THE LIVING WORLD

Unit Four. The Evolution and Diversity of Life

 

14. Evolution and Natural Selection

 

14.4. The Beaks of Darwin's Finches

 

Darwin’s Galapagos finches played a key role in his argument for evolution by natural selection. He collected 31 specimens of finches from three islands when he visited the Galapagos Islands in 1835. Darwin, not an expert on birds, had trouble identifying the specimens. He believed by examining their beaks that his collection contained wrens, “gross-beaks,” and blackbirds.

 

The Importance of the Beak

Upon Darwin’s return to England, ornithologist John Gould examined the finches. Gould recognized that Darwin’s collection was in fact a closely related group of distinct species, all similar to one another except for their beaks. In all, 14 species are now recognized, 13 from the Galapagos and one from far-distant Cocos Island. The ground finches with the larger beaks in figure 14.9 feed on seeds that they crush in their beaks, whereas those with narrower beaks eat insects, including the warbler finch (named for its resemblance to a mainland bird). Other species include fruit and bud eaters, and species that feed on cactus fruits and the insects they attract; some populations of the sharp-beaked ground finch even include “vampires” that creep up on seabirds and use their sharp beaks to drink their blood. Perhaps most remarkable are the tool users, like the woodpecker finch you see in the upper left of the figure, that picks up a twig, cactus spine, or leaf stalk, trims it into shape with its beak, and then pokes it into dead branches to pry out grubs.

 

 

Figure 14.9. A diversity of finches on a single island.

Ten species of Darwin's finches from Isla Santa Cruz, one of the Galapagos Islands. The 10 species show differences in beaks and feeding habits. These differences presumably arose when the finches arrived and encountered habitats lacking small birds. Scientists concluded that all of these birds derived from a single common ancestor.

 

The differences in the beaks of Darwin’s finches are due to differences in the genes of the birds. When biologists compare the DNA of large ground finches (with stout beaks for cracking large seeds) to the DNA of small ground finches (with more slender beaks), the only growth factor gene that is different in the DNA of the two species is BMP4 (figure 14.10 and figure 1.15). The difference is in how the gene is used. The large ground finches, with larger beaks, make more BMP4 protein than do the small ground finches.

 

 

Figure 14.10. A gene shapes the beaks of Darwin's finches.

A cell signalling molecule called "bone morphogenic protein 4" (BMP4) has been shown by DNA researchers to tailor the shape of the beak in Darwin's finches.

 

The correspondence between the beaks of the 14 finch species and their food source immediately suggested to Darwin that evolution had shaped them:

“Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species has been taken and modified for different ends.”

 

Checking to See if Darwin Was Right

If Darwin’s suggestion that the beak of an ancestral finch had been “modified for different ends” is correct, then it ought to be possible to see the different species of finches acting out their evolutionary roles, each using its beak to acquire its particular food specialty. The four species that crush seeds within their beaks, for example, should feed on different seeds, with those with stouter beaks specializing on harder- to-crush seeds.

Many biologists visited the Galapagos after Darwin, but it was 100 years before any tried this key test of his hypothesis. When the great naturalist David Lack finally set out to do this in 1938, observing the birds closely for a full five months, his observations seemed to contradict Darwin’s proposal! Lack often observed many different species of finch feeding together on the same seeds. His data indicated that the stout- beaked species and the slender-beaked species were feeding on the very same array of seeds.

We now know that it was Lack’s misfortune to study the birds during a wet year, when food was plentiful. The size of the finch’s beak is of little importance in such flush times; slender and stout beaks work equally well to gather the abundant tender small seeds. Later work revealed a very different picture during dry years, when few seeds are available.

 

A Closer Look

Starting in 1973, Peter and Rosemary Grant of Princeton University and generations of their students have studied the medium ground finch, Geospiza fortis, on a tiny island in the center of the Galapagos called Daphne Major. These finches feed preferentially on small, tender seeds, abundantly available in wet years. The birds resort to larger, drier seeds that are harder to crush when small seeds are hard to find. Such lean times come during periods of dry weather, when plants produce few seeds, large or small.

By carefully measuring the beak shape of many birds every year, the Grants were able to assemble for the first time a detailed portrait of evolution in action. The Grants found that beak depth changed from one year to the next in a predictable fashion. During droughts, plants produced few seeds, and all available small seeds quickly were eaten, leaving large seeds as the major remaining source of food. As a result, birds with large beaks survived better, because they were better able to break open these large seeds. Consequently, the average beak depth of birds in the population increased the next year because this next generation included offspring of the large-beaked birds that survived. The off spring of the surviving “dry year” birds had larger beaks, an evolutionary response which led to the peaks you see in the graph in figure 14.11. The reason there are peaks and not plateaus is that the average beak size decreased again when wet seasons returned because the larger beak size was no longer more favorable when seeds were plentiful and so smaller-beaked birds survived to reproduce.

 

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Figure 14.11. Evidence that natural selection alters beak size in Geospiza fortis.

In dry years, when only large, tough seeds were available, the mean beak size increased. In wet years, when many small seeds were available, smaller beaks became more common.

 

Could these changes in beak dimension reflect the action of natural selection? An alternative possibility might be that the changes in beak depth do not reflect changes in gene frequencies but rather are simply a response to diet, with poorly fed birds having stouter beaks. To rule out this possibility, the Grants measured the relation of parent beak size to offspring beak size, examining many broods over several years. The depth of the beak was passed down faithfully from one generation to the next, suggesting the differences in beak size indeed reflected gene differences.

 

Support for Darwin

If the year-to-year changes in beak depth can be predicted by the pattern of dry years, then Darwin was right after all— natural selection influences beak size based on available food supply. In the study discussed here, birds with stout beaks have an advantage during dry periods, for they can break the large, dry seeds that are the only food available. When small seeds become plentiful once again with the return of wet weather, a smaller beak proves a more efficient tool for harvesting smaller seeds.

 

Key Learning Outcome 14.4. In Darwin's finches, natural selection adjusts the shape of the beak in response to the nature of the food supply, adjustments that are occurring even today.