UCLA

Phenotypic variation may arise due to the action of natural selection or the plastic response of individuals to environmental changes. In addition, stochastic processes also play an important role in shaping the evolutionary history of species and populations. Both high phenotypic and genetic variation occur in areas with high climatic diversity or with abundant geographic barriers, which usually are biologically diverse. The Neotropical and Neartic biogeographical realms in Central and North America are among the regions with the highest biodiversity in the world. However, very limited genetic studies have been performed in wild populations of mammals to study the evolutionary process shaping their diversity. In this dissertation we explore the phenotypic diversity and gene expression pattern in the Virginia opossum (Didelphis virginiana) and the phylogeographic strucutre in the white-nosed coati (Nasua narica), two mammal species inhabiting Central and North America. In chapter 1 we determined the association of phenotypic variables for body size, extremity size, skin and coat coloration, and environmental variables using a random forest algorithm. This analysis show the phenotypic geographic variation in opossums follows three ecogeographic rules and may be driven by natural selection in response to selective pressures to adapt to seasonal and colder environments in North America. In chapter 2 we focus on the variation in skin pigmentation. We use a gene coexpression approach to analyze the skin gene expression differences between tropical and temperate opossum populations, which indicates pigmentation differences arise from expression changes in melanocytic and immune related genes. These findings suggest two possible explanations: 1) an adaptive explanation that there is a trade-off between high production of eumelanin in tropical populations for protection against pathogens and allocation of energy towards growth and energy storage, which are probably more immediate and greater selective pressures to survive in temperate environments than pathogen resistance; 2) a plastic explanation that depigmentation is the result of an autoimmune response against melanocytes triggered by low temperatures in northern latitudes. Finally, in chapter 3 we analyzed several nuclear and mitochondrial loci in the white-nosed coati under a phylogeographic framework. We found strong genetic structure across the distribution range of the species. The phylogenetic, population structure and migration rate analyses indicate the Panama population is the deepest split, which diverged 4 million years ago, and the migration of the species occurred mainly northwards into North America. These results challenge previous hypotheses about the time of emergence of the Isthmus of Panama and the origin and diversification of extant procyonid species. This dissertation contributes to better understand the evolutionary processes that have shaped the phenotypic and genetic diversity of mammal species in Central and North America.