Galapagos: Islands Lost in Time
Isolated oceanic islands are the great natural laboratories of evolution. They enjoy this status for two rather different reasons. For the first, practicality, they provide nature’s closest approach to the controlled and manageable conditions that every scientist tries to create in laboratory experiments. They are usually small, isolated, and tenanted by a unique and poorly or imperfectly developed fauna; the hurly-burly of a rich continental community of plant and animal life, or “biota,” yields to tractable complexity—nature is never simple.
For the second reason, peculiarity, island biotas are replete with uniquenesses that both fascinate for their own sake and test general principles by probing the limits of their applicability. They are often populated not by a well-balanced set of species sifted in the fine meshes of continental competition, but by a peculiar association of forms that happened, by chance, to cross vast oceanic spaces. Major ecological roles are often without occupants—many islands lack large predators, for example. Or, in the absence of usual tenants, animals may evolve so that they occupy roles that would never be open to them on continents.
Many features conspire to turn islands into crucibles for profound and rapid evolutionary change among their inhabitants: absence of competitors, ecological availability of unconventional niches, the small size of founding populations. Islands therefore become homes for bizarre versions of animals that may be stereotyped in continental situations throughout the world. Sicily, for example, once was host to an elephant that stood less than one meter tall, yet bore all the defining features of its group. The extinct dodo of Mauritius was a giant flightless pigeon.
Since our minds prefer not to deal in abstractions, we tend to create prototypes for categories: Brontosaurus (or is it Tyrannosaurus?) becomes the dinosaur, and Einstein becomes a synecdoche for human intelligence. Among islands, the Galapagos enjoy an unquestioned status as the Mecca of biologists. They are, to be sure, archetypical oceanic islands, replete with all the beauty and fascination of their class. But, like Mecca, they owe their status not to their intrinsic character, but rather to the visit of a primary saint, in this case Charles Darwin, who spent in month in the Galapagos on the first stop of the Beagle’s long voyage home. Conventional mythology proclaims that Darwin finally saw the truth of evolution amid the finches and tortoises of these islands, hence their status as a site for secular pilgrimage. As usual, conventional mythology is largely wrong (see below), but it hardly matters. For if the Galapagos are not transcendent among islands for their own merits, they are certainly among the most fascinating.
The Galapagos Islands lie just below the equator, in the Pacific Ocean 500 to 600 miles west of Ecuador. They have formed over a “hot spot,” an area where magma generated deep within the earth’s mantle intersects the surface and builds volcanoes reaching from the floor of the ocean to its top. The hot spot has not changed its position recently, but …