Agata Plesnar-Bielak

Agata Plesnar-Bielak

The effects of sexual selection on purging mutation load, adaptation rate and reproductive isolation in the bulb mite Rizoglyphus robini.

Sexual selection may play an important role in numerous evolutionary processes. Condition dependence of male reproductive success has been hypothesized to lead to an increase in both the effectiveness of purging mutation load and the rate of adaptation to a new environment in populations in which sexual selection is intense. On the other hand, sexual selection decreases effective population size, making populations more susceptible to genetic drift and hence hampering adaptation and decreasing the efficiency of purging mutation load. In addition, traits increasing male competitiveness may trade-off with other traits related to fitness and, what is more, adaptations for sexual selection in males may impose costs on females in terms of lifetime fecundity and lifespan, decreasing their fitness. In such cases direction of sexual selection may not be in line with other selection factors, making adaptation to a new environment harder. This makes the role of sexual selection in both adaptation and mutation load reduction difficult to predict and experimental approach is needed to determine its net effects on these processes. Such approach is used in this study using bulb mite Rhizoglyphus robini.

In Chapter 1 of this thesis I investigate the role of sexual selection in purging genome of mutations induced by ionizing radiation. I conclude that in the presence of other natural selection components its impact is not detectible, even though when ecological selection is eliminated it can be significant as shown in a previous study. This suggests that the impact of sexual selection is relatively small and does not add to natural selection in purging mutation load.

In Chapter 2 I use experimental evolution approach to test the prediction that sexual selection facilitates adaptation to a novel environment. I show that populations with intense sexual selection not only adapt faster to new thermal conditions than populations in which sexual selection is eliminated, but also that mating system may have drastic effects on the probability of extinction, making polygamous populations more likely to survive environmental challenge.

Sexual selection may influence speciation. In addition to affecting the rate of local adaptation and the risk of extinction, it may also lead to reproductive isolation in allopatric populations subjected to sexually antagonistic coevolution. Theoretical models predict that reproductive barriers can evolve rapidly through sexual conflict. However, the question how widespread and general this phenomenon is, remains controversial.

In Chapter 3 I investigate this problem using experimental evolution with populations evolving under different levels of conflict. I demonstrate that after 35 and 45 generations of the experiment neither populations subjected to high sexual conflict nor those in which the conflict was eliminated evolved significant reproductive isolation. This indicates that sexual conflict does not necessarily drive fast reproductive isolation and it may not be a ubiquitous mechanism leading to speciation, which seems to be in line with those models that incorporate the cost of traits under sexual selection.