The ecology of evolution: the role of environmental heterogeneity in evolutionary dynamics.

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Although we have a good general understanding of how evolution operates, we have only limited insight into its detailed workings in natural populations. A prominent challenge is to understand the extent to which variation in environmental conditions affects evolutionary change within natural populations. Improved knowledge in this area will greatly advance our understanding of how genetic diversity is maintained in natural populations and of how populations are likely to respond to predicted changes in climate. The characteristics of individuals in a population are a consequence of their genes, the environment in which they live and the evolution that has occurred up to that point. So much is clear, but this simple statement conceals much complexity. Specifically, the consequences of carrying particular genes can vary with environmental conditions; it is possible (but not generally proven) that organisms express more of their genetic potential under good rather than bad environmental conditions. Similarly, the strength of natural selection, whereby some individuals have higher survival or reproductive success than others, may vary with environmental conditions. Since evolutionary change occurs when natural selection acts on inherited characters, environmental fluctuations can therefore have a substantial impact on evolution. Finally, because some of the genes underlying a particular character (e.g. body size) may be the same as, or associated with, the genes underlying another character (e.g. parasite resistance), characters are not free to evolve independently of one another. Instead the effect of natural selection on one character will be constrained by any selection that is also occurring on other characters to which it is genetically tied. Recent research suggests that these constraints also vary with environmental conditions. Currently, we have very little understanding of how these various processes interact to shape evolution in natural populations. In this study we will investigate the effect of environmental conditions on evolutionary processes using data from an unmanaged study population of Soay sheep on the island of St Kilda, NW Scotland. The data set is particularly useful for this study because, firstly, we have records on the individual life histories of several thousand sheep measured across 23 years. DNA profiling has been used to determine paternity and provide a family tree suitable for the kind of genetic analyses proposed here. Secondly, our previous research has demonstrated that there are four kinds environmental variation which show strong temporal variation affecting sheep performance, and we will be investigating all four kinds of variation in this study: the weather, sheep population density, food availability and parasite abundance. We will address the effect of the environmental conditions described above on the expression of genetic variation, the strength of natural selection and the genetic relationships between characters. Specifically we will ask how the environmental affects first, the expression of genetic variation in single characters like body weight: is more genetic variation always expressed when conditions are good? Second does environmental variation affect genetic relationships between characters? For example, are body size and parasite resistance freer to evolve separately under good or bad conditions? Third, is selection stronger or weaker under good environmental conditions? Together these analyses will allow us to predict the course of evolution under specific time series of environmental conditions - for example systematic climate change. Long-term studies of individually-recognised wild animals such as these are an important source of information on many aspects of ecology and evolutionary biology, and the proposed grant would guarantee continuity of data collection for one of the UK's most valuable ecological and evolutionary field data sets.