Encyclopedia of Evolution.pdf - Online Reading Center

A map of the Galápagos Islands, off the coast of Ecuador
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Galápagos Islands The Galápagos Islands are volcanic
islands that have played and continue to play a very important
role in evolutionary science. They belong to Ecuador and
are about 600 miles (1,000 km) off the coast of that country
(see figure above). Much of the volcanic activity that produced
them occurred about five million years ago, which is
very recent in evolutionary time.
Being directly on the equator, these islands experience
almost no variation in sunlight. They are right in the path of
oceanic currents that influence temperature and moisture conditions.
There is an annual fluctuation of temperature caused
by the ocean currents. Because of this, the cactuses can use
a temperature cue that allows them to bloom every January.
The El Niño Southern Oscillation alternates between dry
conditions, when the ocean water near the South American
coast is cool, and wet conditions, when the water is warm.
During an El Niño year, large amounts of rainfall stimulate
plant growth, particularly of grasses, vines, and weeds, but
diminish the productivity of the oceans. The food chain of the
ocean depends upon the minerals brought up from cool, deep
ocean waters; when the water is warm, these minerals are not
available. However, periods of heavy rainfall are uncommon,
Galápagos Islands
and the lower slopes of the Galápagos Islands are usually hot
and dry, with scattered trees and a lot of hot, bare ground.
The upland areas of the larger islands are frequently shrouded
in mist, which provides enough moisture for forests to grow.
The islands are famous for their unusual plant and animal
life. There are no large native grazing animals or predators. The
native animals have evolved, until recently, in the absence of
predators and human contact. From the fur seals to the groundnesting
birds, they show no fear of humans. Many of the species
are unique to these islands, or almost so. This is because
they evolved on the islands from small founding populations
of mainland species. All over the world, islands are home to
unique species (see biogeography). The unique species found
on islands was one of the observations that led Charles Darwin
(see Darwin, Charles) to begin thinking about the process of
evolution. During his round-the-world voyage on HMS Beagle,
he visited many islands, including the Galápagos. His 1835
visit to the Galápagos turned out to be perhaps the single most
important event that sparked Darwin’s understanding that evolution
occurred by means of natural selection.
Among the plants and animals that have undergone adaptive
radiation on the Galápagos Islands are:
• Finches. These birds have evolved from a single founding
population into about 14 species that specialize on different
ways of life and types of food (see Darwin’s finches).
Ever since David Lack studied them in 1947, they have
Iguanas bask in the sunlight on the stone marker of Ecuador’s Galápagos
Islands National Park. These iguanas are just one of many species of
organisms unique to these islands. (Photograph by Stanley A. Rice)