"We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages," he wrote in the "On the Origin of Species." "... We only see that the forms of life are now different from what they formerly were."

Perhaps Darwin just didn't linger in the Galapagos long enough. Married biologists Peter and Rosemary Grant have worked on the islands for more than 40 years, and they've witnessed evolution unfolding right before their eyes.

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A decade ago, they saw a drought devastate Daphne Major Island, a scrubby volcanic cinder cone home to little more than some cacti and two species of bird: the medium ground finch and the large ground finch. Because of the resulting food shortage, the smaller-beaked medium ground finches — who didn't try to compete with the bigger birds for large seeds — were outliving their larger-beaked brethren. Within a year, average beak size among medium ground finches had shrunk noticeably.

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And unlike Darwin, who developed his theory of natural selection without the benefit of knowing about modern genetics, the Grants were able to figure out exactly how this evolution happened.

In a study published Thursday in the journal Science, they report that they've pinpointed the bit of finch DNA behind the swift transition: a gene called HMGA2.

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In finches, HMGA2 seems to be the primary factor in beak size — like a really good group project leader, it orchestrates the expression of a number of other genes, each of which tweaks the size of the bird's beak. The same gene also appears in dogs, horses, even humans, holding sway over body size and stature.

Looking at the DNA of medium ground finches who lived (and died) during the drought, they found that the birds who starved to death were more likely to have a version of the gene that leads to big beaks. Those who survived were saved by their small-beak variation.

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The exact mechanism by which HMGA2 influences a finch's beak or a human's height remains unknown. But it's clearly very powerful — the Grants believe that the shift from bigger to smaller beaks they witnessed during the Daphne Major drought is one of the most dramatic examples of natural selection ever recorded.

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The location of HMGA2 adds to work that the Grants (both emeritus professors at Princeton University) and colleagues at Uppsala University in Sweden have already done on Galapagos finches. In a study published last year, the team sequenced the DNA of 120 birds — including members of all known species of Darwin's finches and two closely related species — to study the variation among them.

They found that the birds have been evolving, interbreeding and specializing in response to their environment ever since their common ancestor first arrived on the islands 2 million years ago. Sometimes the traits that make them so distinctive are a result of selection, other times they're a result of hybridization — two species interbreeding to produce something slightly new.

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The Grants even witnessed an example of hybridization during their time on Daphne Major: In the early '80s an immigrant to the island named Big Bird mated with a medium ground finch, producing a new lineage that is now some seven generations strong. The Grants are reluctant to say whether they actually witnessed the origin of a new species — the hybrid (can we call it "medium Bird?") is largely the result of inbreeding, they told Princeton Alumni Weekly two years ago, and could ultimately die out from the resulting "fitness problems."

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Another find from the 2015 study was the gene responsible for beak shape, a bit of genetic code involved in the facial development of vertebrates called ALX1. Depending on which variation of the gene a finch has, its beak may be narrow and pointed for spearing insects or broad and blunt for cracking seeds.

"There are multiple genes that contribute," Leif Andersson, an animal geneticist who worked on the study from Uppsala University, told the BBC at the time. "But we think that ALX1 is one of the most important, if not the most important factor that has changed on the island."

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Combine that with the latest study of HMGA2, and biologists have a pretty thorough blueprint of how these famous finches evolve. Dolph Schluter, a biologist at the University of British Columbia, told Nature that the discovery allows scientists to ask a whole new set of questions about how species split and how new gene variants arise.

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“On the one hand it doesn't change anything, in that we already knew there was an evolutionary response to competition during that drought,” he said. “But on the other hand, it changes everything, because we can point to a physical, material basis for that change.”