Recurrent origin of a sexually selected trait in Xiphophorus fishes inferred from a molecular phylogeny

Nature. 1994 Apr 7;368(6471):539-42. doi: 10.1038/368539a0.


Darwin believed that sexual selection accounts for the evolution of exaggerated male ornaments, such as the sword-like caudal fin extensions of male fishes of the genus Xiphophorus, that appear detrimental to survival. Swordtails continue to feature prominently in empirical work and theories of sexual selection; the pre-existing bias hypothesis has been offered as an explanation for the evolution of swords in these fishes. Based upon a largely morphological phylogeny, this hypothesis suggests that female preference to mate with sworded males arose in ancestrally swordless species, thus pre-dating the origin of the sword itself and directly driving its evolution. Here we present a molecular phylogeny (based on mitochondrial and nuclear DNA sequences) of Xiphophorus which differs from the traditional one: it indicates that the sword originated and was lost repeatedly. Our phylogeny suggests that the ancestor of the genus is more likely to have possessed a sword than not, thus questioning the applicability of the pre-existing bias hypothesis as an explanation for the evolution of this sexually selected trait.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Base Sequence
  • Biological Evolution*
  • Cyprinodontiformes / classification
  • Cyprinodontiformes / genetics
  • Cyprinodontiformes / physiology*
  • Cytochrome b Group / genetics
  • DNA / genetics
  • DNA Primers
  • Female
  • Male
  • Molecular Sequence Data
  • Phylogeny
  • Protein-Tyrosine Kinases / genetics
  • Selection, Genetic*
  • Sensation
  • Sex Characteristics*


  • Cytochrome b Group
  • DNA Primers
  • DNA
  • Protein-Tyrosine Kinases