Effect of Predation on Morphological Traits in Swordtails

In collaboration with William Wagner, I have investigated the effect of natural selection via predation in the green swordtail, X. helleri, by examining geographic variation in body size, fin length, sword length and predation risk in 15 field populations in Mexico and Belize. Sexual selection is known to favor longer male swords and larger body size. Males and females from populations with predators were found to be larger than those from populations without predators. When the effect of body size on sword length was statistically controlled, the data indicated that males from populations lacking predators have relatively longer swords.  These results suggest that natural selection due to predation and sexual selection due to female choice have opposing effects on sword evolution; natural selection favors shorter swords while sexual selection favors longer swords. Natural selection due to predation on body size, however, appears to act in conjunction with sexual selection; both favor the evolution of larger body size. Small body size in low predation populations may be a consequence of the more rapid maturation and thus faster generation time of smaller individuals. While one male trait, the sword, appears to be a compromise between natural and sexual selection, another, body size, appears to be favored by both sources of selection.

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