An Integrative Approach to the Ecological and Evolutionary Causes of Geographic Range Limits

这项拟议的研究将调查鱼类物种对盐度变化的生理反应。淡水生态系统盐碱化已被确定为对生态系统商品和服务的一个重要全球威胁。然而，关于盐度增加将如何影响淡水社区和生态系统，人们知之甚少。这项拟议的研究将有助于从机制上理解盐度如何改变一系列生理和行为特征，以及这些变化如何反过来塑造地理分布和鱼类群落结构。拟议的研究还包括一个以中学生为重点的教育推广部分。课堂上将使用活鱼，通过基于探究的方法向学生传授生物概念。对试点项目的评估表明，学生对生物学概念的理解有所提高，考试成绩也有所提高。自然界中的所有物种都显示出有限的地理分布。事实上，生态学和生理学研究最基本的目标之一就是解释为什么某些生物会出现在某些环境中，而不是其他环境中。地理范围的限制也提出了一个基本的进化问题；为什么种群不随着时间的推移进化到占据更广泛的环境。对于大多数生物体来说，这些简单的问题在很大程度上仍然没有答案，这限制了我们预测生物体如何应对环境变化的能力。这一挑战之所以持续存在，部分是因为地理范围限制的原因需要了解环境中多种非生物和生物因素是如何变化的，以及不同特征的套件如何应对和适应这些挑战。在本文提出的研究中，调查了近缘种热带鱼的分布范围。当这些鱼一起出现时，它们沿着盐度梯度分开，但当它们单独出现时，它们跨越了广泛的盐度范围，这表明耐盐性和竞争性相互作用共同塑造了分布界限和群落结构。这项拟议的研究侧重于渗透调节、行为攻击、荷尔蒙、代谢范围和运动能力如何相互作用。这些特征通常是单独研究的，然而，他们的联合研究揭示了关于共享机制如何以导致地理范围限制的方式影响多个特征的新假设。这项拟议的研究将首先测试1)盐度压力、物种间竞争和捕食压力的高能量成本如何使某些环境过于昂贵而无法维持，以及2)与渗透调节相关的荷尔蒙变化如何影响竞争能力和食物获取。其次，拟议的研究将结合实验室育种实验、基因组方法和人工选择实验，探索基因流、加性遗传变异和负遗传相关性作为渗透调节和竞争能力的进化制约因素的作用。总而言之，拟议的研究不仅提供了一个测试地理范围限制的生态生理和进化原因的机会，而且还提供了一个可以更广泛应用的综合框架。

The proposed research will investigate the physiological responses of fish species to altered salinity. Salinization of freshwater ecosystems has been identified as an important global threat to ecosystem goods and services. Yet very little is known about how increased salinity will impact freshwater communities and ecosystems. The proposed research will contribute a mechanistic understanding on how salinity alters suites of physiological and behavioral traits and how these changes in turn shape geographic distributions and fish community structure. The proposed research also includes an educational outreach component focused on middle school students. Live fish will be used in classrooms to teach students biological concepts through inquiry-based approaches. Assessments of a pilot program have shown improved student understanding of biological concepts and increased test scores.All species in nature show a limited geographical distribution. Indeed, one of the most fundamental goals of ecological and physiological research is to explain why some organisms occur in some environments and not others. Geographic range limits also pose a fundamental evolutionary question; why don't populations evolve over time to occupy a wider range of environments. Such simple questions remain largely unanswered for most organisms, limiting our ability to predict how organisms respond to environmental variation. The challenge persists in part because the causes of geographic range limits requires an understanding of how multiple abiotic and biotic factors covary in the environment, and how suites of different traits respond and adapt to these challenges. In the research proposed here, the distributional limits of closely related tropical fish species in the genus Poecilia are investigated. When these fish occur together they segregate along a salinity gradient, but when they occur alone they span a broad range of salinities, suggesting salinity tolerance and competitive interactions jointly shape distributional limits and community structure. The proposed research focuses on how osmoregulation, behavioral aggression, hormones, metabolic scope, and locomotor performance interact with each other. These traits are typically studied separately, however, their joint study reveals new hypotheses on how shared mechanisms can influence multiple traits in ways that lead to geographic range limits. The proposed research will use lab and field experiments to first test 1) how elevated energetic costs of salinity stress, interspecific competition, and predation pressure may make some environments too expensive to persist in, and 2) how hormonal changes associated with osmoregulation may compromise competitive ability and food acquisition. Second, the proposed research will explore the role of gene flow, additive genetic variation, and negative genetic correlations as evolutionary constraints on osmoregulation and competitive ability using a combination of lab breeding experiments, genomic approaches, and artificial selection experiments. Collectively, the proposed research not only offers an opportunity to test the eco-physiological and evolutionary causes of geographic range limits, but also provides an integrative framework that can be applied more broadly.