The Drivers of Microevolutionary Divergence in Avian Systems

Abstract

The divergence of conspecific populations is a central process in evolutionary ecology, with relevance to speciation, range boundaries, and local adaptation. Advances in our understanding of avian ecology and understanding at the species level have led to substantial insights into the constraints and drivers of macroevolutionary diversification in birds, but comparative trait-based study of diversification within species remains rare. In this dissertation I use population genomics and phylogenetic comparative methods alongside measurements of phenotypic and niche variation to examine population divergence in three avian systems. In Chapter 2, I examine the species-level drivers of gene flow in 44 passerine species along the elevational gradient of the Ecuadorian Andes. I found that detectable genomic divergence with elevation was widespread, and was stronger in taxa that lived at high elevations and had lower dispersal capabilities. I also found evidence of morphological differentiation that was largely decoupled from genomic differentiation. These results highlight the potential for multidimensional environmental gradients to drive microevolutionary diversity. In Chapter 3, I explore comparative patterns of population structuring across fragmented habitat at the southern edge of the Chocó biodiversity hotspot. I found that the 15 forest-dependent species I studied all showed some evidence of population structure across regions of unsuitable habitat. I found that population differentiation was stronger in large understory insectivores, primarily antbirds (Thamnophilidae). I found strong evidence for isolation-by-distance, but little evidence for isolation-by-resistance: associations between population differentiation and traits were generally weak. In Chapter 4, I conduct a case study of population differentiation in a putative recent range expansion in the barred owl (Strix varia) across the continental United States from an ancestral range in the eastern half of the continent. I found strong population differentiation between individuals on the west coast and in the rest of the range, suggesting a considerably older origin for western populations than previously thought. This result was borne out by demographic history analyses. Taken in concert, my thesis chapters showcase the power of comparative population genomics, highlighting the potential for environmental heterogeneity to drive population differentiation. This work simultaneously showcases the ways in which a complex evolutionary history can create idiosyncratic outcomes, reinforcing the need for taxonomic breadth and diversity in trait-space for comparative studies