Scientist’s say they have discovered the genetic “smoking gun” that explains how different species of Galapagos finches came into being and how they manage to evolve so rapidly in response to changes in their environments. Their findings, published in the journal Nature on February 19, are based on genomic sequencing of 120 finches and reveal the genetic mechanism responsible behind the finches’ famously ever-changing beaks.

The researchers found that one gene in particular, known as ALX1, plays a critical role in determining beak shape. They also found that hybridization between species mixes the different variants of ALX1 — that the gene itself has been shaped by interbreeding between species. As a result, not only do different species of finches have different variants of the gene, but different individuals within a species have their own distinct variants.

Here’s where it gets interesting: the ALX1 gene also helps determine the facial structures of humans and other vertebrates. Even slight alterations of the gene in humans can cause severe birth defects, including cleft palate and skull malformation. But in finches, variations in the gene are actually helpful. They contribute to the rapid diversification of beak shapes in response to changes in the environment and allow different species of finch to specialize in utilizing different food sources.

The fifteen currently recognized species of Galapagos finches famously helped inspire Darwin’s theory of evolution. He postulated that the different species of finches descended from a common ancestral species and developed in response to the the particular environmental variations found throughout the archipelago. This process, now known as adaptive radiation, resulted in some species having fine, pointy beaks — perfect for picking tiny insects off of branches, while others developed thick, blunt beaks, which are better suited for crushing nuts and seeds. But until now, no one understood how the process actually worked.

Enter Peter and Rosemary Grant, the married Princeton University researchers who have dedicated their lives to studying the adaptations of finch populations on the island of Daphne Major and who coauthored the new study. The Grants’ decades-long research, made famous in Jonathan Weiner’s Pulitzer Prize winning book The Beak of the Finch, turned the widely held belief that natural selection is a gruelingly slow, impossible-to-see process on its head.

During a severe drought in the 1970s, the medium ground finches on Daphne Major had to compete for an an ever-dwindling supply of seeds. Larger, tougher seeds were better suited to survive the dryer conditions, which put birds with smaller beaks at a distinct disadvantage. When the Grant’s measured the beak sizes of the survivor’s offspring, they found a marked increase in size. Within the span of a single generation the finches of Daphne Major had already started to noticeably adapt to the new realities of their environment.

During a particularly wet El Nino year a few years later the process was reversed; beak sizes became smaller within a single generation.

The new study help explain how such fundamental changes can happen so quickly. The genes responsible for distinctive traits aren’t static, but flow back and forth between species and populations over time. Hybridization has actually helped spawned genetic diversity and allowed the various species of finches to adapt quickly and effectively to the their ever-changing world.

“Both selection and hybridization affected beak shape,” the Grant’s recently told The Washington Post. “Several findings surprised us, but perhaps the greatest surprise was finding a ‘smoking gun’ of hybridization going all the way back to near the beginning of the radiation, a period spanning nearly a million years.”