Differences in beak shapes tell us that all the finches eat the same type of food

Different finch beak shapes are evidence that all Galapagos finches shared a common ancestor a long time ago

Different finch beak shapes are evidence that over time, finch species adapted to different food sources on the islands

Different finch beak shapes are evidence that several finch species with different beak shapes came to the Galapagos Islands form the mainland

Each of the 13 species was founded by the independent migration of a different species from the mainland to the islands.

Many years ago, more than 13 different species of birds migrated to the islands. The current 13 finch species are the only species that survived.

It's not clear based on the DNA evidence whether the 13 species arose from a single migration of one species or multiple migrations of several species from the mainland to the islands.

Many years ago, a small population of a single finch species migrated to the islands and evolved into the current 13 species.

The most common beak depth of the initial population (red bars) was 8.8 mm. A very small proportion of individuals with this beak depth survived the drought.

More than half of the initial finch population died during the drought.

The most common beak depth of the surviving population (black bars) was 10.3 mm. Less than a quarter of individuals with this beak depth from the initial population survived the drought.

Finches with larger beaks had a survival advantage in the 1977 drought.

The average beak depth of the 1978 offspring population is larger than that of the original 1976 population.

The finches that survived the drought (black bars in first graph) had offspring (red bars in second graph) with on average much larger beaks than their parents.

The offspring (red bars in second graph) had a smaller range of beak depths - from smallest to largest - than their parents (black bars).

The size of the offspring population in 1978 is much larger than the initial 1976 population, before the drought.

In both droughts, larger-beaked medium ground finches had the highest survival rates.

Both droughts resulted in strong natural selection on medium ground finch populations.

Both droughts resulted in changes to available food, which favored survival of some medium ground finches over others.

They observed which birds were mating with one another and listened to the songs the birds were singing.

They recorded birds singing on the island of Daphne Major to see which type of song was sung more often by each species.

They placed stuffed females of different finch species on branches to see which males would respond.

They played the songs of medium ground finches and cactus finches through a loudspeaker to see which species responded to each song.

Individuals only recognize and respond to songs of their own species.

One population eats mostly small, soft seeds and the other population eats mostly large, hard seeds.

Geographic isolation and different environments led to changes in traits that affected mating.