Ontogeny, plasticity and phenotypic evolution in the wild

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Evolution is expected to occur when natural selection acts on a trait that has a genetic basis of determination. For example, in many organisms survival and reproductive success depend on body size, a trait which is known to be determined, at least in part, by an individual's genetic makeup. If we can measure the way that selection is acting and also quantify the amount of variation in the population that has a genetic basis, then theoretical models can be used to predict how the trait will evolve over time. While such predictions work well in laboratory studies, results have been more mixed in studies of natural populations. One reason for this is that existing theoretical models do not really capture the true complexities of wild systems. In particular they tend to ignore changes that may occur over ontogeny (or age), and they also assume that the population is in a stable environment. The second of these might well be a reasonable assumption under artificial conditions but is likely to be violated, to a greater of lesser extent, in almost all natural systems. Importantly, age and environmental variation can not only influence the trait itself (e.g., body size increases with age and is often higher in a better environment), but also the way in which genes act to determine the trait. While experimental work has shown that these 'genotype-by-environment' interactions are common, they have rarely been quantified in nature and very little is known about the role they play in determining evolutionary processes in the wild. The primary aim of the proposed project is to address this gap in our knowledge. To do this I will firstly develop and extend existing analytical models, in order to simultaneously incorporate ontogeny and environmental variation. Secondly I will apply these models to data from a long-term study of feral sheep on the Scottish island of Hirta in the St. Kilda archipelago. In particular, I will study the traits of body size and female reproductive strategy which are already known to be under selection in this system. This is a population of long-lived animals that is characterised by very dramatic fluctuations in environmental conditions (climate and density), and consequently it represents an ideal case-study for examining ontogenetic and environmental influences on evolution. By analysing the way in which the trait, its genetic basis of variation, and the nature of natural selection on it may all change with age and environment, it is expected that this work will provide great insights into the evolutionary processes that shape biodiversity in the wild.