THE LIVING WORLD
Unit Four. The Evolution and Diversity of Life
14.5. How Natural Selection Produces Diversity
Darwin believed that each Galapagos finch species had adapted to the particular foods and other conditions on the particular island it inhabited. Because the islands presented different opportunities, a cluster of species resulted. Presumably, the ancestor of Darwin’s finches reached these newly formed islands before other land birds, so that when it arrived, all of the niches where birds occur on the mainland were unoccupied. A niche is what a biologist calls the way a species makes a living—the biological (that is, other organisms) and physical (climate, food, shelter, etc.) conditions with which an organism interacts as it attempts to survive and reproduce. As the new arrivals to the Galapagos moved into vacant niches and adopted new lifestyles, they were subjected to diverse sets of selective pressures. Under these circumstances, the ancestral finches rapidly split into a series of populations, some of which evolved into separate species.
The phenomenon by which a cluster of species change, as they occupy a series of different habitats within a region, is called adaptive radiation. Figure 14.12 shows how the 14 species of Darwin’s finches on the Galapagos Islands and Cocos Island are thought to have evolved. The ancestral population, indicated by the base of the brackets, migrated to the islands about 2 million years ago and underwent adaptive radiation giving rise to the 14 different species.
Figure 14.12. An evolutionary tree of Darwin's finches.
This family tree was constructed by comparing DNA of the 14 species. Their position at the base of the finch tree suggests that warbler finches were among the first adaptive types to evolve in the Galapagos.
The descendants of the original finches that reached the Galapagos Islands now occupy many different kinds of habitats on the islands. The 14 species that inhabit the Galapagos Islands and Cocos Island occupy four types of niches:
1. Ground finches. There are six species of Geospiza ground finches. Most of the ground finches feed on seeds. The size of their beaks is related to the size of the seeds they eat. Some of the ground finches feed primarily on cactus flowers and fruits and have longer, larger, more pointed beaks.
2. Tree finches. There are five species of insect-eating tree finches. Four species have beaks that are suitable for feeding on insects. The woodpecker finch has a chisel-like beak. This unique bird carries around a twig or a cactus spine, which it uses to probe for insects in deep crevices.
3. Vegetarian finch. The very heavy beak of this budeating bird is used to wrench buds from branches.
4. Warbler finches. These unusual birds play the same ecological role in the Galapagos woods that warblers play on the mainland, searching continually over the leaves and branches for insects. They have a slender, warblerlike beak.
Key Learning Outcome 14.5. Darwin's finches, all derived from one similar mainland species, have radiated widely on the Galapagos Islands, filling unoccupied niches in a variety of ways.