CAUSES AND CONSEQUENCES OF FLUCTUATING SELECTION IN A STOCHASTIC ENVIRONMENT

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Selection, i.e. the process by which individuals are sorted according to their differences in fitness, is the major force for causing evolution of phenotypic characters. In spite of its importance for structuring biological diversity at all organismic lev els, it is therefore surprising that we still lack understanding of factors influencing variation in the strength of selection in natural populations1. The purpose with the present project is to derive general principles for the operation of selection in natural populations of house sparrows by analysing stochastic processes acting at both ecological and evolutionary time scales using a novel theoretical framework. Such a stochastic approach is necessary because all natural populations are subject to a fl uctuating environment. Here we examine the relationship between the magnitude of environmental stochasticity and the strength of fluctuating selection in 18 island populations of house sparrows. Two alternative hypothesis are likley for such a relati onship: (1) There should be a positive relationship between the environmental variance and the strength of fluctuating selection because deviant phenotypes are more strongly selected against in flcutuating environments. (2) A negative relatioship is likel y if fluctuations in the environment produce large temporal variation in phenotype that maximizes fitness. A problem for estimating the relative contribution of fluctuating environments to ecological and evolutionary processes has been to account for flu ctuations in age structure. Here we will use a novel modelling framework based on the concept of the total reproductive value of the population, which originates back to Fisher. The advantage of this approach is that fluctuations in population size are no t inflated by transient fluctuations in the age structure. This represents an important advance because it allows to introduce stochastic fluctuations at different levels of the selection process.