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Content archived on 2024-05-29

Consequences of strongly blased sex ratios for evolution and ecology

Final Activity Report Summary - SEX RATIO AND EVOLUT (Consequences of strongly blased sex ratios for evolution and ecology)

Many natural plant and animal populations harbour selfish genetic elements that distort the sex ratio of the host population. By their effect on the sex ratio, these elements alter the direction and intensity of competition and conflict between male and female hosts and are hence expected to have a profound influence on host traits associated with reproduction. This project investigated the effect of sex ratio distorters on the evolution of reproductive characters of their hosts. Taking an experimental approach, we subjected populations of a standard laboratory model organism, the fruit fly Drosophila melanogaster, to different degrees of sex ratio bias. This was done over roughly thirty generations, during which the populations could adapt to the altered conditions.

Our work has shown that sex ratio distortion has a significant effect on host reproductive biology. Despite the relatively short duration of our experiment, we observed changes in both behaviour and reproductive morphology of the flies in response to sex ratio bias. These changes were found to increase the overall productivity of populations harbouring sex ratio distorters. This means that adaptation to a biased sex ratio tends to stabilise populations and reduce elevated extinction risk otherwise associated with sex ratio bias. Importantly, our results also showed that adaptation can be impeded by the increase of genetic drift associated with sex ratio bias. In population with a very uneven sex ratio, evolutionary change can be dominated by stochastic fluctuations in gene frequencies. In small populations or with extreme sex ratio bias, these random effects can outweigh directional evolutionary change and prevent a response to selection. Our experiments demonstrated that, in agreement with this theoretical expectation, adaptation to sex ratio bias requires a large population size.