Secondary sexual characteristics such as peacocks' tails and the male external genitalia of insects are known to be among the fastest evolving animal body parts.

It is thought that this is driven by sexual selection including through female choice and the different evolutionary needs of each sex to find the right mate and maximize their fitness.

Now scientists at Durham University and Oxford Brookes University, UK, have found one of the genes that contributes to genital evolution between two closely related species of fruit fly. The study is published in the journal Current Biology.

The researchers looked at the posterior lobes of the male genitalia of Drosophila simulans and Drosophila mauritiana. Both are species of the Drosophila melanogaster subgroup of fruit flies.

These lobes have rapidly changed in shape and size in less than 240,000 years and those of Drosophila simulans are much bigger than those of Drosophila mauritiana.

They found that evolved higher levels of the gene Sox21b repressed the size of the genital lobes in Drosophila mauritiana, contributing to them having smaller genital lobes than Drosophila simulans.

As the posterior lobes are used by the male to grasp female flies during sex, the researchers say their findings could now help to better understand the effect sexual selection has on the genome to drive changes in genitalia shape and size.