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I study the interactions between marine plants and their environment, focusing on how evolutionary differences among populations and phenotypic/genotypic differences among individuals act to shape and modify the outcome of these interactions. The questions that fascinate me lie solidly at the interface between ecology and evolution, and I use an integrated approach, taking advantage of tools from a variety of different subdisciplines in biology (e.g. physiology, molecular genetics, demography and mathematical modeling) as well as experimental field ecology. Evolution of species’ distributions
Understanding the factors that shape species’ geographic distributions is a fundamental problem in both ecology and evolutionary biology, with implications for ecological theory (e.g. patterns of biodiversity) as well as important applied issues (e.g. invasive species, global climate change). Genetic factors are likely to play a strong role in determining population dynamics at range margins, but their influence is complex, and seemingly contradictory. For example, low dispersal and gene flow into edge populations may prevent range expansion, if edge populations lack adaptive genetic variation in ecologically limiting traits. However, theory also suggests that relatively high gene flow may act to constrain distributions: gene flow across an environmental gradient can counter local adaptation in edge habitats, generating source-sink dynamics that ultimately limit how far the species can spread.
I have developed one of the first model systems to test this idea empirically, examining the effects of dispersal and gene flow on adaptation to, and distribution across, the intertidal gradient in the brown alga Silvetia compressa. I’ve found evidence of local adaptation to tide height within populations of S. compressa over extremely small spatial scales (meters). One trait involved is embryo emersion tolerance: controlled breeding experiments indicate that this is under partial genetic control, providing one of few examples of local adaptation in a marine system on any spatial scale. In addition to a clear genetic influence, a maternal environmental effect conveys greater emersion tolerance to the progeny of mothers in the center of the intertidal distribution. This complicates simple expectations about the consequences of dispersal and gene flow for local adaptation to tide height, because genetic and environmental sources of phenotypic variation act to maximize the trait at different points along the species’ intertidal distribution. Individual variation across a geographic range An emerging theme from recent theoretical and empirical studies in a variety of systems is the observation that individual-level variation can have profound effects on processes occurring at the population and community level (e.g. resilience and resistance to perturbation; coexistence of competitors; etc.). Such effects, in conjunction with patterns of dispersal and population connectivity, may be key to understanding range dynamics. If populations are increasingly isolated towards the edges of a species’ range, this may result in lower genotypic and phenotypic variability among individuals at peripheral sites. My work with S. compressa suggests there is a positive relationship between individual variation in emersion tolerance and the width of the species’ vertical distribution; thus it seems possible that low variability in functional traits may limit the ability of S. compressa populations to spread across the (extremely heterogeneous) intertidal environment. Low dispersal also increases the probability of inbreeding. The strength of inbreeding depression is environment-dependent, and often more severe under stressful conditions. Thus, even if population connectivity is equivalent across the geographic range, the fitness cost of inbreeding may be greater at the edges of species' distributions than the center, if edges represent ecologically marginal habitats. In S. compressa as in virtually all species, the role that population genetic factors play in determining demography at the edges of the geographic range is unknown. In my current position at BML, I am exploring these issues, investigating how dispersal and gene flow among sites may influence S. compressa’s larger geographic distribution and patterns of abundance.  In the field with my phycology students.
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