Doctor of Philosophy (PhD)


Biological Sciences

Document Type



Three fundamental processes are thought to underlie biodiversity patterns: speciation, dispersal, and extinction. Biodiversity patterns in species-rich tropical mountains vary along horizontal (barriers), and vertical (elevation) dimensions. In this dissertation, I set out to explore the interplay between historical and ecological mechanisms hypothesized to shape diversification of Neotropical cloud forest birds. In the second chapter, I conducted comparative analyses using species-specific ecological traits, and 3,175 mtDNA sequences collected across the range of 45 co-distributed birds. First, I found that niche breadth, dispersal ability, and clade age influence lineage diversity (population differentiation). In particular, species that are old, poor dispersers, and specialized to narrow elevational zones, accumulate greater lineage diversity with time. These results are consistent with predictions of the time-for-speciation effect, Janzen’s climatic zonation model, and niche conservatism. Second, I found weak support for simultaneous divergence between multiple population-pairs separated by the same barriers, suggesting temporal idiosyncrasies in differentiation. In the third chapter, I explored geographic patterns of evolutionary assembly using the same dataset. Overall, birds that presently co-occur in the same mountain range represent avian groups that originated in disjunct geographic areas (e.g., Southern Andes, Northern Andes, or Central American mountains), and subsequently occupied the entire Neotropical montane region. In the fourth chapter, I conducted a more in-depth investigation of the role niche breadth and elevational distribution on multi-species divergence. Using 2,475 mtDNA sequences, I estimated genetic divergence between population pairs (287 comparisons) divided by three major Andean barriers: the North Peruvian Low, the Táchira Depression, and the Motilones Pass. I found that genetic divergences were not randomly distributed with respect to minimum elevation (i.e., elevational distribution) and elevational amplitude (i.e., niche breadth). Elevation specialists (i.e., birds with narrow amplitudes) and the upper limits of elevational distributions (i.e., high minimum elevations) exhibited deeper genetic divergences. These results support the hypothesis of greater genetic divergence with increasing elevational distribution and decreasing niche breadth. In summary, my dissertation suggests that the present-day assemblages of Neotropical montane birds resulted from localized speciation in isolation and subsequent colonization. The effect of physical barriers is modulated by niche specialization along elevational gradients, dispersal ability, and time.



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Committee Chair

Brumfield, Robb T.