Through their more than 40 year study of Darwin's finches on the Island of Daphne Major in the Galapagos, the Grants demonstrated how natural selection can drive rapid changes in body and beak size in response to changes in the food supply. In the process the Grants elucidated the mechanisms by which new species arise and how genetic diversity is maintained in natural populations. Their results, which show that the effects of natural selection can be seen within a single lifetime, or sometimes within a couple of years, are in distinct contrast to the theories of Charles Darwin who thought that natural selection required extensive periods of time for its operation.

(Thanks to Juan Weiss for this reference and its interpretation.)

The authors sequenced the genome of 120 individuals representing all of Darwin’s finches. They found that a 240 kilobase haplotype encompassing the ALX1 gene, which encodes a transcription factor affecting craniofacial development, is strongly associated with beak shape diversity across Darwin’s finch species, as well as the medium ground finch (Geospiza fortis), a species that the Grants observed undergoing rapid evolution of beak morphology in response to the environmental changes described in their 2006 paper.
The authors saw variants of this gene in the finches, each of which encoded for a different type of beak morphology. These variants had arisen during natural selection processes.

Order of authorship in the original publication: Lamichhaney, Berglund, .... Grant, Grant.

(Thanks to Juan Weiss for this reference and its interpretation.)