Evolutionary and ecological implications of sexual reproduction

Abstract

For a large part of our evolutionary history, all life on earth reproduced without sex. Today, the vast majority of visible life is dependent on sexual reproduction. My thesis examines the consequences of this change in reproductive mode from an evolutionary and ecological perspective, using a combination of mathematical models, empirical data and literature reviews. Why sex is so common is still something of a mystery, as it seems less efficient than asexual reproduction. The first chapter in the thesis takes a closer look at the various fitness costs that make the persistence of sexual reproduction so difficult to explain. For example, if a female simply made female clones of itself, it would, in principle, have a twofold reproductive advantage over its sexual competitors. Sexual reproduction does not imply the existence of males and females. Males are defined as the reproductive type that produces the smaller gametes, and in ancestral sexual reproduction all gametes were of equal size, as is the case in many extant species. The divergence of males and females is therefore synonymous with the evolution of anisogamy, or divergence of gamete sizes. In the second chapter, I develop a mathematical model on the evolution of anisogamy, unifying some aspects of earlier work into one mathematical framework. In many sexual processes there can be more than one alternative evolutionary outcome. The divergence of the sexes is one example: either one of the ancestral reproductive types could end up being the male or female. In the third chapter, I discuss several other examples of alternative evolutionary stable states and the processes that lead to them. I also develop theory on the evolution of parental care, and show that this too is a trait that can reach different evolutionary outcomes, depending on the initial conditions and the sequence of evolutionary events. Sexual conflict, the subject of the fourth chapter, is a result of different evolutionary optima in males and females. This can lead to conflict because the majority of the genome is shared by the two sexes. The problem I address in this chapter is the maintenance of sexually antagonistic genes. In many cases, the theoretically expected outcome is that one allele is driven to fixation for a trait under sexually antagonistic selection, yet allelic variation exists in nature. Using a combination of empirical and theoretical work I show that negative frequency dependent selection of sexually antagonistic genes is one process that can maintain this variation. In the final chapter I review an unusual class of organisms that fall between purely sexual and asexual reproduction. Sexually parasitic animals and plants engage in mating that seems superficially normal, but at the genetic level they propagate their own genome in an unfair manner, cloning either all or half of their genome. This can lead to complicated evolutionary and ecological host-parasite dynamics, and in many cases both the host and parasite are theoretically predicted to be driven to extinction. Show more