Research Interests
I study the ecological and evolutionary mechanisms that generate and maintain biodiversity, and how human activities are altering not only patterns of species distribution, but patterns of intraspecific variation in adaptive traits. For many species, this variation represents the best form of bet hedging in the face of an environmentally uncertain future, especially with respect to global climate change. Loss of adaptive variation will reduce species’ potential to hedge in a rapidly warming world, ultimately leading to extinctions and loss of biodiversity above and beyond that directly caused by exploitation and habitat loss. The net effect of such biodiversity loss will be cascading shifts in community structure that alter ecosystem functioning, likely putting additional species at risk.
Overall, this adds urgency to understanding the causes of biodiversity patterns, and fundamental aspects of the evolutionary process. Assessments of the relative importance of neutral and adaptive evolution at micro and macro-evolutionary timescales, and the geographic-environmental axes along which diversification plays out are two areas in which much work remains to be done. Ideally, by identifying regions and landscapes important to the process of diversification, we may be able to implement conservation strategies that protect them.
In light of these concerns, my research program encompasses a variety of ecological and evolutionary studies. I focus on natural populations in the tropics, undertaking integrative analyses that may include genome scans, multi-locus sequence data, statistical models of gene-environment relationships, phenotypic variation, environmental data from remote sensing, species distribution databases and historical specimens from museums, and ecological niche models. In addition to specific projects comprising my dissertation research (listed below), I am involved in collaborative research on the landscape genetics of California mule deer (In Press, Molecular Ecology) and speciation in East African giraffes (In Review, BMC Evolutionary Biology).
I will be finishing my Ph.D. in the summer of 2009, and am actively seeking a postdoctoral research position, as well as other research collaborations.
Savanna Snake Invasions into African Rainforest
Snake species adapted to African savanna have recently started to appear deep in the rainforest zone of Cameroon. Reported occurrences were proximal to disturbed habitats, suggesting the potential for a broader invasion from the adjacent savanna biome. Using species locality data obtained from country-wide reptile surveys by CAMHERP, a Cameroon-based NGO, and remote sensing data, I used ecological niche models to predict the relative roles of vegetation and climate on invasion potential in the rainforest zone for three snakes adapted to savanna and open, disturbed habitats: the night adder (Causus maculatus), the olympic lined snake (Dromophis lineatus), and the African house snake (Lamprophis fuliginosus). With these models, I determined that (i) rainforest habitats are typically unsuitable for these snakes, but that removal of rainforest vegetation has apparently created suitable habitats which these species can invade, (ii) that all three species were predicted across large areas of the rainforest zone, suggesting high invasion potential, and (iii) based upon projections of future climate from the CCM3 global circulation model, cliimate change will have heterogeneous effects on invasion potential. This work has recently been published in the February 2009 issue of Conservation Biology.
Environmental Modeling of Partitioned Genome Scans
The traditional view of diversification in tropical Africa has been that cool, dry periods associated with glacial periods led to the contraction of rainforest into isolated refugia, and that population differentiation, and ultimately speciation are primarily caused by stochastic genetic divergence between them. However, recent work by Dr. Tom Smith and other CTR researchers has called the generality of this hypothesis into question, suggesting that divergent selection along the rainforest-savanna gradient may be more important to speciation than refugial isolation. To date, no studies evaluating these competing claims have looked at patterns of adaptive genetic variation, let alone distinguished them from presumably neutral patterns. To address this I contrast neutral and adaptive genetic variation in a rainforest lizard, Trachylepis affinis, to evaluate simultaneously the importance of rainforest refugia and ecological gradients. I do this by performing a genome scan on 12 populations sampled in Cameroon, in order to generate a large set of amplified fragment polymorphism (AFLP) markers. I then partition this genome scan into neutral loci, and those bearing a signature of divergent natural selection among either habitats or populations. In conjunction with traditional genetic structure analyses, I link variation from these two partitions to climatic and vegetation patterns from satellite remote sensing.
While there is some evidence for neutral genetic differentiation between proposed refugia, evidence for introgression between these areas in both AFLPs and mitochondrial sequence data suggest this differentiation is ephemeral. In contrast, loci under selection are differentiated along lowland rainforest-montane and rainforest-savanna gradients. Environmental modeling indicates neutral differentiation occurs along the latter gradient as well, most likely due to the effects of repeated colonization along the front of increasing suitable rainforest habitat following the cold, dry portion of global climate cycles. This expansion is confirmed with an ecological niche model for T. affinis, that by back-projecting this species environmental preferences to the Last Glacial Maximum, predicts post-glacial range expansion. Overall, this research reinforces the importance of ecological gradients, and suggests that refugia play a secondary role as the repository of variation upon which selection acts when populations are distributed into novel environments during range expansions.
Human-mediated Loss of Adaptive Diversity
My work and that of my CTR colleagues confirms that ecological gradients between tropical rainforests and savannas are important generators of biodiversity. However, the impact of human activity on these gradients remains unexamined. To examine the evolutionary implications of deforestation, I use data collected on morphological traits from Andropadus virens—a rainforest bird species with demonstrated phenotypic divergence across the gradient between intact rainforest and the rainforest-savanna ecotone—and a time series of satellite-based estimates of forest cover across West and Central Africa, to show that anthropogenic changes along the this gradient are altering traits typically shown to be important in fitness. In West Africa, where deforestation has been greater than Central Africa, the rainforest-ecotone gradient has become more homogeneous, resulting in a greater loss of phenotypic diversity. I predict that continuing deforestation will likely lead to further homogenization of the gradient across tropical Africa, resulting in additional loss of diversity. Such losses will make it less likely that at least some populations will contain the adaptive variation necessary to persist in the face of ongoing climate change. The practical consequence of these findings is that remediating for such losses requires the conservation of landscapes that comprise intact ecological gradients.
Education
University of California, Los Angeles CA
Currently enrolled in Ph.D. program in the Department of Organismic Biology, Ecology & Evolution,
University of Florida, Gainesville, FL
M.S., Department of Wildlife Ecology & Conservation, May 2000
University of Massachusetts, Amherst, MA
B.S., Environmental Science, Biology concentration, May 1995
The George Washington University, Washington, D.C.
B.A., Philosophy, May 1991
Research Grants
• UCLA/Department of Ecology and Evolutionary Biology Research Fellowship, 2009
• Environmental Protection Agency STAR Fellowship, 2006-2009
• UCLA/Biology Department Research Fellowship, Summer 2006
• UCLA/Biology Department Research Fellowship, Fall 2005
• Fulbright Fellowship for research in Cameroon, 2004-2005
• UCLA/Biology Department Research Fellowship, 2003
• UCLA Non-Resident Tuition Fellowship, 2002-2003
• Jennings Memorial Wildlife Ecology & Conservation Scholarship, 1998
Publications
• Pease, K.M., A.H. Freedman, J. Pollinger, W. Buermann, and R.K. Wayne. In Press. Landscape genetic structure of Calfornia mule deer (Odocoileus hemionus): the role of contemporary ecological and historical factors. Molecular Ecology.
• H.A. Thomassen, A.H. Freedman, D.M. Brown, W. Buermann, and D.K. Jacobs. In Review. Seasonal timing of rainfall sustains reproductive isolation in East African giraffes. BMC Evolutionary Biology.
• A.H. Freedman, W. Buermann, M. Lebreton, L. Chirio, and T.B. Smith. 2009. Modeling the effects of anthropogenic habitat change on savanna snake invasions into African rainforest. Conservation Biology 23:81-92.
• Funk, D.W., L.E. Noel, and A.H. Freedman. 2004. Environmental gradients, plant distribution and species richness in Arctic salt marsh near Prudhoe Bay, Alaska. Wetlands Ecology and Management 12:215-233.
• Freedman, A.H., K.M. Portier, and M.E. Sunquist. 2003. Life history analysis for black bears (Ursus americanus) in a changing demographic landscape. Ecological Modelling 167:47-64.
• Hamilton, A.M., A.H. Freedman, and R. Franz. 2002. Influence of deer feeders, habitat, and sensory cues on predation rates on artificial turtle nests. American Midland Naturalist 147:123-134.