Amy Angert

We are just beginning to understand population-level responses to extreme climatic events. Populations of the same species from differing climatic regions can have different traits and adaptations that have locally evolved, and this can alter their evolutionary trajectories when exposed to similar selection pressures. My thesis examines how populations of Mimulus (Erythranthe) cardinalis (scarlet monkeyflower) from historically different climates respond to a severe drought, using a resurrection approach to grow ancestral (pre-drought) and descendant (peak-drought) individuals in a common environment and growing them in either wet or dry treatments. In chapter two, I report that populations in the north (historically wetter and less variable) did not show evolutionary changes at the macro-morphological scale (e.g., date of flowering, specific leaf area), while southern populations (historically drier and more variable) evolved toward trait values that promote dehydration avoidance. In chapter three, I find that macro-morphological changes (or lack thereof) can be better understood by examining more cryptic micro-morphological alterations in leaf architecture and physiology. Here, northern populations evolved greater plasticity in mesophyll composition, stomatal density and assimilation rate, trending toward dehydration avoidance and potentially allowing macro-morphological traits to remain stabilized. Southern populations began with trait values that were more drought avoidant and evolved further along this trajectory, resulting in a trade-off for photosynthesis under wet conditions. Finally, differences in evolutionary trajectories between northern and southern populations can also be attributed to phenotypic legacies from non-genetic inheritance. In chapter four, I grew northern and southern populations for 3 generations under either wet or dry treatments to assess epigenetic stress memory. Northern populations exhibited greater epigenetic-attributed plasticity than the southern populations, but the southern populations evolved epigenetic plasticity, which likely assisted in vegetative avoidance of drought. Thus, although at the macro-scale northern populations did not show a clear trend, anatomical features and grandparental histories show both regions rapidly evolve toward dehydration avoidance and non-genetically respond to drought stress, but through different mechanisms. These results indicate that northern populations are becoming more like ancestral southern populations, while southern populations might be nearing the edge of their adaptive capacity.  

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Understanding the evolution of species distributions is a fundamental goal of evolutionary ecology. With global change comes new challenges in predicting the movement of species ranges and the re-assembly of ecological communities. Importantly, these rapid changes in species distributions can provide unique opportunities for ecology to shape evolution, and for evolution to govern population and community dynamics across space and time. However, much remains unknown about how rapid feedbacks between population ecology and genetics (i.e., eco-evolutionary feedbacks) scale up to govern species distributions and community patterns. To make progress, this thesis tests a series of predictions on how evolution can alter species distributions and community assembly, using the evolution of duckweed species in experimental mesocosms. In Chapter 2, I test how evolution during range expansion alters our ability to predict population spread. With selection from an environmental gradient in space, I found that range expansion speed was more variable, and hence less predictable, across replicate populations. This occurred despite slower population spread and higher population densities at the range front – conditions in which models otherwise predict less variable range expansion speeds due to greater selection over genetic drift. In Chapter 3, I test how community context, specifically interspecific competition, alters adaptation during climate-driven range shifts. I found feedbacks between competition and evolution differed across a species experimental range, with competition limiting adaptive evolution at the range core, but promoting adaptation to climate stress at the range edge. I suggest that competition therefore has the potential to promote evolutionary rescue in response to global change. Lastly, in Chapter 4, I test whether and how mechanisms of coexistence evolve over time, from identical lineages to distinct species. I parametrized coexistence of genetically divergent lineages within and across duckweed species to show that coexistence mechanisms evolve in concert with lineage divergence. While evolutionary biology has mainly focused on mechanisms of genetic isolation, I suggest that feedbacks between genetic isolation and ecological differentiation is key for the origins of coexisting species. Overall, my thesis unpacks the diverse, and often surprising, pathways by which eco-evolutionary feedbacks govern population and community trajectories across space and time.

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The ecological niche is an essential concept for studies in ecology, evolution and biogeography. Geographic distributions are largely determined by species’ ecological niches. In turn, niches evolve via selection stemming from where species occur, which has implications for coexistence and the breadth of environmental tolerance. With modern comparative methods, we can improve our understanding of interactions among niche, range and diversification across spatial scales. To select variables for quantifying niche properties, I first applied generalized linear models with occurrence data of 71 western North American monkeyflowers (Mimulus sensu lato). Then I evaluated the relative importance of four bioclimatic variables by ranking them based on the magnitudes of model-averaged regression coefficients. Thus three out of four bioclimatic variables were identified as important predictors in determining geographic distributions of Mimulus species, while one variables was negligible due to its small effect. To determine how geographic overlap affects niche divergence, I quantified niche divergence for 16 closely related Mimulus species pairs. I found that macrohabitat niche divergence decreased with increasing range overlap, consistent with environmental filtering operating in sympatry and divergent selection operating in allopatry. For species pairs with partially overlapping ranges, greater microhabitat niche divergence was found in sympatry, consistent with competition driving divergence where species interact. Phylogenetic distance was positively related to niche divergence for two macrohabitat axes but negatively related for one microhabitat axis. This suggests increasing coarse-scale niche similarity with increasing sympatry following allopatric speciation, while greater local-scale niche divergence accumulates through time. Given differences in evolutionarily lability of niche axes across spatial scales, I next examined evolutionary trends in niche breadth. For 82 Mimulus species, I converted niche breadths into binary states, generalist or specialist. Then I tested whether niche breadth affected diversification rate and explored evolutionary transitions. My results showed higher diversification rates for generalists and weak generalist-to-specialist trends for three bioclimatic variables, but higher diversification rates for specialists and weak specialist-to-generalist trends for two microhabitat variables. Together, these results suggest that ecology plays an essential role in diversification processes, but underlying mechanisms might differ across spatial scales.

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Every species experiences limits to its geographic distribution on the landscape. Sometimes the barriers that limit geographic ranges are obvious. For example, oceans and topographic features may prevent a species from colonizing the areas beyond them. However, species' distributions frequently end at places on the landscape where no obvious barrier or abrupt shift in the environment occurs, and this raises the question of what limits the range at these edges, both proximately and in evolutionary time. This thesis investigates the contributions of pollination, climate, and gene flow to limiting range edge populations of an annual wildflower, Clarkia pulchella. Pollinators may be important at range edges because many of the proposed characteristics of edge populations (small, isolated, or low density) are also features that might make pollination less reliable and in some cases favour the evolution of self-pollination. I found that climate influences floral morphology and that the capacity of plants to set seed in the absence of pollinators was slightly higher in northern range edge populations. All populations benefit from the service of pollinators. Another factor that may limit populations at geographic range edges is the influence of asymmetric gene flow from central populations, which could prevent local adaptation in range edge populations. Alternatively, edge populations might have low genetic variance and therefore might benefit from gene flow. I tested these competing predictions by simulating gene flow between populations from across the species' range in the greenhouse and planting the progeny into common gardens at the northern range edge. This experiment took place during an extremely warm year. As a result, gene flow from warmer provenances improved performance. I also found a small benefit of gene flow independent of climate. Finally, I found no evidence that environmental differences contribute to genetic differentiation of populations, though geographic distance is a strong predictor of genetic differentiation. Contrary to expectations, genetic variation was higher near the northern range edge. Together, these chapters shed light on important drivers of reproductive success and local adaptation in this species and allow for insights into what processes are likely (or unlikely) to generate range limits.

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