Figure 16.8: The Origin of a Sexually Selected Trait The long tail of male swordtail fishes (genus Xiphophorus) apparently evolved through sexual selection, with females mating preferentially with males with a longer “sword.” Phylogenetic analysis reveals that the Priapella lineage split from the swordtails before the evolution of the sword. The independent finding that female Priapella prefer males with an artificial sword further supports the idea that this appendage evolved as a result of a preexisting preference in the females.