The genetics of competition: does resource limitation constrain evolution?

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Competition occurs among animals when resources (e.g., food, mates, space) are limited. Although their impacts on growth and reproduction have long interested population biologists, competitive interactions may also have profound, but largely unrecognised, implications for our understanding of how evolution works. This is because while we normally think of competition as arising from purely environmental effects (e.g. lack of food), its outcome (i.e., which animal 'wins') may depend critically on genes. This causes problems for biologists by breaking down the traditional separation of genetic and environmental effects which is the starting point for most studies of evolution. Competition can be thought of as an environmental effect, but we need to acknowledge that it is one with the potential to evolve in its own right. This possibility is not usually accounted for by the theory underpinning evolutionary studies, and may actually have enormous consequences for many fields of pure and applied research - from ecology and animal behaviour, to livestock breeding and the promotion of animal welfare. For example, imagine a gene that influences an animal's growth through effects on foraging - with some versions of the gene (termed 'alleles') making individuals better than average foragers, and therefore likely to grow faster. We might expect that selection of individuals carrying these alleles would result in evolution of faster growth. However, if food is limited, then a gene that increases one individual's growth by allowing it to dominate access to a resource, would actually cause other individuals to get less to eat, and therefore grow more slowly. Seen from this point of view, selection of faster growing individuals would lead to a more competitive environment which would act to counter (and possibly even reverse) our initial expectation that growth rate will increase. This type of process could impose limits on the extent to which traits respond to selection, either natural selection occurring in a wild animal population, or artificial selection being imposed by an animal breeder. Additionally, since competition often involves aggressive behaviours (e.g., animals fighting over a limited resource), any selection that inadvertently leads to a more competitive environment may also be detrimental to animal welfare. Although it is therefore clear that competition could have enormous implications for the evolution of resource-dependent traits, we currently have very little information about how widespread or large these effects actually are. While methodological limitations have restricted progress to date, a number of recently developed statistical approaches mean that these questions can now be addressed. I therefore propose to conduct a comprehensive investigation of the genetics of competition, assessing the role that it plays in shaping, and constraining the evolution of growth and other resource-limited traits. To do this I will take two approaches. Firstly I will perform a series of experiments on a laboratory population of fish (namely the green swordtail, Xiphophorus helleri). This will allow me to directly manipulate key variables such as food availability, the number of competing individuals, and relationships among competitors thereby testing several hypotheses that stem from theoretical predictions. Secondly, I will apply novel statistical methods to test for genetic variation in competitive ability in two commercial livestock populations (Atlantic salmon, chickens). Use of these two high quality data sets will complement the experimental work, and also allow a direct assessment of the applied potential for this direction of research.