13.1: The Evolution of Self-Incompatibility
Most people have not spent a lot of time thinking about the sex lives of plants. The classic mode of sexual reproduction in angiosperms (flowering plants) involves pollen (the male gametophyte stage of the plant life cycle). Pollen lands on the pistil (the female reproductive structure) and produces a pollen tube. Sperm cells move down the pollen tube, and one sperm cell unites with the egg to form a new zygote in the ovule.
As you might imagine, plants have little control over what pollen grains land on their pistil (although plant species do have some remarkable adaptations to control pollination by animals; see Anders Nilsson 1992) . In particular, this "standard" mode of reproduction leaves open the possibility of self-pollination, where pollen from a plant fertilizes eggs from the same plant (Stebbins 1950) . Self-fertilization (sometimes called selfing) is a form of asexual reproduction, but one that involves meiosis; as such, there are costs to self-fertilization. The main cost is inbreeding depression, a reduction in offspring fitness associated with recessive deleterious alleles across the genome (Holsinger et al. 1984) .
Some species of angiosperms can avoid self-fertilization through self-incompatibility (Bateman 1952) . In plants with self-incompatibility, the process by which the sperm meets the egg is interrupted at some stage if pollen grains have a genotype that is the same as the parent (e.g. Schopfer et al. 1999) . This prevents selfing – and also prevents sexual reproduction with plants that have the same genotype(s) at loci involved in the process.
Species of angiosperms are about evenly divided between these two states of self-compatibility and self-incompatibility (Igic and Kohn 2006) . Furthermore, self-incompatible species are scattered throughout the phylogenetic tree of angiosperms (Igic and Kohn 2006) .
The evolution of selfing is a good example of a trait that might have a strong effect on diversification rates by altering speciation, extinction, or both. One can easily imagine, for example, how incompatibility loci might facilitate the evolution of reproductive isolation among populations, and how lineages with such loci might diversify at a very different tempo than those without (Goldberg et al. 2010) .
In this chapter, we will learn about a family of models where traits can affect diversification rates. I will also address some of the controversial aspects of these models and how we can improve these approaches in the future.