Boughman JW. (2007) Condition dependent expression of red color differs between stickleback species. Journal of Evolutionary Biology. 20 (4): 1577-1590.
Sexual isolation may arise when male mating traits and female preferences differ between species. Such divergence in mating traits is likely to occur when the strength or targets of sexual selection differ. Therefore, by comparing the traits under sexual selection in closely related species and the nature of preference for those traits we can gain insight into when sexual selection contributes to sexual isolation and how it does so. Collecting these data is no easy undertaking. To simplify this comparison, I use the presence and extent of condition dependence in traits to determine whether directional sexual selection is acting on them. Condition dependence thus serves as a signature of sexual selection. I investigate differences in sexual selection on red nuptial color in limnetic-benthic species pairs of threespine sticklebacks. I evaluate condition dependence by comparing the strength of the relationship between color and condition, and the magnitude of variance in red nuptial color to other color traits and to nonsexual traits. I find that limnetic males have strong condition dependent expression of red nuptial color whereas benthic males have at most weak condition dependent expression. Ancestral anadromous males show no condition dependence. This suggests that color is under strong directional sexual selection only in limnetics and that this is the derived state. Moreover, I find that the strength of female preference for red is related to the extent of condition dependence. The extent of condition dependence is also associated with the importance of color differences to mate recognition. These results show that differences between these species in the action of sexual selection underlies their sexual isolation.