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.)