The Effect of Sexual Selection on Cline Patterns in Biological Traits

Abstract

Natural selection makes it possible for biological populations to adapt to their local environmental conditions. A species living in habitats with contrasting conditions may evolve differences in biological traits in the different habitats. If gene flow exists between two differently adapted populations a biological trait that is under selection may exhibit a gradual change in space. This gradual change in a biological trait over space is commonly referred to as cline in the trait. Examples include clines in size or thickness of shell of individuals of the marine snail Littorina saxatilis. Empirical studies of clines in different traits in L. saxatilis have found that the midpoint of the cline in size is significantly shifted from the geographic position of the environmental change, whereas no significant shift is found in other clines. This is puzzling since typically theoretical studies find that on average the midpoint of a cline coincides with the geographic position of environmental change. The shift occurs in the same direction in three independent populations. Therefore it is unlikely that it is detected by chance. The aim of this Master thesis is to investigate whether such a shift could result from sexual selection. To this end, two models with different types of sexual selection are presented. One model is based on assortative mating, and the other on that the reproductive success of males depends on size. The use of these models is motivated by data from recent laboratory experiments performed within the Linnaeus Centre for Marine Evolutionary Biology, University of Gothenburg (data obtained and analysed with permission from Kerstin Johannesson). In this thesis it is found that assortative mating does not produce a shift of the midpoint of a cline. By contrast, it is shown in this thesis that the model in which the reproductive success of males increases with increasing size gives rise to a shift of the midpoint of the resulting size cline. The shift appears during a transient state which lasts for up to 105 generations under the parameters tested here. The shift, however, disappears when the system reaches a steady state. Considering that Littorina saxatilis colonised its current habitat after the last glacial period, which ended about 10000 years ago, L. saxatilis is likely to be in the transient state. If so, a shift caused by sexual selection is expected to be observed today. It remains to be understood whether the midpoint of the clines in other traits is affected by a physical linkage of the genes underlying them to the genes underlying the trait that is under sexual selection.