coloration, and an extremely rare “yellow morph” with yellow ventral coloration. This polymorphism also extends to mating behavior, such that black morph males perform a display prior to copulation whereas plain and yellow morph males almost exclusively sneak copulations. Polymorphism persistence requires some form of evolutionary explanation. A powerful hypothesis to explain polymorphism is negative frequency dependent sexual selection (NFDSS), under which rare males have a mating advantage of some kind due to their rarity. Another frequently invoked hypothesis for polymorphism maintenance is opposing selection pressures, under which opposing sexual and survival selection each favor a different morph, so that both are maintained in the population. A previous study has shown that female G. metallicus fish do not favor black morph males; however, this hypothesis may be rescued if the black morph display is actually an aggressive signal to other males, allowing black morph males to out-compete plain morph males and gain a mating advantage. We used our Baker and Koob endowment funds to test both of these hypotheses in a laboratory setting and present our preliminary results at a conference in January. These two experiments (discussed in more detail below) will constitute the bulk of my Masters thesis at Cal Poly, and would not have been possible without the Baker and Koob endowment.
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