LEADING IDEAS. “Two major hypotheses have been proposed to explain the evolution of separate sexes. The first hypothesis is that there are trade-offs between male and female function, such as when mating displays enhance male fitness but decrease female fitness. In this case, individuals can gain reproductive advantages by specializing as a male or female.”
http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899