Figure 21.9 The Origin of a Sexually Selected Trait The tail extension of male swordtails (genus Xiphophorus) apparently evolved through sexual selection, as females mated preferentially with males that had long “swords.” Phylogenetic analysis reveals that Priapella split from the swordtails before the evolution of the sword. The independent finding that female Priapella prefer male Priapella with an artificial sword further supports the idea that this appendage evolved as a result of a preexisting preference in females.