Darwin's Finch Beak Evolution

In 1977, a severe drought struck Daphne Major, a tiny volcanic island in the Galápagos archipelago. Rainfall that year was less than 24 mm — a fraction of the normal level — and plant life collapsed. The small, soft seeds that the medium ground finch (Geospiza fortis) preferred as food disappeared almost entirely. What remained were the hard, large seeds of Tribulus cistoides (puncture vine) and other drought-resistant plants, seeds that required considerable force to crack open.

Peter and Rosemary Grant of Princeton University had been monitoring the Daphne Major population since 1973, meticulously measuring the beak depth of hundreds of individual finches. When the drought struck and food supply shifted dramatically toward large seeds, they documented one of the most rapid and clear cases of natural selection ever recorded in a wild population. The population crashed from roughly 1,200 birds to fewer than 200 by early 1978. Crucially, the survivors were not random: birds with deeper, stronger beaks were significantly more likely to survive, because they could crack the large seeds that smaller-beaked birds could not.

The statistical result was striking. Mean beak depth in the surviving population increased from approximately 9.4 mm to 9.96 mm — a shift of about 0.5 mm. While that sounds small, it represents a highly significant change in the distribution of a heritable trait over just one generation of selection. The offspring of survivors born in 1978 inherited their parents' deeper beaks, shifting the population distribution rightward on a histogram of beak depth.

This simulation models that exact phenomenon. Adjust the Large Seed % slider to simulate drought (high %) or a wet season (low %), then run successive seasons and watch the beak size histogram and mean-beak-depth line chart respond. The distribution you observe is not random — it is the statistical signature of natural selection acting on heritable variation.