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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.