Recent and Ongoing Research Projects

I study the genetics of colonization and its evolutionary and ecological outcomes. My work draws largely on the natural experiments provided by human-mediated species introductions and I'm working to understand how the genetic variation in these populations translates into phenotypic diversity, adaptation, and changes in ecology. 

The genetic basis of adaptation
Sources of genetic variation and hybridization
Evolution of inbreeding depression
Evolution of 'invasiveness'

Genetic basis of adaptation

During my dissertation, I found evidence of adaptation in recently introduced populations that had experienced strong genetic bottlenecks, highlighting the need to understand how genetic architecture makes such rapid evolution possible. I developed phylogeographic, population genetic, and phenotypic studies of the shrub Hypericum canariense (Canary Island St. John's wort), which was introduced from the Canary Islands into California and Hawaii ~50 years ago. My work revealed that these populations have lost about half of their neutral genetic diversity relative to native source populations. Nevertheless, introduced populations displayed adaptive evolution of increased growth and a latitudinal cline in flowering phenology, with no detectable declines in phenotypic variation for either trait.

'Next-generation' sequencing technologies are providing the opportunity to begin making the connection between such rapid phenotypic evolution and its molecular genetic basis in non-model organisms. As part of my postdoctoral work with Loren Rieseberg, I am identifying loci associated with adaptation in another introduced plant, Centaurea solstitialis (yellow starthistle). This species has spread from eastern Europe into large regions of western Europe, North America and South America, creating many opportunities for adaptive differentiation.  Updates of our work on the genomics of weedy an invasive plants can be found here.

Sources of genetic variation, hybridization, and novelty

It is a simple truism that evolution requires genetic variation. For colonizing populations, the source(s) of individuals and genes that contribute to a new population will determine its raw material for future evolution. Among introduced species in particular, mixing of material from different source locations and hybridization with related species are both hypothesized to enhance adaptation and establishment success. However, we know little about how often or how much these processes actually contribute to variation in founding populations, and whether such intra- or inter-specific hybridization results in novel traits that are not already found in the initial source of an introduction.

In Centaurea solstitialis, genomic data suggest cryptic hybridization in invading genotypes. By examining the distribution of divergence times (measured as synonymous substitution rates) between members of gene families, my collaborator Mike Barker and I have identified anomalous gene variants which are derived from a separate, divergent genome. I am currently in the process of evaluating both the geographic extent of this introgression, and the identity of the unknown parent. If introgression is confirmed, I will be evaluating whether particular classes of genes are introgressing preferentially, and/or if particular alleles are sweeping through introduced populations. 

Evolutionary loss of inbreeding depression

In plants, the ability to self pollinate would seem to confer a tremendous advantage during colonization, and indeed many studies have found an association between self pollination and colonization success of some kind.  Selfing may be especially beneficial to invading species, because opportunities to establish a new population from a single individual may abound in unoccupied (but suitable) habitat.  Whether inbreeding lineages succeed during colonization is expected to be a function of the need for reproductive assurance and the ability to evolve reduced inbreeding depression.  I am particularly fascinated by the evolution of inbreeding, because this single trait affects the evolution of all other traits by generating linkage disequilibrium across the genome.

Hypericum canariense invasions provide a truly exceptional study system in which to investigate the adaptive evolution of inbreeding and inbreeding depression.  I have found a dramatic shift from inbreeding depression to outbreeding depression (in seed set, germination and/or growth) during the recent expansion of individual introductions in this species.  This is one of the first studies to demonstrate that inbreeding depression can be lost rapidly in natural populations. The pattern suggests that self fertilization has become an important mode of reproduction in each invasion, and that selection has favored an increase in the success of inbred progeny through purging of genetic load and/or development of co-adapted gene complexes. I am currently collaborating with Eduardo Cuevas (Univ. of Morelia) and Ingrid Parker (Univ. of CA Santa Cruz) to quantify realized changes in outcrossing rates in these populations.

The evolution of 'invasiveness'

There is a growing appreciation that adaptive evolution might contribute directly to invasive behavior in introduced species, allowing them to overcome environmental obstacles and exploit ecological opportunities. Nevertheless, adaptation is a process of relative changes in the fitness of different lineages, and this may have no impact on vital rates in the population overall. For instance, an increase in the rate of seed production will aid the evolutionary dominance of a lineage, but will have no effect on a safe-site-limited population.

Models of demography and spread rate have already proven useful for understanding the potential impacts of ecological changes on invasive plants, such as biocontrol introductions or changes in pollinator service. I am working to make links between genetic variation, adaptation, and colonization success by collaborating with demographers on a variety of projects following the performance of multiple populations across the range of a species.