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多样性，超过7000种。然而，我们对导致这种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一类常见的新热带淡水鱼的进化，并将其作为模型来理解塑造新热带淡水鱼进化的力量。通过多样化的形式和生态习性，慈鱼已经适应了几乎在南美洲和中美洲的所有环境中生活。我们假设，慈鱼的多样性和生态多样性源于早期扩展为新的、可用的生态资源，如食物和栖息地(生态机会)，这些资源起源于独特的、生态专业的属。相比之下，在大多数这些属中，物种往往彼此相似，既有它们的形态(如身体大小和形状)，也有它们的生态偏好(包括栖息地和饮食)。属内物种之间缺乏多样性可能源于生态机会的对立面，即适应性限制。一旦一个血统成为生态专家，它可能无法在与其专门化的环境太不同的条件下生存。这迫使专家的进化路径保持在所谓的“适应高峰”附近，在这个高峰附近，所有的生态、形态和生理条件都有利于其生存。如果进化路径离顶峰太远，物种就无法生存。在这个项目中，我们想要了解生态机会和适应性约束如何产生不同的进化结果。我们将通过研究美洲丽鱼属的主要进化线(属)，并通过比较类群中的两个属：Crenicichla属的“pike cichlids”和Apistogramma属的“Dwarf Cichlids”来实现这一点。梭鱼的生态环境和生态环境截然不同，而侏儒则具有相似的生态环境和形态。虽然这两个属的物种数量相当，但梭鱼可能通过生态机会而多样化，而矮小叶蝉可能在严格的适应限制下进化。这为研究这两种力量如何通过不同的机制产生多样性提供了一个自然的系统。我们将使用成百上千的DNA序列和化石记录来重建进化关系(家谱)和美洲丽鱼、梭鱼和矮小丽鱼起源的时间。然后，我们将使用过时的家谱作为路线图，以了解这些鱼的不同特征是如何以及何时出现在它们的进化史上的。我们将追踪不同种类的慈鱼在形态(取食机制、游泳属性)和生态方面进化变化的速度和形式。通过比较不同种类的慈鱼，我们将了解生态机会和适应性限制如何有助于美洲最多样化的鱼类之一的多样化。了解进化力量如何塑造生物多样性，对于致力于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来说也是相关的。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程度。因此，属于具有严重适应性限制的群体的物种可能无法容忍其环境的变化，而属于较少限制的群体的物种可能更能容忍生态变化。

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.