| This thesis consists of four models. The first three are modifier models addressing various aspects of the evolution of the sex ratio. In Section 2.1, it is shown that when the sex ratio is determined by a single locus in an effectively infinite population, the even sex ratio predicted by Fisher is unaffected by population subdivision and migration. The possible effect on the evolution of the human sex ratio of a preference for male children is examined in Section 2.2. It is shown that the killing of infant daughters can lead to either a female- or a male-biased sex ratio, the outcome depending on the decision rule used to determine the fate of a child. This reconciles long-standing contradictory results in the literature (Darwin 1874; Shaw 1961; Uyenoyama & Bengtsson 1979; Harada 1989). Finally, it has been argued that hermaphrodites in gynodioecious populations should allocate more resources to male function to compensate for the lack of pollen production in the female part of the population. The model presented in Section 2.3 is a test of this hypothesis. It is found that modifiers affecting the sex allocation in gynodioecious populations can invade only under restrictive conditions. This result is in contrast to the only other previous work on this question. The reasons for this are discussed.; Chapter 3 of this thesis presents the fourth model, which is an extension of the classical two-locus symmetric viability model (Lewontin & Kojima 1960, Bodmer & Parsons 1962). Most classical work on two-locus viability models has assumed that the fitness of double heterozygotes is the same whether they are in cis or trans configuration. This assumption is not likely to be valid, especially if we consider polymorphic sites within the same locus. We study the effects of incorporating cis-trans differences in selection on otherwise standard two-locus models. The main conclusion is that high levels of linkage disequilibrium at equilibrium are much more likely to occur under these conditions than in the standard models. |
