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.