Species Richness in Time and Space: a Phylogenetic and Geographic Perspective

Abstract

Biodiversity varies dramatically across geographic space and across the tree of life, yet active debate among biologists remains regarding the underlying causes of these diversity patterns. By integrating phylogenies with species geographic range information and environmental or climatic datasets, we can explore questions relating to the assembly of communities and diversity gradients at continental to global scales.

In Chapter 1, I introduce major themes uniting macroevolution and macroecology. I describe the underlying conceptual framework that links my different research chapters together. I explain how these efforts advance our understanding of large-scale patterns of diversity, while providing critical assessments of tools and resources that facilitate the study of diversification across environmental and geographic gradients.

In Chapter 2, I highlight the recent availability of several large-scale phylogenies for squamate reptiles, and explore how they might affect macroevolutionary research. Using Australian squamates as a case study, I find that a great deal of conflict exists across phylogenies, both in terms of divergence times and topology. I demonstrate that these differences can be severe enough to alter conclusions drawn from downstream macroevolutionary analyses. I further explore the potential sources of and solutions for these discrepancies.

To properly test hypotheses pertaining to limits on species’ geographic distributions, we need accurate geographic range estimates. A majority of studies currently rely on a set of 19 bioclimatic variables for species distribution modeling and related ecological research. In Chapter 3, I assemble a new bioclimatic dataset from variables described in the literature, in order to increase the number of predictors that are easily accessible to ecologists and evolutionary biologists. I find that incorporating these predictors into species distribution modeling workflows leads to noticeably improved models, and I anticipate that they will prove useful in macroecological studies as well.

In Chapter 4, I evaluate the performance of a number of approaches for estimating species-specific ``tip rates'' of speciation. These metrics, which quantify recent variation in rates of speciation across a phylogeny, are key for the study of trait-dependent diversification as well as spatial variation in rates across biomes and latitudinal gradients. Under a number of simulation scenarios, I assess the performance of three model-free tip rate metrics, and compare them to BAMM, a Bayesian model-based approach for estimating diversification rates. I find that BAMM exhibits the least amount of error in speciation rates in all diversification scenarios evaluated. One of the model-free metrics, DR, also performs well, although its performance is hampered by high variance in rate estimates.

Finally, in Chapter 5, I explore how biogeographic rates of dispersal have contributed to the latitudinal diversity gradient in marine fishes. There are dramatically more species in the tropics than at high latitudes, but prior research has found that speciation rates exhibit an inverse relationship with latitude, with the lowest rates in the tropics. I sought to determine whether or not global patterns in biogeographic immigration in marine fishes conform to an “out of the tropics” scenario, where lineages disperse out from the tropics and enrich higher latitude assemblages. I find that dispersal rates are strongly biased in a poles-to-tropics direction. However, given the strong latitudinal species richness gradient, estimated per-lineage rates of dispersal translate to greater net movement from the tropics to high latitudes, confirming that high latitude assemblages are enriched by tropical diversity over macroevolutionary timescales.