Theoretical Studies in Ecological and Population Genetics

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Asmussen 980462 A high degree of genetic variation is a striking and universal feature of natural populations. A hint of this variability is easily seen in the world around us. For instance, there is a wide diversity in such characters as flower color in plants, wing patterns in butterflies, and disease susceptibility in humans and other organisms, all of which have a genetic basis. The full extent of genetic variation is found at the molecular level, where laboratory analyses of gene products and DNA sequences reveal that a single gene may have dozens or even a hundred or more different forms in a population. This great genetic diversity is of vital evolutionary importance because it determines the ability to adapt to changing environmental conditions, which is critical to the long-term survival of species. Variation in flower color, for example, can allow plants to attract new insect pollinators, thereby ensuring successful reproduction, while variation in butterfly wing patterns can reduce detection and predation by visual predators such as birds, which tend to focus on a limited set of search images. In the same way, variation in immune-related genes can allow a population to survive an epidemic induced by a new pathogen. Because genetic diversity plays such a critical role, a major goal of evolutionary genetics is to identify and understand the forces that account for the vast genetic variability in natural populations. The proposed research will provide a new approach to this fundamental question by comprehensively evaluating the potential for permanent genetic variation when the rate of survival and reproduction of individuals changes through time as a function of the changing genetic composition of the population. Such frequency-dependent selection is ecologically important and thought to facilitate the maintenance of genetic diversity in a wide range of organisms. To date, however, little is known about the true potential of this widespread form of selection to maintain th e high genetic variation found in natural populations. The present proposal will initiate a long-term, theoretical investigation of this key issue, based on the development and extensive analysis of a series of novel mathematical models and computer simulations. A detailed characterization will be made of the patterns and levels of genetic diversity, both under frequency-dependent selection alone and in the biologically realistic scenario in which new forms of genes constantly arise in the population. The final, critical step will be to compare the resulting behavior to observed data. Together, the results will provide a much-needed evaluation of the extent to which frequency-dependent selection can explain the high levels of genetic variation found in nature.