Research

Over long timescales, biological diversity is shaped by trait evolution within populations and by the speciation and extinction of lineages. Research in the lab centers around plant mating systems and geographic ranges as two examples of characters influenced by both these microevolutionary and macroevolutionary forces. Our work generally includes the development and application of mathematical models, usually in collaboration with experts in various empirical systems.

Plant mating systems

The amount of outcrossing (versus self-fertilization) in a population significantly shapes effective population size, inbreeding depression, and spatial distributions of genetic diversity. It thus has important consequences for population dynamics and, on longer timescales, for speciation and extinction.

Reproductive systems and lineage diversification

Solanum crispum

Self-incompatibility (SI) is a genetic mechanism that causes a hermaphrodite plant to reject its own pollen. SI systems are widespread in flowering plants, but they have been frequently lost due to a variety of conditions under which selfing is favored. We have found that in the nightshade family Solanaceae, despite transitions to self-compatibility (SC) in hundreds of lineages, species with SI have a higher net rate of diversification and are thus maintained within the family over long time-scales by species selection. In contrast, we’ve found different macroevolutionary dynamics for dioecy—in which individual plants are either male or female. Transitions from dioecy to hermaphroditism seem to be much more common than from SC to SI, and dioecy does not seem to have a consistent effect on lineage diversification.

Breeding or sexual system is surely not the only trait that affects speciation and extinction, however, and it is correlated with many other life-history and morphological traits. In particular, our related work includes investigations of the macroevolution of flower color and polyploidy.

This empirical work has also inspired methods development, especially trying to figure out how phylogenies can reliably be used to test whether traits affect rates of speciation and extinction.

Geography of mating system

Clarkia xantiana

The relationship between mating system and geographic distribution may eventually be one piece of understanding how mating system affects speciation and extinction, and it’s of course interesting in its own right. One productive local collaboration has used sister species comparisons to show that mating system predicts range size but not range overlap. With a big collaborative group, we have also compiled a large dataset that provides strong support for the long-held idea that species capable of self-fertilization would be overrepresented on islands because they avoid mate limitation during colonization. Continuing interests include models of mating system evolution at range limits set by abiotic factors or by interactions with other species.

Biogeography

Regional diversity dynamics

Acropora nasuta, photo by Danwei Huang

In all plant and animal groups, species richness and composition vary dramatically across the globe. To identify the large-scale drivers of diversity patterns, we work with dynamic models of three fundamental processes: speciation, extinction, and range shifts. Regional differences in these forces are the essence of the classic biogeographic concepts of “centers of origin,” “museums of diversity,” and “centers of accumulation,” among others. Disentangling the contributions of lineage diversification and dispersal to observed spatial variation in richness and endemism requires thinking carefully about how these processes operate together to give rise to species’ geographic distributions, stratigraphic ages of lineages, and phylogenetic trees.

Collaborative projects have investigated the latitudinal diversity gradient in marine bivalves, California plant diversity in different habitat types, and coral diversity in the Indo-Pacific. Current interests include the potential for geographic range to interact with intrinsic species’ traits (including mating system—see above) to affect speciation and extinction.

Coevolution and range limits

Phylloscopus chloronotus, photo by Thor Veen

Thinking about geographic ranges on a smaller scale, one can consider in more detail spatial variation in the environment and the processes acting within and between species. Our work here has focused on the connection between coevolutionary interactions and geographic range limits along environmental gradients. Population-dynamic and population-genetic models allow us to investigate how dispersal, resource competition, hybridization, local adaptation, and environmental conditions work together to limit species ranges.

Work so far includes the spatial signatures of ecological and reproductive character displacement, the effects of partial dispersal barriers, and the evolutionary and population-dynamic consequences of ecological differences between competitors. Continuing considerations include the roles of plasticity and mating systems, and the effects of environmental change on range limits and spatial distributions of traits.

Collaborators: Russ Lande, Trevor Price