New research suggests that the changes are due to epigenetics.
As a report posted on GenomeWeb puts it, “epigenetic variation between urban and rural populations of Darwin's finches … could underlie their adaptation to a new environment.”
The article goes on to say:
“The Galápagos Islands only recently underwent urbanization, leading the researchers to wonder how organisms there are coping with speedy environmental change. By examining populations of two species of Darwin's finches, researchers from Washington State University and the University of Utah uncovered morphological differences between urban and rural populations of Geospiza fortis as well as epigenetic differences between urban and rural populations of G. fortis and G. fuliginosa. However, as they reported in BMC Evolutionary Biology last night, they found little genetic variation.”
They did not find much morphological change in the birds.
“But when the researchers compared DNA methylation patterns — generated using methylated DNA immunoprecipitation (MeDIP) sequencing — they did find differences between rural and urban populations in both species.
The genes associated with the differentially methylated regions the researchers identified were typically involved in metabolism, cell signaling, and transcription, though they also differed by species. In particular, they noted that some differentially methylated regions were associated with genes in BMP/TGF-beta pathway. BMP4 expression, they added, has previously been linked to beak shape in Geospiza.”