Of flies, fitness and fluctuating environments

Environmental heterogeneity may be important in determining the amount of genetic variation within a population. Previous theoretical studies have analysed the importance of spatio-temporal variablity for ecological genetics within a very general framework. The present study attempts to analyse the consequences of environmental heterogeneity for a particular ecologically important character. The study is concerned with the evolution of body size in an 'r-selected' poikilotherm. Is body size. The measure of fitness, r, is determined by the fecundity of the organism and its development time. These two life history characters are correlated to body size and hence the latter may be used as a measure of changes in life history parameters, whether or not selection acts directly upon body size. A model is presented that relates the effect of spatial and temporal variation on body size. This effect may be due to direct effects on body size as with size selective predation or due to effects upon other characters such as development time. To demonstrate that the behaviour of the model does not result from implausible assumptions or parameter values the model is developed with reference to an organism for which these factors have been reasonably well studied. This group is the Drosophila and most particularly, Drosophila melanogaster. The conclusions drawn from the model are that spatial and temporal variability can determine both the optimum body size and the range in body size and that 'rare' events may have significantly more effect on the evolution of body size than the most frequently occurring conditions.Science, Faculty ofZoology, Department ofGraduat