Evolution of reproductive isolation in a long-term evolution experiment with Drosophila melanogaster: 30 years of divergent life history selection.

We ask if three decades and over 1 500 generations of divergent life history selection on age at reproduction has resulted in the evolution of reproductive isolation (RI) between laboratory populations of Drosophila melanogaster. We tested for premating, postmating-prezygotic and postzygotic reproductive isolation between 3 replicate population pairs. Large evolved differences in body size between selection treatments suggested the potential for prezygotic barriers driven by sexual selection or physical incompatibilities between the sexes. Although a simple prediction would be preference for larger size, creating directional isolation, our results from individual mate choice trials indicate that populations from both selection treatments show a marked bias towards homotypic mate choice; indicative of prezygotic RI driven by sexual selection or sexual conflict. Hybridization between the focal populations resulted in the production of viable adult flies with intermediate size and developmental traits. We observed a suggestive but statistically non-significant trend of fitness decline in the F2 generation of hybrids, but no significant evidence suggesting the evolution of postmating-prezygotic or postzygotic RI. Our findings are in accord with extant literature that posits that premating RI evolves before postmating forms of RI.