Research

Types and sources of adaptive genetic variation for colonizers

Introduced species offer excellent opportunities to investigate the factors that shape species ranges because their introduction history (i.e., source population, founding population size, and sites of introduction) is often well documented. As a PERT postdoctoral fellow at the University of Arizona, I conducted a population genomic study of the hairy weevil (Eustenopus villosus), an insect which is native to Europe and introduced to the western US to control invading yellow starthistle (Centaurea solstitialis; right). Introduced weevils are descended from a single source population in Greece, and may therefore have insufficient genetic variation to adapt to climate in the western US.

I am using landscape genomics approaches to explore genetic adaptation to climate in the native range of hairy weevils, and to assess how much of this variation is present in introduced populations. The results will improve our understanding of the evolutionary potential (i.e., ability to adapt) of small populations in novel environments, as well as the role of adaptive genetic variation in shaping species distributions. In collaboration with Katrina Dlugosch, I also reconstructed colonization pathways and identified sources of adaptive genetic variation for invading yellow starthistle (Barker et al. 2017). Additionally, we reported on one of the first empirical tests of the joint effects of expansion dynamics and environment on effective population size variation during invasive range expansion (Barker et al. 2019).

Historical and topographic drivers of tropical insular diversity

Island ecosystems, or those which are geographically isolated, are “natural laboratories” for studying evolution. Despite a rich history of research on island evolution and biogeography, the effects of topography and past climate change in generating island biodiversity are not well understood. My PhD dissertation explored the role of topography and range shifts resulting from past sea-level changes and climate suitability in shaping genetic diversity of two frogs in the Puerto Rican Bank, an archipelago in the eastern Caribbean Sea.

Basins and past climate change: impacts on genetic variation in montane island endemics

To better understand lineage diversification of tropical montane species within islands, I studied the historical biogeography and consequences of palaeoenvironmental change on patterns of genetic diversity in the mountain coquí (above), Eleutherodactylus portoricensis, a frog endemic to montane rain forests of Puerto Rico. I found that the low-elevation Caguas Basin in eastern Puerto Rico is a long-term barrier to gene flow between populations in the Luquillo and Cayey Mountains, with population divergence beginning more than 75 ka (Barker et al. 2011). Stable population sizes over time indicate a lack of demographic response to climatic changes during the last glacial period. The results highlight the importance of topographic complexity in promoting within-island vicariant speciation in the Greater Antilles, and indicate long-term persistence and lineage diversification despite Quaternary climatic oscillations.

Sea level, topography, and human introductions: impacts on genetic variation in widespread island endemics

Quaternary climatic oscillations caused changes in sea level that altered the size, number, and degree of isolation of islands, particularly in land-bridge archipelagos such as the Puerto Rican Bank. To increase our understanding of the role of climate change in shaping evolutionary processes in tropical insular systems, I used multiple DNA loci to examine demographic phenomena and assess the effect of sea-level changes on populations of the red-eyed coquí, E. antillensis across the Puerto Rican Bank. A hypothesis of colonization of the Eastern Islands from sources in eastern Puerto Rico during the penultimate and last glacial period, when a land-bridge united the Puerto Rican Bank, was supported (Barker et al. 2012). The Río Grande de Loíza Basin in eastern Puerto Rico delineates a phylogeographic break. Haplotypes shared between the Puerto Rican Bank and St. Croix (an island ca. 105 km southeast of this archipelago) represent human-mediated introductions (Barker et al. 2017).

Climatic stability and genetic endemism in tropical islands

The ecological requirements of species largely determine what geologic and climatic factors constitute barriers to gene flow, and ultimately play a role in divergence. There is remarkably little known, however, about the interplay of historical events and ecological requirements driving intra-specific diversification within tropical island systems. Using multi-locus sequence data and spatially explicit models of population history developed from ecological niche models, I explored the role of late Quaternary climate change in shaping population structure and historical demographics of E. portoricensis, a narrowly distributed montane forest specialist, and E. antillensis, a widespread habitat generalist. I found that areas of high habitat stability fostered population persistence and the accumulation of genetic endemism, with E. antillensis experiencing greater climate-driven fluctuations in population size than E. portoricensis (Barker et al. 2015). Unstable habitat in the Río Grande de Loíza Basin is a historical barrier to dispersal and levels of genetic isolation in each species likely track species-specific physiological requirements. Understanding how late Quaternary climatic oscillations and topography affect gene flow and past population dynamics provides a framework for identifying evolutionary processes that underlie speciation in tropical islands, and can inform predictions of potential responses of endemic tropical insular biodiversity to future habitat alterations.

Frog tissues and both traditional and photographic voucher specimens associated with my dissertation research are archived at the Museum of Southwestern Biology, University of New Mexico, Albuquerque or the Museum of Vertebrate Zoology, University of California, Berkeley. Collaborators and co-authors Joe Cook (University of New Mexico), Bob Waide (University of New Mexico), and Javier Rodríguez-Robles (University of Nevada Las Vegas) were instrumental in completing this research. Multiple colleagues, undergraduates, and friends in both New Mexico and Puerto Rico also made this work possible!

Long-term bird population dynamics

Tropical ecosystems are changing rapidly in response to deforestation, climate change, urbanization, and other drivers. Unfortunately, a lack of long-term data on the effects of these changes hinders our ability to predict how ecosystems will respond to future perturbations. I collaborated with Principal Investigator Bob Waide at the University of New Mexico on a project investigating how change in climate and disturbance regimes affect long-term population dynamics of birds in the Luquillo Mountains of Puerto Rico. This work, which is part of the Luquillo Long-Term Ecological Research Program (Luquillo LTER), involves estimating the density and frequency of 54 bird species across a 16 hectare study plot known as the "Big Grid" over a 24 year time period.