title: Evolution of the elaborate male intromittent organ ofXiphophorusfishes.
abstract: Internally fertilizing animals show a remarkable diversity in male genital morphology that is associated with sexual selection, and these traits are thought to be evolving particularly rapidly. Male fish in some internally fertilizing species have "gonopodia," highly modified anal fins that are putatively important for sexual selection. However, our understanding of the evolution of genital diversity remains incomplete. Contrary to the prediction that male genital traits evolve more rapidly than other traits, here we show that gonopodial traits and other nongonopodial traits exhibit similar evolutionary rates of trait change and also follow similar evolutionary models in an iconic genus of poeciliid fish (Xiphophorusspp.). Furthermore, we find that both mating and nonmating natural selection mechanisms are unlikely to be driving the diverseXiphophorusgonopodial morphology. Putative holdfast features of the male genital organ do not appear to be influenced by water flow, a candidate selective force in aquatic habitats. Additionally, interspecific divergence in gonopodial morphology is not significantly higher between sympatric species, than between allopatric species, suggesting that male genitals have not undergone reproductive character displacement. Slower rates of evolution in gonopodial traits compared with a subset of putatively sexually selected nongenital traits suggest that different selection mechanisms may be acting on the different trait types. Further investigations of this elaborate trait are imperative to determine whether it is ultimately an important driver of speciation.
authors.affiliation: Lehrstuhl für Zoologie und Evolutionsbiologie Department of Biology University of Konstanz Universitätstraße 10 78457 Konstanz Germany; Zukunftskolleg University of Konstanz Konstanz Germany; Present address: Evolution, Behaviour and Environment School of Life Sciences University of Sussex Brighton UK.
authors.forename: Julia C
authors.initials: JC
authors.lastname: Jones
authors.affiliation: Lehrstuhl für Zoologie und Evolutionsbiologie Department of Biology University of Konstanz Universitätstraße 10 78457 Konstanz Germany; School of Earth, Environmental & Biological Sciences Queensland University of Technology Brisbane Qld 4000 Australia.
authors.forename: Carmelo
authors.initials: C
authors.lastname: Fruciano
authors.affiliation: Lehrstuhl für Zoologie und Evolutionsbiologie Department of Biology University of Konstanz Universitätstraße 10 78457 Konstanz Germany.
authors.forename: Anja
authors.initials: A
authors.lastname: Keller
authors.affiliation: Physiological Chemistry, Biozentrum University of Würzburg Am Hubland 97074 Würzburg Germany; Comprehensive Cancer Centre University Clinic Würzburg Josef Schneider Straße 697074 Würzburg Germany.
authors.forename: Manfred
authors.initials: M
authors.lastname: Schartl
authors.affiliation: Lehrstuhl für Zoologie und Evolutionsbiologie Department of Biology University of Konstanz Universitätstraße 10 78457 Konstanz Germany.
authors.forename: Axel
authors.initials: A
authors.lastname: Meyer
keywords: Male intromittent organ; Xiphophorus fish; reproductive character displacement; sexual selection; species diversification