Lecture 13: Darwin and Natural Selection
When Darwin left on the Beagle, he shared the orthodox belief that living
species had been made by the creator, more or less in their existing forms.
What he saw on his travels undermined this conviction. First, he observed
the staggering variety of living organisms and the many subtle variations
between species. Second, he noted many examples of similar but not identical
species living close to each other, such as the ¿ nches, tortoises, and iguanas
of the Galapagos Islands. In South America, he also found fossils (such
as those of armadillo-like creatures) that were similar but not identical to
species still living in the same areas. He concluded that all species, including
humans, must be the products of slow, continuous change. In a letter to a
friend, he wrote, “I am almost convinced (quite contrary to the opinion I
started with) that species are not (it is like confessing a murder) immutable”
(Eldredge, Darwin, p. 58). Third, Darwin understood that few would accept
this conclusion unless he could explain how species changed. In other words,
he had to solve the riddle of adaptation.Darwin stumbled on the solution two years after returning home, after
reading Thomas Malthus’s Essay on the Principle of Population (1798).
Malthus was a pioneer of demography, or the study of populations. He
pointed to the terrifying fact that in every generation and every species,
most individuals die before they can reproduce. Darwin immediately saw
an analogy with pigeon breeding. Breeders only allowed individuals with
particular features to breed, in the expectation that these features would
become more common in subsequent generations. Darwin concluded that
nature “selected” individuals to breed in a similar way.But what were the criteria for selection used by nature? His answer was
“¿ tness”—how well they ¿ tted their environment. Those individuals whose
features best ¿ tted them for their environment would survive and reproduce
just as those best ¿ tted to the speci¿ cations of the breeder survived in the
arti¿ cial world of pigeon breeding. In his autobiography, he wrote, “it at
once struck me that under these circumstances favorable variations would
tend to be preserved, and unfavorable ones to be destroyed. The result would
be the formation of new species” (Eldredge, Darwin, p. 52). Repeated over
many generations, this mechanism could explain why species changed and
why those changes that were preserved tended to be “adaptive”: They tended
to aid the species’ survival.