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

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.