Macroevolution and Biogeography

I am interested in models that aid inference of macroevolutionary processes from available data on extant species, especially with regard to characters that may affect rates of speciation and extinction (such as mating system type or geographic location).

In collaboration with Boris Igic, I am developing a method to assess the effect of self-incompatibility on diversification rate in Solanaceae, allowing for incomplete sampling and lack of branch lengths in the phylogeny.

Because there is strong evidence that self-incompatibility in this system is not regained once it has been lost, Boris and I are also interested in phylogenetic methods for testing Dollo's Law. We showed that standard methods of rate estimation and ancestral state reconstruction frequently incorrectly reject irreversibility, and we described a more effective procedure.

For a geographic character, I developed a Markov chain model to show how the age distributions of extant taxa can yield information on rates of macroevolution in different regions and range changes. We used this model to examine the relationship of regional diversity and endemism to rates of speciation, extinction, and dispersal. We then applied it to marine bivalves, inferring an out-of-the-tropics mechanism driving diversity patterns in that system.

We also used this model, and a simulation-based extension of it to molecular phylogenies, to emphasize that dispersal must be explicitly included in attempts to estimate origination and extinction rates of different regions. Comparing our model results to recent work on marine bivalve and terrestrial avian biogeography suggests that the processes of origination, extinction, and dispersal have operated differently in these systems.

In collaboration with Rick Ree, I am now working on modifying this geographic model to allow simultaneous estimation of speciation, extinction, and dispersal rates from molecular phylogenies. We are also pursuing an extension to allow inferences about the biogeographic mode of speciation.


Dispersal and Demography in River Networks

With Bill Fagan, Heather Lynch, and Mike Neubert, I am looking at the sensitivity of population growth rate to various aspects of demography and dispersal for species that live in river networks. We are particularly interested in the differing effects of the branching structure and dispersal biases inherent in a river system on species with various life histories.

Collaboration with Evan Grant on an application to stream salamanders is in the works.


Coevolution and Biogeography

I have also done some work on the effects of population-level processes on biogeographic patterns.

Are borders between species expected to coincide with geographic barriers to dispersal? I used spatial models of competition, hybridization, and local adaptation to examine the effects that a region of reduced dispersal can have on the location of a border between two similar species. I found that when interspecific competition or detrimental hybridization are strong, a border is attracted to a partial dispersal barrier, but when local adaptation and gene flow are important, a border is repelled by a barrier.

In character displacement, coevolution of two competing or hybridizing (when hybrids are less fit) species causes their phenotypes to be displaced from what they would otherwise be. What observable patterns can this process create? For two-species systems, the classic pattern is greater phenotypic difference in sympatry than in allopatry, but other outcomes are possible, too. I investigated this by using spatial models of both ecological and reproductive character displacement, with the goal of broadening the ways in which we look for this phenomenon in nature.