"They chose the larger one over and over," Langerhans said. "All females had the same preference." A tenet of Langerhans's study was his belief that the gonopodium could be viewed as a secondary sexual trait ?similar to a peacock's tail, meant to enhance reproductive opportunity and selected by the female ?rather than a primary sexual trait ?which is the actual reproductive organ itself, simply meant to transfer sperm. Historically, genitalia were not believed to be subject to such sexual selection.
"Male genitalia can be seen as just another morphological character, and if you think of them that way, females might be choosing just as they would for any morphological trait," he said. "This at least partially explains the variability in male genital morphology."
Using this study as a springboard, Langerhans is exploring the role of genital divergence in the process of speciation. Divergence in a copulatory organ might be especially important in speciation -- owing to its obvious link to reproduction, which largely defines species boundaries. Evolutionary changes in male genital morphology between populations may result in reproductive incompatibility when populations merge again in the future, resulting in the generation of new species.
"Since gonopodium size is highly variable among livebearing fish species -- ranging from less than 20 percent of the body length to more than 70% -- I am extending the results reported in the PNAS paper -- divergence of genital size within species -- to an investigation of diversification in genital size between species, inferring possible modes of speciation. Since variation in predator regime exists between as well as within species, we can test specific theoretical predictions regarding genital evolution to evaluate what processes might have caused the patterns."
Langerhans also plans to examine whether other species of livebearing fish also exhibit female mating preference for males w
Source:Washington University in St. Louis