Adaptation Across Ecological Gradients
Fig. 1: Topographic profiles of the two cordilleras separating sampling sites in the Vilcanota region and along the Manu Road (see Fig. 1c). Arrows with red lettering indicate mean migration rates (m). b Bill length differences among the four Cusco localities. Blue represents the two humid sites and brown the two drier sites.
Fig 2: Mean differences effect sizes for each trait based on t-tests between California salt marsh and interior population (0 line) compared to t-test between Mexico salt marsh and interior.
Exploration in the Andes inspired a major theme of my research: adaptation across environmental gradients. Andean topography creates an ecologically complex landscape with dramatic turnover in climate, vegetation, and other ecological factors across short distances. My research on adaptation across these gradients centers on understanding patterns of phenotypic divergence in association with spatially varying selective pressures, and how factors such as gene flow and colonization history interact with ecological pressures to ultimately shape patterns of adaptive divergence.
My Master's research with Chris Witt at UNM examined the influence of the Andean landscape on bill morphology in the hummingbird Metallura tyrianthina. We found a strong association between variation in seasonality and bill length with length increasing in seasonally drier environments. Although we found increases in bill length associated with three independent colonizations of west-facing Andean slopes, the degree of divergence in bill length was found to depend on topography. Fig.1 shows how higher elevation cordilleras in the Andes contribute to reduced gene flow, which in turn corresponds to greater divergence in bill length. I have also been involved in projects associated with adaptation to hypoxia along elevation gradients in the Andes (see Galen et al. 2015).
The influence of gene flow and colonization history on adaptation across salinity gradients is a central question in my dissertation research. Salt marsh populations of the Savannah Sparrow derive from two independent colonizations of salt marshes by freshwater-adapted populations. Salt marsh populations along the central California coast exhibit little to no genetic differentiation, whereas Mexican salt marsh populations exhibit a longer history of isolation. Both of these populations exhibit divergence in a suite of physiological traits associated with osmoregulatory performance in high-saline environments. However, in all of these traits Mexican populations are more divergent than California populations (Fig. 2). This suggests that either differential levels of gene flow and/or divergence history play an important role in shaping adaptive divergence in salt marsh Savannah Sparrows. Stay tuned for more as I pursue my research in this vein!
Benham, P.M & Witt, C.C. 2016. BMC Evolutionary Biology. Open Access. PDF
Galen, S.C., et al. 2015. PNAS 112: 13958-13963. PDF