An outstanding goal in speciation research is to trace the mode and tempo of the evolution of barriers to gene flow. Such research benefits from studying incipient speciation, in which speciation between populations has not yet occurred but where multiple potential mechanisms of reproductive isolation (RI: i.e. premating, postmating-prezygotic (PMPZ), and postzygotic barriers) may act. We used such a system to investigate these barriers among allopatric populations of Drosophila montana. In all heteropopulation crosses we found premating (sexual) isolation which was either symmetric or asymmetric depending on the population pair compared. Postmating isolation was particularly strong in crosses involving males from one of the study populations, and while sperm were successfully transferred, stored and motile, we experimentally demonstrated that the majority of eggs produced were unfertilized. Thus, we identified the nature of a PMPZ incompatibility. There was no evidence of intrinsic postzygotic effects. Measures of absolute and relative strengths of pre- and post-mating barriers showed that populations differed in the mode and magnitude of RI barriers. Our results indicate that incipient RI among populations can be driven by different contributions of both premating and PMPZ barriers occurring between different population pairs and without the evolution of postzygotic barriers.

Jennings et al 2014 associated DATA DRYAD deposit in Tab delimited formatThis data file consists of the data used for analysis and subsequent display. For each table/figure generated, the spreadsheet indicates the data that were used. The ReadMe file describes the data for each set of columns corresponding to display items in the paper. Read me text is listed for rows and columns when imported into Excel as a comma delimited file.Jennings et al 2014 Dryad text.txt