Theories of the Evolution of Sexual Dimorphism

Abstract

In this section of the book (Chaps. 4–8), we turn our attention to the evolution of sexual dimorphism, having earlier considered theoretical models (Chap. 2) and empirical patterns (Chaps. 1 and 3) concerning gender dimorphism. Sexual dimorphism refers to differences between individuals of differing gender in traits other than gender itself (Chap. 1, Table 1), and can be found in both primary and secondary sex characters. Primary sex characters are traits, such as gonads and copulatory organs, which are essential for reproduction (Darwin 1871). In plants, primary sex characters include the androecium and gynoecium (Lloyd and Webb 1977; see Harris and Harris 1994 for definitions of floral parts). Primary sex organs are necessarily dimorphic between male and female sex functions within flowers. In addition, dimorphism is often conspicuous between functionally male and female flowers in diclinous populations, because each flower type fully expresses the sex organs of only one sex function (Chap. 1). Less commonly, functionally male and female flowers and plants retain well-developed sex organs of the opposite sex function, and dimorphism between flower types (and between individual plants) is less obvious (Chaps. 1 and 5).Secondary sex characters are traits that are not “directly connected with the act of reproduction” (Darwin 1871). Darwin recognized two kinds of secondary sex characters, those relating more directly to reproduction and those pertaining to the “different habits of life” of the sexes. In angiospersms, reproductive sex characters encompass features of flowers and inflorescences that affect male and female mating success or fertility (Chap. 5). Ecological sex characters include life history traits (Chap. 6), vegetative morphology and physiology (Chap. 7),and ecological relations such as competitive ability and susceptibility to herbivores and pathogens (Chap. 8). Secondary sex characters are often shared by individuals of different gender class, but may nevertheless differ between them. They sometimes achieve only rudimentary expression in one sex, or are entirely limited in their expression to one sex.