Galapagos Finches

Different finch species have beaks of different shapes and sizes. What do these beak differences tell us?

• 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

Answer explanation

Having a beak shape well-adapted for obtaining and manipulating/eating a particular type of food in a given environment (i.e. a seed of a certain size and shape or nectar from certain plants) is critical to survive. The second answer choice is incorrect because it is similarities in traits, rather than differences, that would provide evidence for shared ancestry.

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Genetic evidence supports which of the following explanations for the presence of 13 different finch species on the Galapagos Islands?

• 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.

Answer explanation

The Galápagos finch DNA reveals that the finch species are more related to each other than to any species on the mainland.

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In 1977, Daphne Major experienced a severe drought. The figure shows the beak depths of the initial population of medium ground finches before the drought (red bars), and of the drought survivors (black bars). What do the data show? Select all that apply.

• 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.

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The top figure shows the beak depths of the 1976 finch population (red bars) before the drought, and the population after the drought (black bars). The lower figure shows the beak depths of the offspring of the drought survivors in 1978. What do these figures tell us?

• 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.

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The Grants witnessed strong selection during the droughts in 1977 and 1985. Compare the two droughts. Select all that apply.

• 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.

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How did the Grants test their hypothesis that differences in the finch songs can keep different species of finches from breeding with one another?

• 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.

Answer explanation

The Grants’ experiments using recorded finch songs showed that finch species respond to their own song and not to the songs of other species. The fact that species differences in finch song keep species apart is an example of reproductive isolation.

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The film defines species as populations whose members don't interbreed. What keeps different Galápagos finch species from mating? Select all that apply.

• 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.

Answer explanation

In general, female finches only recognize and respond to males that sing their species’ song. They will ignore singing males of other species. Also, the Grants and their team gathered evidence to show that finches choose mates on the basis of certain traits that differ among species. These differences arose as geographically isolated populations changed over time in different ways (i.e., without mixing their genes).

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