Australasian Science: Australia's authority on science since 1938

The Evolution of Sexes

Credit: Ezume Images

Credit: Ezume Images

By Jack da Silva

Sex does not depend on the existence of different sexes. Instead, males and females may be the result of genetic conflict arising from the evolution of large and complex individuals.

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In the early 1970s, Geoff Parker of The University of Liverpool and colleagues proposed a radical and elegant new theory to explain the evolution of males and females. Then, after some early refinements, the idea settled into a mature theory.

This disruptive selection theory posits that when sexually reproducing organisms become large and complex they benefit from producing large, single-celled embryos that can store nutrients to enable rapid initial development. This provides the opportunity for the evolution of two mating types that produce sex cells of different sizes. One mating type produces many small sex cells that compete for sexual fusions with a second type of individual that produces only a few large sex cells. In the extreme, this results in sperm-producers and egg-producers.

This has been described as the original conflict between the sexes. It is also known as the cost of males, because females make all of the investment in reproduction: a species without sexes would have equal investment from both mating types.

This theory has received some support but has been difficult to distinguish from related competing theories. Recently, however, I derived a specific prediction from a complete mathematical model of the theory and tested it on a group of algae renowned for their cross-species variation in size and complexity. While the test...

The full text of this article can be purchased from Informit.