Genes gives clues to outcome of species interbreeding
Conflicts in DNA may explain why some hybrids are evolutionary dead ends

CRISSCROSS Freshwater fish Xiphophorus birchmanni (top left) can breed with sister species Xiphophorus malinche (top right) to form hybrids (bottom three rows), but the hybrids have hundreds of incompatible stretches of DNA in their genomes that may keep them from becoming new species.
H. S. Fisher et al./Proceedings: Biological Sciences 2006