The Evidence for Natural Selection ..................................................

LECTURE

If you look at a lot of species, one of the strange things you ¿ nd

is you can often ¿ nd very similar features across a wide range of

different species.

B

ecause his ideas seemed so radical to his contemporaries, Darwin
defended them with great care. Yet he did so in language accessible
to a general readership, which is why On the Origin of Species is
one of the most readable of all scienti¿ c classics. We will ¿ nd that many of
Darwin’s arguments are easier to understand if we appreciate that he was
arguing against an alternative theory of life: the idea that all living things
had been created by a deity. This lecture discusses the evidence available
in Darwin’s time, as well as the evidence that has become available
since his death.

For Darwin himself, the ¿ rst argument for his theory was that it made sense
of what he had encountered on his travels. Darwin was repeatedly puzzled
by ¿ nding many neighboring species that were clearly related but also
slightly different. These differences were particularly striking among the
birds, tortoises, and lizards of the Galapagos Islands, a group of 19 small
volcanic islands in the Paci¿ c Ocean about 600 miles west of Ecuador. For
example, there seemed to be 14 distinct species of ¿ nches, each exquisitely
adapted to one of the islands. In 1846, Darwin wrote, “Seeing this gradation
and diversity of structure in one small, intimately related, group of birds, one
might really fancy that from an original paucity of birds in this archipelago,
one species had been taken and modi¿ ed for different ends” (Eldredge,
Darwin, p. 89).

Only on his return did Darwin realize how similar the different species of
¿ nch were. This suggested they had all once belonged to a single species. Yet
in each island, slightly different beaks must have been favored because of
slight differences in the trees and fruit of the island. Over many generations,
those individuals with the best-adapted beaks would have fed and bred
more successfully—and left more offspring, with similar beaks. Eventually,