The finches of the Galapagos Islands were central to Darwin's development of the theory of evolution through natural selection, and they haven't stopped evolving since. The astonishing adaptations of these finches have seen a new species appear, literally while researchers were studying it, and island urbanization is producing its own effects. A new study credits epigenetics for these rapid changes.
Genetic variation is normally considered the fuel for evolution. A species rich in diversity can adapt when circumstances favor particular behaviors or forms. However, when studying two species of Galapagos finch, Sabrina McNew, a PhD student at the University of Utah, found them to be genetically quite homogenous.
This ought to have made the ground finch (Geospiza fortis) and small ground finch (G. fuliginosa) ill-suited to new circumstances, such as the rapid urbanization that has occurred in certain Galapagos islands in recent years. Yet in BMC Evolutionary Biology, McNew reports plenty of variation in the finches’ epigenetics, creating an alternative path to adaptation.
McNew examined 1,097 birds from urban and rural locations 10 kilometers (6 miles) apart on the island of Santa Cruz. The city ground finches were larger than their country cousins, but G. fuliginosa lived up to their name by being consistently small. Differences were also observed by location in G. fortis' beaks. Both differences may be a product of access to human foods in urban areas.
Changes in gene function that don't involve variation in DNA sequences have gone from the periphery of biology to a central topic of research in just a few decades. Most prominently, methylation – or the addition of methyl groups (CH3) to DNA – can change the extent to which a gene is expressed, fundamentally altering living things’ development.
“We believe that the epigenetic differences may be a heritable component that might explain the rapid adaptation of Darwin’s finches to an urban environment,” said senior author, Washington State University's Dr Michael Skinner in a statement. “These species of finch have distinct diets, which could explain the differences in methylation patterns as diet is known to influence epigenetics. This is a novel mechanism which is not seriously considered in evolutionary biology at this time.”
McNew admitted the relationship between methylation differences and the shape or function of the finches has yet to be demonstrated, let alone studied in detail. “However, it's exciting to document epigenetic variation on a population-level. This project is a first step in understanding if epigenetic changes are involved in adaptation to changing environments," she said.
Just how important epigenetic variation actually is for evolutionary change remains to be seen, but the authors note methylation modifications are more common than genetic mutations, and the effects of their variation at a species level have barely been studied.