Natural selection, in its most basic form, is described as a process in which traitsincrease or decrease in frequency depending on their fitness, and only the trait withthe highest fitness will remain in the population. Yet, populations rarely have asingle `optimal' trait. The way natural selection maintains this observed variationwithin populations has been a keen focus of evolutionary biologists. In the followingchapters, I focus on how natural selection maintains a form of phenotypicvariation referred to as `partial migration'. Partial migration is the coexistence ofmigratory and non-migratory phenotypes, and is found in a wide variety of taxa. Ifound that some, but not all forms of density-dependent competition can lead to theevolution and maintenance of partial migration (i.e., partial migration as an evolutionarilystable strategy (ESS) and convergent stable strategy (CSS)). Whetherdensity-dependent competition allows for partial migration as an ESS and a CSSdepends on how it influences the relative fitnesses of the phenotypes. If competitionchanges the relative fitnesses in opposing directions, then it will allow forpartial migration. If it affects the relative fitnesses in the same direction, it willnot. I then apply these results to a fish species of conservation and commercialconcern: Oncorhynchus mykiss, or steelhead and rainbow trout. I demonstratehow female steelhead and rainbow trout competing separately for spawning habitatcan still be subject to frequency-dependent selection and how this allows forpartial migration. The frequency-dependent selection also results in strong feedbacksbetween survival and reproduction, which produces a non-linear response inthe migration propensity ESS and CSS. In practical terms, this means that conservationor management actions may not affect the population as expected, andmeasuring the propensity for migration in wild populations is notoriously difficult.To address this difficulty, I develop a method to measure the propensity for migrationin wild populations that can be used to test the predicts I generated in thetwo previous chapters. The method is called sex-ratio balancing and it relies on afundamental relationship between sex ratios and the propensity for migration. Sexratios are much easier to measure than the propensity for migration and the easeof measurement makes this method valuable for studying many different partiallymigratory taxa.
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