Many species occasionally hybridise and share genetic material with related species; however, interspecific gene flow may be counteracted by natural selection at particular 'barrier loci.' As a result, a pair of species can end up sharing more genetic variation in some parts of their genome than in others, and the tree of relationships in a group of species can consequently differ from one part of the genome to another. This study by Martin et al. studied relationships and barriers among three species of Heliconius butterflies using whole-genome sequences from nine populations. The authors find that species relationships vary dramatically and predictably across the genome because the species barriers are more porous in genomic regions with higher recombination rates. This occurs because recombination determines how broadly the surrounding genome is affected by a barrier locus. The study reveals how hybridisation, recombination and selection interact to reshape species' relationships. The image shows two of these closely related species, Heliconius cydno (left) and Heliconius melpomene (right), which co-occur in the forests of Central America. They will occasionally interbreed in captivity as seen here but hybridisation between the two species is extremely rare in the wild due to strong preference for mates of their own species. The hybrid offspring are not only ill-adapted to their environment, but female hybrids are sterile.

Image Credit: Chris Jiggins