Ongoing Evolution Among Darwin's Finches
A particularly interesting example of contemporary evolution involves the 13 species of finches studied by Darwin on the Galapagos Islands, now known as Darwin's finches. A research group led by Peter and Rosemary Grant of Princeton University has shown that a single year of drought on the islands can drive evolutionary changes in the finches.6 Drought diminishes supplies of easily cracked nuts but permits the survival of plants that produce larger, tougher nuts. Drought thus favors birds with strong, wide beaks that can break these tougher seeds, producing populations of birds with these traits. The Grants have estimated that if droughts occur about
once every 10 years on the islands, a new species of finch might arise in only about 200 years.7
which have become further reduced in modern whales.5
Another focus of research has been the evolution of ancient apelike creatures through many intermediate forms into modern humans. Homo sapiens, one of 185 known living species in the primate order, is a member of the hominoids, a category that includes orangutans, gorillas, and chimpanzees. The succession of species that would give rise to humans seems to have separated from the succession that would lead to the apes about 5 to 8 million years ago. The first members of our genus, Homo, had evolved by about 1.5 million years ago. According to recent evidence—based on the sequencing of DNA found in a part of human cells known as mitochondria—it has been proposed that a small group of modern humans evolved in Africa about 150,000 years ago and spread throughout the world, replacing archaic populations of Homo sapiens.
Animals and plants do not live in isolation, nor do they evolve in isolation. Indeed, much of the pressure toward diversification comes not only from physical factors in the environment but from the presence of other species. Any animal is a potential host for parasites or prey for a carnivore. A plant has other plants as competitors for space and light, can be a host for parasites, and provides food for herbivores. The interactions within the complex communities, or ecosystems, in which organisms live can generate powerful evolutionary forces.
Evolution in natural communities arises from both constraints and opportunities. The constraints come from competitors, primarily among the same species. There are only so many nest holes for bluebirds and so much food for mice. Genetically different individuals that are able to move to a different resource—a new food supply, for example, or a hitherto uninhabited area—are