A living laboratory
Daphne Major is a tiny volcanic island north of Santa Cruz Island in the Galápagos. It is a tuff cone, devoid of trees, and its steep, eroded slopes make landing difficult. This isolation makes it an ideal natural laboratory for studying evolution without human interference. Since 1973, biologists Peter and Rosemary Grant have conducted a long-term study here, capturing, measuring, and tracking nearly every individual of the island's primary finch populations.
Their main subject is the medium ground finch, Geospiza fortis. The Grants and their team have shown that evolution, driven by natural selection, is not a slow process. It can happen fast enough to be observed in just a couple of years. The main variable on Daphne Major is the weather. The availability of the seeds that the finches eat is entirely dependent on rainfall.
In 1977, a severe drought lasting 551 days devastated the island's plant life. soft seeds became scarce, leaving only large, hard seeds from the caltrop plant. The finch population, once around 1400, crashed to just a few hundred. The Grants' meticulous measurements showed that the survivors were disproportionately those with larger beaks, which were strong enough to crack the tough seeds. In the next generation, they found that the average beak depth of the finch population had increased by 4 percent. This was a measurable evolutionary shift in response to a single weather event.
Selection's shifting pressures
The story did not end with the drought. In 1982, the island experienced the opposite extreme: a season of heavy, prolonged rain caused by an El Niño event. This time, the plants that produce soft seeds flourished. The finches with smaller beaks were more efficient at harvesting this abundant food source. The selective pressure reversed, and within a few generations, the average beak size in the population decreased by 2.5 percent. This oscillation shows that natural selection is a dynamic process, pushing populations in different directions as environmental conditions change.
The Grants also witnessed a rare event: the formation of a new lineage. In 1981, a male Española cactus finch (Geospiza conirostris), a species not native to Daphne Major, arrived from an island more than 100 km away. He was larger and had an unusual song. This immigrant bred with a resident medium ground finch, creating hybrid offspring. This new "Big Bird" lineage became reproductively isolated from the native finches in just two generations, partly because their unique song did not attract native mates. The lineage, which now consists of about 30 individuals, has a distinct beak morphology and occupies its own ecological niche, showing a real-time example of speciation.