Gametic selection favours polyandry and selfing

Competition among pollen or sperm (gametic selection) can cause evolution. Mating systems shape the intensity of gametic selection by determining the competitors involved, which can in turn cause the mating system itself to evolve. We model the bidirectional relationship between gametic selection and mating systems, focusing on variation in female mating frequency (monandry-polyandry) and self-fertilisation (selfing-outcrossing). First, we find that monandry and selfing both reduce the efficiency of gametic selection in removing deleterious alleles. This means that selfing can increase mutation load, in contrast to cases without gametic selection where selfing purges deleterious mutations and decreases mutation load. Second, we explore how mating systems evolve via their effect on gametic selection. By manipulating gametic selection, polyandry can evolve to increase the fitness of the offspring produced. However, this indirect advantage of post-copulatory sexual selection is weak and is likely to be overwhelmed by any direct fitness effects of mating systems. Nevertheless, gametic selection can be potentially decisive for selfing evolution because it significantly reduces inbreeding depression, which favours selfing. Thus, the presence of gametic selection could be a key factor driving selfing evolution. Animals and plants often produce millions of sperm or pollen, of which only a tiny fraction succeed. This means sperm and pollen harbour a lot of evolutionary potential, even though they are generally small and hidden from view. We used mathematical models to understand the evolution of genes that influence the fertilisation success rate of sperm or pollen. A crucial issue is the number of different males who donate sperm or pollen, which is called the mating system. We can predict how genes will evolve with different mating systems. These predictions can then be tested by looking at DNA sequences from genes that are active in sperm or pollen in species with different mating systems. This process allows us to check our understanding of evolution. Secondly, we looked at the evolution of the mating behaviour itself. By changing their mating behaviour, animals and plants can manipulate natural selection on sperm or pollen, thereby evolving to change the way evolution operates. This helps to answer a bigger evolutionary question: why do organisms evolve differences in the way they respond to natural selection?