Most of us take for granted three facts about our gender: (1) that we each produce only male or female gametes, not both; (2) that about half of us are male and half are female; and (3) that this is a result of the mechanism of sex determination known as male heterogamety. In male heterogamety, possession of two similar (X) chromosomes determines a female individual, whereas possession of two non-identical chromosomes (X and Y) determines a male. Because haploid sperm are produced by fair (Mendelian) segregation of the male sex chromosomes, half produce daughters and half sons. This keeps the sex ratio at approximate equality.
In the natural world as a whole however, none of these facts may be taken for granted; many organisms are cosexual, or display biased sex ratios, or have different sex determining mechanisms. Instead of assuming the above three facts, we should instead ask the following three questions:
1. What makes an organism dioecious as opposed to cosexual?
2. What favours equal or biased reproductive investment in the sexes?
3. How do sex determining systems evolve and why?
This chapter is primarily about answering these three questions. Sex allocation (the first two questions) is often considered the 'jewel in the crown of evolutionary ecology' (West and Herre 2002). It was one of the first fields to clearly match a knowledge of ecological circumstances to evolutionary outcomes. For this reason alone it must be discussed. However, recent advances are suggesting a far more interesting role for sex allocation: as a driver of other evolutionary and ecological phenomena. The chapter will explore this possibility as well.
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