What are sexes, and why are there sexes?

Abstract Sex and genders occupy a central place in the human mind and have a long history of alternative interpretations throughout human cultures. Although intimately linked to reproduction in animals and other multicellular eukaryotes, sex is fundamentally not a reproductive process. This chapter first discusses what sex is and why it evolved, arguing that it is best defined by the presence of meiosis, that it might have originated as a DNA-repair mechanism, and is likely maintained by indirect benefits stemming from recombination.We then present the several kinds of mating categories (mating types, sexes, genders, self-incompatibility systems), discussing their similarities and differences, and the reasons why they exist. We introduce the concepts of sex-ratio selection and sexual selection, which are arguably the main evolutionary forces underlying the dynamics of sex-determination systems. Finally, this chapter we argues against the traditional distinction between sex determination and primary sex differentiation, defining sex determination as the whole process that leads, from undifferentiated gonads or meristems, to the development of differentiated reproductive organs. This broad definition widens the scope of our book, to also include, for example, sex differentiation in simultaneous and sequential hermaphrodites.