Some of the current projects in the lab...

Diversification of the Neotropical spiral gingers

I have been studying a diverse radiation of gingers in the Central and South American rainforests. The genus Costus dispersed from Africa to the Neotropics approximately 1.5 - 7.1 mya and has subsequently radiated into over 51 species, making it one of the fastest known plant radiations. For photos of Costus, click here. All species in the Neotropics use either orchid bees or hummingbirds for pollination, and hummingbird pollination has evolved many times. Plant-pollinator interactions are sufficiently specialized that a difference in syndrome is an effective mechanism of reproductive isolation between sympatric species. I have also found that for species that share pollination syndromes, both mechanical floral isolation and postpollination pollen-pistil incompatibility are important isolating mechanisms. We have strong evidence that the pollen-pistil incompatibility has been reinforced in sympatry. My current work in this system includes: 1) Dissecting the genetic basis of mechanical floral isolation and pollen-pistil incompatibility between C. pulverulentus and C. scaber, 2) Using field experiments to understand how selection acts on individual loci underlying floral isolation, 3) Understanding the history of gene flow between C. pulverulentus and C. scaber with molecular markers, and 4) Resolving phylogenetic relationships among Neotropical Costus.

Cryptic speciation driven by edaphic adaptation in the California goldfields

Nishi Rajakaruna and I are working to understand how adaptation to different soil conditions has driven the evolutionary divergence of different races and species of goldfields (genus Lasthenia). Lasthenia californica and L. gracilis are nearly indistinguishable morphologically, and often co-occur on adjacent soils. Edaphic races characterized by their flavonoid profiles appear to have evolved in parallel within each of these phylogenetic species. What role does edaphic adaptation play in the evolution of reproductive isolation between species and races? How are these cryptic species able to coexist? Our work involves crossing studies among races and species to examine the evolution of postpollination reproductive isolation, transplant studies to better understand edaphic adaptation, and using molecular markers to better understand the genetic structure of species and races across the bay area where they co-occur.

Life history trade-offs and reproductive biology in the California Clarkias

Along with Kjell Bolmgren, I am examining the evolution of life history trade-offs between growth and reproduction, and how these relate to local climatic conditions. When should a plant divert resources away from vegetative growth and begin to flower? What are the repercussions of different flowering strategies for the growth and overall fitness of the plant? How does selection for a change in flowering affect other aspects of the plant? We are using common garden and artificial selection experiments on several Clarkia species to address these questions. I am also using Clarkia as a system to understand how species respond to variation in pollinator assemblage across their geographic ranges and across the diversity of habitats that they occupy.

Population genetics of serpentine endemic Monardellas

We are studying the population genetic structure of two rare mints that occur on the serpentine soils of Plumas National Forest. This study will better elucidate the evolutionary origins of edaphic endemism and provide valuable information for restoration efforts. This is a collaboration with Susan Harrison at UC Davis and Nishi Rajakaruna at COA.

        Research in my lab centers on understanding diversification mechanisms of flowering plants, including adaptation and speciation. I study natural plant populations, from both tropical and temperate systems, and I would like to understand the ecological and geographical settings in which speciation occurs, the divergent adaptations and phenotypic changes that contribute to reproductive isolation, and the genetics underlying those adaptations. I am especially interested in plant adaptations to pollinators and to the edaphic environment, and the role those types of adaptations play in plant diversification at the species and population level. My work combines field and greenhouse observations and experiments with insight from molecular genetics, microscopy, phylogenetic inference, and comparative biology.

        Although my research questions are applicable to all organisms, I think plants are a great study system because of the wide variety of habitats that they occupy, their diversity of reproductive characteristics, and their logistical and experimental tractability. Plus, plants are fun to work with, my field sites are gorgeous, and it’s always sunny and warm in the greenhouse!

Here are some of the questions that interest me:

1. •How do plant-pollinator interactions contribute to plant speciation?
   

2. •How does ecological differentiation, in terms of habitat affinity, contribute to reproductive isolation?
   

3. •Does natural selection reinforce speciation?
   

4. •How does divergence proceed in the face of some interspecific gene flow?
   

5. •How important are pollen-pistil interactions as isolating mechanisms?
   

6. •How do diverse and variable pollinator assemblages exert selection on floral traits?
   

7. •Why is the Neotropical flora so diverse? Why is the California Floristic Province so diverse? Why are some lineages of plants more diverse than others?

Point Reyes National Seashore

La Selva Biological Station, Costa Rica -- one of my main field sites.

Part of a potted plant array to study floral isolation in Costus.

Costus pollen germinating on a stigma.

Clarkia amoena at Pt. Reyes National Seashore

Lasthenia californica on Mt. Tamalpais

Clarkia breweri