Evolution, 4th Edition

EvoluTIonARy BIology 11

a naturalist and captain’s companion on the British Navy ship h.m.s. Beagle, tasked

with charting the coast of South America.

The voyage of the Beagle lasted from December 27, 1831, to October 2, 1836.

The ship spent several years traveling along the coast of South America, where

Darwin observed the natural history of the Brazilian rainforest and the Argentine

pampas, then stopped in the Galápagos Islands, which lie on the equator off the

coast of Ecuador. In the course of the voyage, Darwin became an accomplished

naturalist, collected specimens, made innumerable geological and biological

observations, and conceived a new (and correct) theory about the formation of

coral atolls.

Soon after Darwin returned, the ornithologist John Gould pointed out that

Darwin’s specimens of mockingbirds from the Galápagos Islands were so differ-

ent from one island to another that they represented different species (FIGURE

1.7). Darwin then recalled that the giant tortoises, too, differed from one island to

the next (FIGURE 1.8). These facts, and the similarities between fossil and living

mammals that he had found in South America, triggered his conviction that dif-

ferent species had evolved from common ancestors.

Darwin’s comfortable finances enabled him to devote the rest of his life exclu-

sively to his scientific work (although he was chronically ill for most of his life

after the voyage). He set about amassing evidence of evolution and trying to

conceive of its causes. In 1838, at the age of 29, Darwin read an essay by the Charles Robert Darwin

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(A) Lamarck’s hypothesis

Young adults Older adults

Generation 1

Young adults

Generation 2

Young adults Surviving adults

Generation 1

Young adults

Generation 2

(B) Darwin’s hypothesis

FIGURE 1.6 Contrast between Lamarck’s

and Darwin’s hypotheses for how charac-

teristics evolve, shown across two genera-

tions. (A) Under Lamarck’s hypothesis, traits

change within the lifetime of individuals

because of their needs, illustrated here by

giraffes that need longer necks to reach

high leaves. Changes that are acquired

during this generation are passed on to

the next generation. (B) Under Darwin’s

hypothesis, there is variation among

individuals at the start of each generation.

Individuals with certain traits (e.g., a longer

neck) have a greater chance of surviving.

The variation is inherited, so survivors pass

on their traits to the next generation. Dar-

win was right, but about 50 years would

pass before scientists would understand

how the inherited variations arise.

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