EEB Student Dissertation Defense - Comparing micro- and macroevolutionary diversification dynamics between Neotropical montane and lowland birds

Abstract:
The tropical Andes mountains and adjacent Amazonian lowlands are home to more species than anywhere else in the world. Prior work suggests that avian species richness declines but speciation rates increase as elevation increases in this biodiversity hotspot. Tropical mountains are also characterized by steep elevational turnover in environmental conditions, including the availability of oxygen. My thesis compares diversification and selection dynamics between lowland Amazonian and Andean montane species, with a special emphasis on tanagers, an iconic Neotropical radiation and the most speciose family of songbirds. I focus on how patterns and processes of the early stages of speciation vary across elevation in the Andes-Amazonia system, and on the molecular evolutionary consequences of hypoxic stress at high altitudes.
Whether large macroevolutionary biodiversity gradients emerge from underlying microevolutionary processes is an active area of research in evolutionary biology. For example, elevational gradients in speciation rates could be caused by geographic variation in the tendency for populations to become isolated and diverged from one another. A major focus of my dissertation is evaluating whether Neotropical montane and lowland regions differentially promote incipient speciation, linking microevolutionary processes to broader biodiversity patterns. First (Chapter 2), I aggregated previously published mitochondrial phylogeographic datasets from birds in the Andes-Amazonia system (~7,000 sequences from 103 species), to test whether levels of intraspecific population structure vary across elevation and whether rates of population differentiation predict speciation rates. My results revealed that phylogeographic structuring is higher in montane birds and increases with elevation, but rates of population differentiation did not predict speciation rates in the focal set of taxa.
Next (Chapter 3), I more rigorously explored how Andean and Amazonian landscapes promote population isolation and differentiation using whole-genome comparative phylogeography in 8 species of Tangara tanagers (4 Andean and 4 Amazonian). My results show that Andean species consist of more differentiated, less connected, smaller, and less genetically diverse populations than Amazonian species. This supports a scenario of greater incipient speciation in the mountains, but these same characteristics also reduce persistence in a theoretical metapopulation framework. In conjunction with results from my previous chapter, I suggest that there exists a tension between factors promoting divergence versus persistence, which could contribute to observed elevational biodiversity gradients in Neotropical birds.
Finally, elevational gradients are also marked by increasing hypoxia at higher altitudes, an important physiological stress owing to the essential role that oxygen plays in cellular energy production. In Chapter 4, I use comparative genomic methods across 20 species of Neotropical tanagers to test whether signatures of selection in oxidative phosphorylation genes vary with elevation. I find evidence of stronger purifying selection on components of this metabolic pathway in species with higher elevational distributions, expanding our understanding of how tropical montane species cope with reduced oxygen availability.