The impact of hybridization on morphological variation on Xiphophorus fishes

Hybridization can produce novel phenotypic variation which may allow hybrid populations to evolve in novel directions relative to parentals. Hybridization can produce intermediate as well as extreme phenotypes with respect to the parent species. Furthermore, separate hybridization events may create phenotypically divergent populations compared to the parentals but also to other hybrid zones. The hybrid system of Xiphophorus birchmanni and X. malinche has been shown to be replicated independently in seven streams along an elevation gradient. The sword sexual ornament is an extension of the ventral portion of the caudal fin that has been secondarily lost in X. birchmanni but persists in X. malinche and is polymorphic in hybrids. Female X. birchmanni have lost a preference for the sword and female X. malinche and hybrids are indifferent to males with swords. To determine if these replicated hybrid zones have unique introgression of the sword, morphometric data of the parental species and hybrids were collected across the hybrid zones. Though there was a general trend for the increase of the sword as one moved towards a more X. malinche-like population, the hybrid zones displayed variation for the introgression of the sword