The genetics of speciation: genes of small effect underlie sexual isolation in the Hawaiian cricket Laupala.

Sexual behaviours often evolve rapidly and are critical for sexual isolation. We suggest that coordinated sexual signals and preferences generate stabilizing selection, favouring the accumulation of many small-effect mutations in sexual communication traits. Rapid radiation of a sexual behaviour used in signalling, song pulse rate, has been observed in the Hawaiian cricket genus Laupala. Using marker-assisted introgression, we isolated five known quantitative trait loci (QTL) influencing species-level differences in pulse rate from one species, L. paranigra, into a closely related species, L. kohalensis. All five QTL were found to have a significant effect on song and appear to be largely additive in backcross introgression lines. Furthermore, all effect sizes were small in magnitude. Our data provide support for the hypothesis that stabilizing selection on sexual signals in Laupala creates genetic conditions favourable to incremental divergence during speciation, through the evolution of alleles of minor rather than major phenotypic effects.