System of Study:
The green anole, Anolis carolinensis provides an intriguing example of natural invasion. A. carolinensis is nested within the Cuban green anole clade, dispersing overwater to the establish in peninsular Florida as late as the Pliocene. The only anole native to the continental United States, A. carolinensis has spread from Florida throughout the southeast to occupy a range of environments as far north as Tennessee and North Carolina and west into Texas and Oklahoma. It is hypothesized that winter temperatures limit the northern edge of the species’ distribution. However, northern populations do not hibernate as is common for most reptiles at these latitudes. Despite regular ambient temperatures below freezing, northern populations of this species endure winter by retreating to sheltered sites and basking on rock faces during periods of sun exposure. The recently published genome of the species provides an invaluable resource for understanding the influence of climatic variation on population structure and natural selection at the molecular level. I use this species a model to understand how changes in environment influence the evolution and shape patterns adaptation at phenotypic and genomic level.
Phylogeography:
Gaining a clear view of population relatedness across geographic space plays a key role in understanding the demographic and selective pressures that have shaped the evolutionary history of a species. Towards this aim, I central goal of my research is to gain an in-depth understanding of the phylogeographic history of the green anole. The first phase in this process has utilized extensive sampling of mitochondrial DNA across the range of the A. carolinensis, and its Cuban relatives to identify major genetic groups and date times of divergence within and between these groups. The second phase of this project is using genomic sampling and coalescent analyses to produce more accurate times of divergence between major lineages, gain a better understanding of ancient and contemporary gene flow across the species’ range, and identify regions of the genome that are resistance to introgression between clades.
Evolution, adaptation and range expansion after invasion into novel climate:
Does adaptation to native environments constrain the range of invasive species? This question has important implications for understanding the effects of human-mediated climate change on biodiversity in the coming years, decades, centuries, and beyond. Invasive species are amongst the most severe threats to native biodiversity worldwide. The introduction of invasive predators, competitors and deadly prey species has had severe impacts on biomes around the globe.
Though many species have been established in introduced ranges via human-mediated dispersal for several decades, understanding the effects on niche conservatism and local adaptation in shaping the evolutionary history of these lineages may require longer time periods than have yet been realized. Natural invasion events provide longer timescales with which to study processes of niche evolution and local adaptation associated with introduction into novel habitats. These studies give valuable insight into the long-term implications of invasive species currently beyond the view of human-mediated dispersal events. Using the green anole as a model system, I am combining niche modeling, thermal physiology, and genomic techniques to explore climate-mediated evolution associated with the natural invasion of the ancestrally Cuban lizard into more temperate habitats in mainland North America.
Adaptive plasticity and local adaptation of thermal tolerance:
The biological mechanisms that allow organisms to adapt to temporal and geographic variation in climate play an integral role in the generation of biodiversity over space and time. Phenotypic plasticity and local adaptation may act separately or in tandem to produce observed patterns of adaption to climate. The predictability, magnitude, frequency, and speed of environmental change may have a significant impact of the ability of organisms to acclimate and adapt to changing environments, and the likelihood that these environmental challenges will be met through genotypic specialization or phenotypic plasticity.
While disentangling the effects of phenotypic plasticity and local adaptation on geographic variation is of fundamental importance to many evolutionary questions, achieving this in natural populations can prove challenging. Through controlled experimental manipulations of green anoles sampled across a climatically heterogeneous range, I am currently attempting to disentangle these effects and interactions to better understand how environmental variation generates phenotypic diversity within this species.
Rapid evolutionary response to extreme weather events:
Catastrophic weather events have long been considered strong agents of natural selection, dating back as early as Bumpus’ study (1899) of selection and mortality in house sparrows in response to a winter storm. However, the effects of extreme weather events remain poorly studied. Because extreme weather is rare and unpredictable, collecting necessary data both before and after such an event can prove difficult. Global climate change is expected to increase the probability of extreme weather events. Therefore, measuring organismal response to such events has become increasingly important for estimating the rate of survival and the pace of adaptive response in natural populations.
In 2013 we measured cold tolerance related phenotypes for five populations of green anoles across a north-south transect from southern Texas to southeastern Oklahoma. Shortly thereafter, the disruption of the polar vortex led to an extreme cold event throughout the region. This unusual weather event led these populations to experience 16.7% - 47.5% increase in days where temperatures dropped below their average cold tolerance compared to the previous winter. Additionally, the average daytime temperature during the winter of 2013-2014 was significantly lower than it was the previous year at most sites. I revisited these populations to measure shifts in cold tolerance phenotypes in response to the polar vortex. This project provides a rare opportunity to measure real-time evolutionary response to an extreme weather event and may provide valuable insight into population level responses to climatic perturbations at the phenotypic and genomic level.