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Resource competition and soft selection in Sabatia angularis
Walker, Mark James
Walker, Mark James
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Thesis/Dissertation
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2023-12
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Biology
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http://dx.doi.org/10.34944/dspace/9526
Abstract
Intraspecific competition is a primary ecological interaction in plant populations, and competitive interactions for limited resources in turn are an important driver of natural selection, setting the stage for eco-evo feedbacks. Such feedbacks are likely when the competitive neighborhood individuals experience are heterogeneous and individual fitness is dependent on the competitive neighborhood within which it finds itself. In situations where individual fitness is a function of the density and genetic composition of competitors, selection is considered ‘soft’. The extent to which soft selection occurs in natural populations remains an open question and a critical one, given it can explain phenomena such as the maintenance of genetic load in populations. My dissertation explores the role of competition in regulating fitness by investigating how the density and genetic composition of conspecific competitors impacts juvenile and adult traits in the biennial plant Sabatia angularis. In Chapter 2, I investigate the degree, scale, and nature of density-dependent growth and survival of juveniles in natura. Based on four-years of demographic data, I demonstrate that juvenile size declines significantly with density at the scale of centimeters. Although density did not directly impact juvenile survival, survival was significantly size-dependent. The degree of inequality in juvenile size within ecological neighborhoods did not depend on density, suggesting competitive interactions at this stage are largely symmetric, at least in the population studied. In Chapter 3, I consider whether soft selection can occur via asymmetric competition in the context of inbreeding depression and its population level consequences. As a mixed-mating species with intermediate selfing rates in wild populations and inbreeding depression, S. angularis provides an ideal system to test soft selection. Selfed individuals expressing genetic load due to elevated homozygosity, in theory, should be inferior competitors than outcrossed individuals. However, if soft selection occurs, then demes comprised of purely selfed or purely outcrossed competing individuals should nevertheless exhibit equal fitness. As the importance of competition decreases, so too should the softness of selection such that deme fitness depends more on genetic makeup as density declines. I show that these predictions can be met for juvenile size in S. angularis by experimentally establishing local competitive environments comprising different densities and frequencies of selfed and outcrossed individuals (100% selfed, 100% outcrossed, or 50/50% “mixed”). I found that genetic background strongly predicts mean size at low densities, but that mean size among demes converges as density increases. Additionally, I find that mixed demes exhibit greater size inequality, consistent with asymmetric competition between selfed and outcrossed individuals, providing greater opportunity for selection against selfed individuals. Chapter 4 looks at the role of frequency-dependence and its interaction with density dependence, across both juvenile and adult life history stages. I followed the fate of either selfed or outcrossed individuals in pots at two different densities and a gradient of frequencies of selfed vs. outcrossed competitors (0, 25, 50, 75, 100% selfed). I find evidence of inbreeding depression and frequency-dependence across all adult traits I assessed, such that individuals produced significantly more and larger flowers as the frequency of selfed competitors increased. Moreover, my data suggests that frequency-dependent inbreeding depression may occur in critical floral traits. Consistent with Chapters 2 and 3, juveniles exhibited strong density dependence, but in contrast to Chapter 3, I did not detect frequency dependence among juveniles within the two densities examined. Together, my studies suggest the ingredients for soft selection on juvenile and adult traits– which are subject to pollinator-mediated selection in S. angularis– exist under certain competitive conditions.
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