GALÁPAGOS ISLANDS

On its trip up the western shore of South America, the HMS Beagle
stopped at the Galápagos Islands, which are named after the
large tortoises that inhabit them (Sp. galápago, tortoise). The tortoises
weigh up to 250 kg, have shells up to 1.8 m in diameter, and
live for 200 to 250 years. The islands’ governor pointed out to
Darwin that the shapes of the tortoise shells from different parts of
Albemarle Island differed. Darwin noticed other differences as
well. Tortoises from the drier regions had longer necks than tortoises
from wetter habitats. In spite of their differences,
the tortoises were quite similar to each other and to the tortoises
on the mainland of South America.
How could these overall similarities be explained? Darwin
reasoned that the island forms were derived from a few ancestral animals
that managed to travel from the mainland, across 900 km of
ocean. Because the Galápagos Islands are volcanic
and arose out of the seabed, no land connection with the mainland
ever existed. One modern hypothesis is that tortoises floated from
the mainland on mats of vegetation that regularly break free from
coastal riverbanks during storms. Without predators on the islands,
tortoises gradually increased in number.
Darwin also explained some of the differences that he saw. In
dryer regions, where vegetation was sparse, tortoises with longer
necks would be favored because they could reach higher to get
food. In moister regions, tortoises with longer necks would not necessarily
be favored, and the shorter-necked tortoises could survive.
Darwin made similar observations of a group of dark, sparrowlike
birds. Although he never studied them in detail, Darwin
noticed that the Galápagos finches bore similarities suggestive of
common ancestry. Scientists now think that Galápagos finches also
descended from an ancestral species that originally inhabited the
mainland of South America. The chance arrival of a few finches, in
either single or multiple colonization events, probably set up the
first bird populations on the islands. Early finches encountered
many different habitats, all empty of other birds and predators. Ancestral
finches, probably seed eaters, multiplied rapidly and filled the
seed-bearing habitats most attractive to them. Fourteen species of
finches arose from this ancestral group, including one species found
on small Cocos Island northeast of the Galápagos Islands. Each
species is adapted to a specific habitat on the islands. The most obvious
difference between these finches relates to dietary adaptations

and is reflected in the size and shape of their bills. The finches of the
Galápagos Islands provide an example of adaptive radiation—the
formation of new forms from an ancestral species, usually in response
to the opening of new habitats (figure 4.8).
Darwin’s experiences in South America and the Galápagos
Islands convinced him that animals change over time. It took the
remaining years of his life for Darwin to formulate and document
his ideas, and to publish a description of the mechanism of evolutionary
change.