Evolution World Tour: Galápagos Islands, Ecuador
In Charles Darwin’s day, the Galápagos Islands were perhaps the best place in the world to observe evidence of evolution by natural selection. They still are.
The 19 islands are the tips of volcanoes that began emerging from the ocean some five million years ago, steaming with fresh lava and devoid of life. The plants and animals that dwell there today are descended from castaways that arrived by sea or air. Finches and mockingbirds were blown off course by storms; iguanas floated on rafts of debris; and the tree-like scalesia plants are the overgrown progeny of sunflowers that made landfall via airborne seeds. It’s easy to study the diversity of species here in part because there aren’t all that many species to see.
The islands—separated from one another by distance, deep water and strong tides—isolated the newcomers, preventing many of the plants and animals from breeding with others of their kind that may have colonized other shores. With no place else to go, the Galápagos’ denizens adapted to conditions unique to their new homes.
Consider, for example, a tale of two tortoises. On Santa Cruz Island, with dense forests of scalesia, giant tortoises are built like rounded tanks that can crash through the underbrush. But over at Isabela Island, the dominant plant is the prickly pear cactus, a delicacy to giant tortoises. Some biologists have suggested the cactus adapted to this threat by growing taller, generation after generation, and acquiring a bark-like covering at tortoise-eye level. The tortoises, in turn, apparently evolved to have a notch in the shell behind their head, allowing the animals to stretch their long necks straight up to reach the lowest-hanging prickly pear pads.
The islands, which straddle the Equator, are still relatively pristine; 97 percent of the land is national park, and Ecuador limits who may live there and how many tourists may visit. “Galápagos are a wonderful place to study evolution, still, because, remarkably, several islands and their inhabitants are close to being in the fully natural state, with little or no influence of human activities,” says the evolutionary biologist and Princeton University professor emeritus Peter Grant who, with his wife, Rosemary (also a biologist), has been studying finches there since 1973. Their research has shown that natural selection is frequently at work in the Galápagos: After a drought, finches with larger beaks were able to eat tough seeds and survive; their offspring became predominant. After a particularly rainy year, softer seeds proliferated and smaller-beaked finches were more likely to survive. Grant recommends that a newcomer to the islands “be alert to differences between obviously related organisms” and visit at least two islands to appreciate how descendants of the same founder species adapted to different environments.