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I’m a junior author on a new paper from Erik Hanschen and colleagues, “Multicellularity drives the evolution of sexual traits.”

Figure 3 from Hanschen et al. 2018. Figure 3. Ancestral state reconstructions of six sexual characters. A, Evolution of all (green) and reduced number of (black) meiotic products germinating from a diploid zygospore. B, Evolution of isogamy (green), anisogamy (blue), and oogamy (black). C, Evolution of external (green) and internal (black) fertilization. D, Evolution of normal females (green) and extrafertile females (black). E, Evolution of normal males (green) and dwarf males (black). 

The Am. Nat. website also includes a “lay summary“:

Sex didn’t always involve males and females. In fact, sex existed before males and females, and many species are still doing it without them. Males and females are defined by the different sizes of gametes they produce, and not all species produce two different sizes of gametes. Why sperm-producing males and egg-producing females evolved from ancestors without differentiated gametes is a major problem in evolutionary biology. Work led by Erik Hanschen that involved scientists at the University of Arizona, Georgia Tech, and the University of Tokyo suggests that the evolution of large, multicellular bodies drove the evolution of male and female sexes. Using the volvocine green algae to test mathematical theory dating back to the 1970s, they showed a positive relationship between body size and derived sexual characters, starting with different sized gametes (sperm and eggs). Larger species are more likely to have internal fertilization, as in mammals, while smaller species have external fertilization, as in most fishes. Body size is also related to the evolution of secondary sexual dimorphism, differences between males and females of the same species beyond their gametes. As in many birds, mammals, and other animals, male and female individuals of some large volvocine algae differ in their physical characteristics. The convergence of these patterns between the volvocine algae and other taxa suggests that the same principles apply to the evolution of the sexes in other groups, including plants and animals. The evolution of males and females, and subsequently of secondary sexual dimorphism, may be an inevitable consequence of the evolution of large, multicellular bodies.

Hanschen, E.R., M. D. Herron, J. J. Wiens, H. Nozaki, & R. E. Michod. 2018. Multicellularity drives the evolution of sexual traits. The American Naturalist 192: E93-E105.