The Galápagos finch species that underwent rapid beak evolution during a drought on Daphne Major was Geospiza fortis, the medium ground finch.
In 1977, a severe drought sharply reduced the island’s supply of small, soft seeds. Finches with smaller beaks struggled to find enough food, while birds with relatively larger, stronger beaks could crack the remaining hard seeds. Because beak size is heritable, the surviving population’s average beak size increased in the following generation—a classic example of natural selection observed in real time.
Daphne Major later provided another important lesson. When large ground finches, Geospiza magnirostris, became established on the island, they competed with G. fortis for large seeds. This favored smaller beaks in some medium ground finches, showing that evolution can reverse direction when ecological conditions and competition change.
A common mix-up is confusing G. fortis with G. magnirostris. The latter helped create the competitive pressure, but G. fortis was the population whose beak evolution was measured.