The international team, including scientists based in the UK, Germany and Panama, compared brain morphology of two distinct but closely related lineages of butterfly that occur in distinct tropical forest habitats. The first, including the species Heliconius cydno, lives in deeper forests, where the canopy light levels are low. Its sister lineage, including a species called Heliconius melpomene, lives around the forest edges, where light is much more abundant. Despite their ecological differences, these species are very closely related and can still produce viable offspring, suggesting they sit right at the brink of being new species.
The team found substantial differences in the brains of forest edge and deep forest species, with the latter investing more in parts of the brain that process visual information. By collecting butterflies across south and central America, as well as rearing captive individuals under controlled conditions at the Smithsonian Tropical Research Institute in Panama, the researchers showed that differences in brain morphology have accumulated in a way consistent with natural selection. “These butterflies aren’t separated by huge distances, nor are they distantly related, but their brain structure is finely tuned to the specific habitats they occupy and we think this process helps keep the two lineages apart, allowing them to become distinct species.” said Dr Stephen Montgomery from the University of Bristol who led the project.
Similar differences were seen when the team examined the how highly different genes were expressed in the brain. Matteo Rossi, a PhD student at LMU, Munich explained “based on the pattern of gene expression in brain tissue we can accurately cluster individuals into the correct species. The expression of genes driving these differences evolve fast, and seem to be located in regions of the genome that are most distinct between the two species.”