Female preference predates the evolution of the sword in swordtail fish.

The study of female preferences and the evolution of male traits has until recently centered on genetic coevolutionary mechanisms. An alternative mechanism posits that a preference results from a preestablished bias in the female information-processing system arising from sources independent of sexual selection. Male traits then arise that are selected by this preexisting preference. The genus Xiphophorus consists of swordless platyfish and swordtails. Swordlessness is the primitive state. In this study, female platyfish, X. maculatus, were found to prefer conspecific males with artificial swords over those without swords, despite evidence that the common ancestor of platyfish and swordtails was swordless. These results suggest that the evolution of the sword in the swordtail clade was a consequence of selection arising from a preexisting bias.