Exploring the Opportunities for Divergence in Heterogeneous Environmnets in the Tropical Andes.

Abstract

The high biological diversity associated with environmentally heterogeneous regions reflects the complex interactions between multiple evolutionary and ecological processes. Disentangling this interaction is crucial for understanding a wide variety of fundamental biological questions including the differential accumulation of species across the globe, the influence of geographic context on diversification processes, and the susceptibility of species to environmental change. In this dissertation, I integrate theoretical expectations with comprehensive analyses of species, genetic, and phenotypic variation across the hyper-diverse tropical Andes to help elucidate the role environmental heterogeneity plays in the generation and maintenance of diversity.

First, I explore the association between diversity and spatial and temporal heterogeneity. I show that environmental heterogeneity is a strong predictor of both species and phylogenetic diversity across the entire study region despite significant environmental differences among regions. My results indicate that this association is unlikely to be simply explained by passive accumulation of species or spatial autocorrelation. Instead, active differential diversification across zones may explain this pervasive association.

Second, I investigate if environmentally driven isolation and adaptive divergence drives population differentiation in this system using the soft-grass mouse as a model. Specifically, I test whether local adaptation is prevalent in this species. However, rather than providing strong support for local adaptation, my findings indicate environmental heterogeneity promotes neutral and phenotypic differentiation in this species through genetic drift facilitated by environmental isolation.

Third, I explore the extent of climatic niche differentiation in the soft-grass mouse to assess the role of environmental heterogeneity in promoting ecological specialization. Combining genetic and ecological analyses, I provide evidence of restricted differentiation of the climatic preferences of populations of this species. I show that in spite of marked genetic and geographic structure, this species maintains a common set of climatic tolerances. Initial exploration of plausible causes for this finding hints at selection for ecological broadness, reinforced by frequent range shifts.

Together, this dissertation offers a multi-faceted evaluation of the evolutionary consequences of inhabiting highly heterogeneous environments. Its findings demonstrate the significant role that geographic context plays in structuring diversity, bringing attention to the importance of system-specific characteristics in modulating these effects.