Macroevolutionary patterns and processes in Neotropical freshwater fishes: cichlids as a model

南美洲和中美洲被称为新热带地区，拥有地球上最高的淡水鱼类多样性，超过7000种。然而，我们对产生如此令人难以置信的多样性的进化过程知之甚少。我和我的学生研究了新热带区淡水鱼的一个共同群体--慈鲷科（Cichlidae）的进化，并将其作为一个模型来理解塑造新热带区淡水鱼进化的力量。慈鲷已经适应了生活在南美洲和中美洲的几乎每一个环境，通过多样化成各种各样的形式和生态习惯。我们假设，慈鲷的多样性和生态多样性，导致早期扩展到新的，可用的生态资源，如食物和栖息地（生态机会），起源不同的，生态专业化的属。相比之下，在大多数属中，物种往往彼此相似，共享它们的形态（如身体大小和形状）和生态偏好（包括栖息地和饮食）。这种属内物种多样性的缺乏可能源于生态机会的对立面，适应性约束。一旦一个谱系成为生态专家，它可能无法在与其专业化的条件相差太大的条件下生存。这迫使专家的进化路径保持接近所谓的“适应性高峰”，所有生态，形态和生理条件都有利于其生存。如果进化的道路远离顶峰，物种就无法生存。在这个项目中，我们希望了解生态机会和适应性约束如何产生不同的进化结果。我们将通过研究美国慈鲷科的主要进化系（属），并通过比较组内的两个属：“派克慈鲷”在属Crenicichla，和“侏儒慈鲷”在属Apistogramma。派克慈鲷已经多样化到显着不同的形式和生态位，而侏儒慈鲷共享相似的生态和形态。虽然这两个属的物种数量相当，派克慈鲷可能已经多样化，通过生态机会，而矮慈鲷可能会演变严重的适应性约束。这提供了一个自然的系统来研究这两种力量如何通过不同的机制产生多样性。我们将使用数百个DNA序列和化石记录来重建进化关系（家谱）以及美洲慈鲷，派克慈鲷和侏儒慈鲷的起源时间。然后，我们将使用过时的家谱作为路线图，以了解这些鱼类的不同特征是如何以及何时出现在它们的进化历史中的。我们将追踪慈鲷科不同类群的形态（摄食机制、游泳属性）和生态的进化变化的速度和形式。通过比较不同群体的慈鲷，我们将了解如何生态的机会和适应性的限制有助于多样化的一个最多样化的鱼类在美洲的家庭。了解进化力量如何塑造生物多样性也与从事保护或试图了解气候变化或其他环境变化的长期影响的生物学家有关。物种对环境变化的反应取决于它们对不同生态条件的适应能力。因此，属于具有严重适应性限制的群体的物种可能无法忍受环境的变化，而在限制较少的群体中的物种可能对生态变化更具容忍力。

South and Central America, known as the Neotropical region, have the highest freshwater fish diversity anywhere on the planet, exceeding 7000 species. Yet we know very little about the evolutionary processes that originated such incredible diversity. My students and I study the evolution of a common group of Neotropical freshwater fishes, the family Cichlidae, and use it as a model to understand the forces that have shaped the evolution of Neotropical freshwater fishes. Cichlids have adapted to live in nearly every environment of South and Central America by diversifying into a large variety of forms and ecological habits. We hypothesize that the diversity and ecological versatility of cichlids resulted from early expansions into new, available ecological resources such as food and habitats (ecological opportunity), originating distinct, ecologically specialized genera. Contrastingly, within most of those genera, species tend to be similar to each other, sharing both their morphology (like body size and shape) and their ecological preferences (including habitat and diet). This lack of diversity among species within genera may stem from the opposite of ecological opportunity, adaptive constraint. Once a lineage becomes an ecological specialist, it may not be able to survive under conditions too different from those to which it is specialized. This forces the evolutionary path of the specialist to remain close to a so-called “adaptive peak”, around which all ecological, morphological and physiological conditions favor its survival. If the evolutionary path moves too far away from the peak, the species cannot survive. In this project, we want to understand how ecological opportunity and adaptive constraint produce different evolutionary outcomes. We will do this by studying the major evolutionary lines (genera) of American cichlids, and by comparing two genera within the group: “pike cichlids” in the genus Crenicichla, and “dwarf cichlids” in the genus Apistogramma. Pike cichlids have diversified into remarkably different forms and ecological niches, while the dwarf cichlids share similar ecologies and morphologies. Although both genera have comparable numbers of species, pike cichlids may have diversified through ecological opportunity, whereas dwarf cichlids may be evolving under severe adaptive constraint. This provides a natural system to study how these two forces can generate diversity through different mechanisms. We will use hundreds of DNA sequences and the fossil record to reconstruct the evolutionary relationships (the family tree) and the when American cichlids, pike cichlids, and dwarf cichlids originated. We will then use the dated family trees as roadmaps to understand how and when different features of these fishes appeared in their evolutionary history. We will trace the speed and form of evolutionary change in morphology (feeding mechanisms, swimming attributes) and ecology of the different groups of cichlids. By comparing different groups of cichlids, we will learn how ecological opportunity and adaptive constraint contributed to the diversification of one the most diverse families of fishes in the Americas. Understanding how evolutionary forces shape biological diversity is also relevant for biologists working on conservation or trying to understand the long term effects of climate change or other environmental transformations. How species respond to environmental changes depends on how adaptable they are to varying ecological conditions. It follows that species belonging in groups with severe adaptive constraints may not be able to tolerate changes to their environment, whereas species in less constrained groups may be more tolerant of ecological transformations.