The Ecology Book

27

of 1835 provided especially strong

evidence for his later theory of

evolution by natural selection. Here,

he observed that the shape of the

carapaces (shells) of giant tortoises

varied slightly from island to island.

Darwin was also intrigued to find

that there were four broadly similar,

yet clearly distinct, varieties of

mockingbirds, but that no single

island had more than one species

of the bird. He saw small birds,

too, that looked alike but had a

range of beak sizes and shapes.

Darwin deduced that each group

possessed a common ancestor but

had developed diverse traits in

different environments.

Darwin’s conclusions

On Darwin’s return to England, the

differing beaks of the small birds

he had found on the Galapagos,

usually called “finches” although

they are not in the true finch family,

set him thinking. He knew that

a bird’s beak is its key tool for

feeding, so its length and shape

offer clues to its diet. Later research

revealed that there are 14 different

finch species on the Galapagos

islands. The differences in their

beaks are marked and significant.

For example, cactus finches have

long, pointed beaks that are ideal

for picking seeds out of cactus

fruits, while ground finches have

shorter, stouter beaks that are

better suited for eating large seeds

on the ground. Warbler finches have

slender, sharp beaks, which are

ideal for catching flying insects.

Darwin speculated that the

finches were descended from a

common ancestral finch that had

reached the archipelago from the

mainland of South America. He

concluded that a variety of finch

THE STORY OF EVOLUTION

See also: Early theories of evolution 20–21 ■ The rules of heredity 32–33 ■ The role of DNA 34–37 ■ The selfish gene 38–39

■ The food chain 132–133 ■ Mass extinctions 218–223 ■ Population viability analysis 312–315

populations had evolved in different

Galapagos habitats, each group

adapted for a more or less specialist

diet by a process that he would

later call “natural selection.” Over

time, the finch populations had

become distinct species.

In the early 21st century,

researchers at Harvard University

uncovered new evidence of how

this happens at a genetic level.

Their findings, published in 2006,

showed that a molecule called

calmodulin regulates the genes

involved in shaping birds’ beaks,

and is found at higher levels in

longer-beaked cactus finches than

in shorter-beaked ground finches.

Refining the theory

Darwin was influenced by Thomas

Ma lthus’s An Essay on the Principle

of Population (1798), in which

Malthus predicted that population

growth would eventually outstrip

food production. This idea matched

the evidence Darwin had observed

of ongoing competition between

individual animals and species for

resources. This competitive aspect

formed the backbone of Darwin’s

coalescing theory of evolution.

By 1839, Darwin had developed

an idea of evolution by natural

selection. He was, though, reluctant

to publish because he understood

that the theory would unleash a

storm of controversy from those

who would view it as an attack

on religion and the Church. When,

in 1857, he began receiving

communications from fellow British

naturalist Alfred Russel Wallace,

who had independently arrived at

very similar conclusions, Darwin

realized he had to publish his ❯❯

Comparison of Galapagos finch

bill structure

Geospiza magnirostris

The short, sharp bill of the Large

Ground Finch, the biggest of Darwin’s

finches, enables it to crack nuts.

Geospiza fortis

The bill of the Medium Ground Finch

is variable, evolving rapidly to adapt

to whatever size seeds are available.

Geospiza parvula

The stubby bill of the Small Tree Finch,

which forages in foliage, suits its diet

of seeds, fruits, and insects.

Certhidea olivacea

The slender, probing bill of the Green

Warbler-finch helps it catch small

insects and spiders.

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