Ecological constraints on phenotypic evolution: a case study with Lake Malawi's 'prototype' cichlid fish

通过研究“模型系统”，我们正在更好地理解生物多样性是如何通过性选择和自然选择进化和维持的。然而，我们对为什么新物种的进化在某些环境中发生，而在其他环境中却失败知之甚少。例如，许多鱼类在湖泊中进化出了新的形态，但在河流中却没有，包括棘鱼、鳟鱼、鲈鱼和鲶鱼。了解这一点可以帮助我们了解环境如何调节发生的进化类型，并解决未来的环境变化，例如极端事件的频率和外来物种的存在，如何改变进化途径。调节种群规模的过程影响后代之间的竞争强度，并决定哪一个个体最终可以交配。如果种群数量受到空间或食物的限制，那么种群成员之间的竞争将会很激烈，而在这种环境下，产生良好竞争对手的幼鱼应该是值得的，从而导致当地专门鱼类的进化。在这样的栖息地，分散到这些专业化效率较低的地方不一定值得。如果种群数量受到偶发的灾难性事件（如干旱）的限制，自然选择就会青睐那些在机会出现时抓住机会的通才。在这些生境中，选择可能有利于高度分散的进化，以防止雌性在不利条件出现时失去所有后代。在马拉维湖，数百种慈鲷鱼在湖中进化，但在周围的河流中却没有发生这种情况，但我们对原因没有明确的解释。拟议的研究将探讨由对比环境施加的选择类型是否影响了进化的模式和速度。这项工作将研究马拉维鱼鱼辐射中唯一存在于河流和湖泊中的鱼，Astatotilapia calliptera（=“原型”），这是一种在种群之间具有功能形态学差异（牙齿，颌骨）的物种。该项目将包括四个组成部分：1)实地调查，以测试河流和湖泊的慈鲷种群数量是否受到不同的控制，河流鱼类种群数量受到年度灾难性事件的影响，而湖泊鱼类种群数量在很长一段时间内是稳定的，受食物和空间可用性的控制。2)利用遗传（微卫星）标记定量分析河流和湖泊种群的扩散。3)河湖种群的形态和生态专门化分析。这将包括在野外收集鱼并测量功能特征。为了检验这些差异是否是自然选择的结果，来自四个种群的鱼将在实验室水族箱的一个普通花园实验中饲养，从而量化由潜在遗传差异解释的变异程度。4)利用遗传（AFLP）标记对种群的进化关系进行研究，从而能够测试相同的形态是否只进化了一次，或者可能是并行进化了多次。综合这些信息将：a)有助于解释为什么新物种在某些环境中进化，而在其他环境中却没有；B)有助于研究环境变化如何影响进化的模式系统的发展；C)有助于解释区域空间尺度上生物多样性的模式，d)有助于确定进化过程如何被现代环境变化所改变。布里斯托尔大学和自然历史博物馆的三位相对较新任命的学者的技能，经验和热情将为学生提供一个积极的学习环境。除了将提供科学专门知识方面的一般培训外，还将获得许多技能方面的具体培训，包括热带实地考察、数量形态学、生态调查、人口遗传学、实验生态学和分子系统遗传学。

By studying 'model systems' we are gaining a better understanding of how biological diversity has evolved and is maintained by sexual and natural selection. However, we know little about why evolution of new species takes place in some environments, but fails in others. For example, many fishes have evolved new forms in lakes, but not in rivers, including sticklebacks, trout, perches and catfishes. Understanding this may help us to know how environments mediate the type of evolution that takes place, and address how future environmental changes, for example in frequency of extreme events and presence of alien species, may change evolutionary pathways. The type of process that mediates population size influences the strength of competition among offspring, and decides which individuals eventually get to mate. If populations are limited by space or food, then competition between members of the population will be strong, and it should pay to produce young that are good competitors for that environment, leading to the evolution of locally specialised fish. In such habitats it would not necessarily pay to disperse to locations where these specialisations are less effective. If populations are limited by occasional catastrophic events, such as droughts, selection will favour generalists that take opportunities as and when they arise. In these habitats selection may favour evolution of high dispersal, to prevent females losing all offspring should unfavourable conditions arise. In Lake Malawi hundreds of cichlid fish species have evolved in the lake, but this has not happened in surrounding rivers, but we have no clear explanation for why. The proposed research will explore if the types of selection imposed by contrasting environments have influenced patterns and rates of evolution. The work will study the only fish in the Malawi cichlid radiation that is present in both rivers and the lake, Astatotilapia calliptera (=the 'prototype'), a species with functional morphological differences (tooth, jaw) between populations. The studentship will involve four components: 1) field surveys to test if river and lake cichlid population sizes are controlled differently, with river fish populations being subject to annual catastrophic events, while lake fish are stable over long periods of time and controlled by food and space availability. 2) an analysis of dispersal of river and lake populations, by quantifying gene flow using genetic (microsatellite) markers. 3) an analysis of morphological and ecological specialisation in river and lake populations. This will involve collecting fish in the field and measuring functional traits. To test if differences are a consequence of selection, fish from four populations will be reared in a common garden experiment in laboratory aquaria, enabling quantification of the extent of variation that is explained by underlying genetic differences. 4) A study of evolutionary relationships of populations using genetic (AFLP) markers, enabling tests of whether the same morphology has evolved only once, or perhaps multiple times in parallel. Together this information will: a) help explain why new species evolve in some environments, but not in others; b) contribute to the development of a model system for studying how environmental variability influences evolution; c) help explain patterns of biodiversity over regional spatial scales, and d) help determine how evolutionary processes may be altered by modern environmental changes. The skills, experience and enthusiasm of three relatively newly appointed academics of the University of Bristol and the Natural History Museum will provide a positive learning environment for the student. In addition to generic training in scientific expertise that will be provided, specific training will be gained in numerous skills, including tropical fieldwork, quantitative morphology, ecological surveys, population genetics, experimental ecology and molecular phylogenetics.