The role of climate and geochemistry in diversification of North American plant radiations
It is a well accepted hypothesis that the ability of species' to occupy and adapt to multiple and varied niches is a likely path to the generation of species and diversity as local populations become geographically or adaptationally separated from each other. Testing this hypothesis is difficult and requires either long-term breeding experiments or collection of data that is logistically only feasible in modest sample sizes, limiting the breadth at which conclusions can be drawn. However the relatively recent advent of large readily-available databases of sequence data (GenBank), distributional data (GBIF, BISON, IdigBio) and soil and geochemistry data (USGS/USDA) allows us to test this hypothesis and ask further questions such as how innovation to extreme environments may drive diversification, how diversity relates to specific environmental variables, whether related species occupy similar niches etc.
To explore these questions this project focuses on the massively diverse Compositae/Asteraceae (daisy) family, representing 1 in 10 flowering plants, and with large recent species' radiations in the environmentally extreme and varied US SW and northern Mexico. It is a collaboration between researchers at the Smithsonian, USGS, USDA, University of Florida, and private industry, coordinated through the USGS Powell Center for collaborative synthesis and analysis.
Hybrid sterility on the X chromosome
"Haldane's Rule" is a longstanding observation that when hybridizing species produce offspring where one gender is infertile, it is almost invariably the heterogametic sex (e.g. in mammals or insects the XY (= male)). Further data shows that when the genetic factors responsible for hybrid sterility are mapped to the genome, they are disproportionately found on the X chromosome (the large X effect). Why this is so is still not entirely understood and more data, for example on the frequency of sterility factors on different chromosomes, relationship to selection, and ultimately the function of genes responsible for incompatibility, are required to better address hypothesis for these observations.
I am working to locate the factors responsible for male sterility on the X chromosome in hybrids between Drosophila simulans and D. mauritiana, two relatively recently diverged fruit fly species.
Species limits in a complex of Australian tropical trees (Melaleuca leucadendra species group)
The Melaleuca leucadendra complex (Broadleaved Paperbarks) is an important component of the tropical and sub-tropical Australian flora, with 14 currently recognized morphospecies distributed almost continuously across monsoonal (seasonally dry/wet) savannah habitats.
Various taxonomists have attempted to confidently delimit these morphospecies with limited success: species are poorly diagnosable with a range of variable and overlapping diagnostic characters, and morphology is noted to vary considerably across and between populations. This, combined with the observation that 'species' often appear to segregate locally across water availability gradients has raised the suggestion that species limits within the group may be imperfect, either in the early stages of differentiation (possibly ecologically mediated), or subject to relatively frequent introgression (hybridization).
The large population sizes and huge geographic distances covered by these species provides a challenge for sampling representative genetic diversity and includes geography as a major player in the possible speciation history of the group.
To date we have established that the complex is a recent rapid radiation, consistent with the likely expansion of savannah habitat in response to increased aridification about 4-2 Ma (estimated crown ages for the complex of 3.6 Ma and 6.7 Ma (Cook et al., 2008).
Unpublished data indicates that there is surprisingly good molecular differentiation between species, but with signatures of incomplete lineage sorting (retention of ancestral genotypes) suggesting only recent divergence. A strong cryptic geographic signal was also detected.
Two particularly indistinguishable species may have diverged in allopatry across the Great Dividing Range, with subsequent contact due to decay of the barrier between them (Edwards et al., 2013).
Future research will focus on increasing the number of loci available, strengthening our phylogenetic reconstructions and better testing hypotheses of isolation with low level introgression or slow sorting of very large population sizes.
Common biogeographical barriers to species distributions in the Australian Monsoon Tropics
Australia has long been considered an exemplar of the role of division between geographical regions (barriers) in promoting the divergence and diversification of flora and fauna (allopatric speciation). Much anecdotal observation has proposed that barriers are due to regions of aridity becoming exaggerated with the drying of the climate during the Pleistocene (especially glacial maxima), dividing the continent and associated communities of plants and animals. Some molecular evidence (particularly in birds) supports historical isolation, however there is little quantification of what proportions of species may respond to such changes, how these putative barriers may influence the distribution of species in the present, and little work at all on the response of plants to these barriers.