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Genital Evolution
Relevant Publications:
Langerhans, Layman, DeWitt. 2005. PNAS
Langerhans in press (Chapter in Ecology and Evolution of Poeciliid Fishes)
See a video clip illustrating the choice between males of varying gonopodium size (digitally altered).
See some of the media attention this work has received.
Male genitalia may experience more rapid, divergent evolution than any other animal character-but why? Research during the past several decades has culminated in the view that genital diversification primarily results from postmating sexual selection (e.g. sperm competition, cryptic female choice). However, the potential roles of premating sexual selection (e.g. mate choice) and natural selection have received little attention. To investigate the possible importance of these mechanisms in genital evolution, I examine genital diversification in livebearing fish (Family Poeciliidae). Male poeciliid fishes possess a gonopodium, a modified anal fin that serves as a sperm transfer organ. We have found that gonopodium size diverges between predator regimes within two species of livebearing fish (Gambusia affinis in Texas, USA and G. hubbsi in The Bahamas). Controlling for body size, males exhibit a larger gonopodium in predator-free environments than in predatory environments-a trend that persists across space (multiple populations), time (multiple years), and species. We have further shown that G. affinis offspring raised in a common laboratory environment retain differences in gonopodium size observed in the wild, suggesting a heritable component to the genital divergence. By conducting laboratory experiments with G. affinis, we have found that premating sexual selection appears to favor larger male genitalia (females exhibit mating preference for males with larger gonopodia), but natural selection in the presence of predatory fishes seems to favor reduced genital size (larger gonopodium size is associated with reduced burst-swimming performance, an important antipredator behavior). While postmating sexual selection is widely presumed to be the most important mechanism driving genital diversification, our findings suggest that alternative mechanisms, particularly for organisms that cannot retract their genitalia, may also prove important.
(Representative laboratory-reared G. affinis males derived from predator-free (A) and predator (B) populations. Arrows indicate the gonopodium. Note the larger gonopodium in (A). The scale bar represents 5 mm.)
Future work:
Since gonopodium size is highly variable among poeciliid fishes (see figure to the right), I am extending the results described above regarding intraspecific divergence to an investigation of interspecific divergence in gonopodium size. Such divergence in a copulatory organ might be especially important in the process of speciation. Since variation in predator regime exists among as well as within species (particularly within the genus Gambusia), interspecific patterns may parallel the observed intraspecific patterns. Thus, macroevolutionary trends in the family may mirror microevolutionary patterns that we have discovered, providing a more general understanding of the causes of the remarkable diversity of male genitalia and mechanisms of speciation. I am currently planning a study to test this hypothesis using detailed comparative studies conducted in a phylogenetic context.
Female mating preference based on gonopodium size might help explain the evolution of swords (elongate, sword-like projections of the caudal fin) in male swordtail fishes. That is, swords, due to their resemblance to gonopdodia in overall shape, might effectively represent gonopodium mimics and exploit a preexisting sensory bias in females. I am currently planning studies to investigate this hypothesis.
Gonopodium morphology, other than size, also exhibits conspicuous variation among species (e.g. hooks, spines, serrae). These more subtle aspects of variation seem likely to be related to postmating sexual selection (e.g. cryptic female choice). In addition, gonopodium size might be related to sperm competition if larger gonopodia exhibit greater sperm transfer performance or traumatize female genitalia, restricting subsequent matings. I am currently planning studies to investigate these additional and alternative hypotheses.
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(Male G. hubbsi chasing a female in a blue hole on Andros Island, The Bahamas)
(from Rosen and Gordon 1953)
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