Biology (Holt)

Darwin visited the Galápagos Islands, located about 1,000 km (620

mi) off the coast of Ecuador. Darwin was struck by the fact that many

of the plants and animals of the Galápagos Islands resembled those

of the nearby coast of South America. Darwin later suggested that

the simplest explanation for this was that the ancestors of Galápagos

species such as those shown in Figure 3,migrated to the islands from

South America long ago and changed after they arrived. Darwin later

called such a change “descent with modification”—evolution.

When Darwin returned from his voyage at the age of 27, he con-

tinued his lifelong study of plants, animals, and geology. However,

he did not report his ideas about evolution until many years later.

During those years, Darwin studied the data from his voyage. As

Darwin studied his data, his confidence that organisms had evolved

grew ever stronger. But he was still deeply puzzled about how evo-

lution occurs.

Growth of Populations

The key that unlocked Darwin’s thinking about how evolution takes

place was an essay written in 1798 by the English economist Thomas

Malthus. Malthus wrote that human populations are able to increase

faster than the food supply can. Malthus pointed

out that unchecked populations grow by geometric

progression, as shown in Figure 4.Food supplies,

however, increase by an arithmetic progression at

best, also shown in Figure 4. He suggested that

human populations do not grow unchecked

because death caused by disease, war, and famine

slows population growth.

The term population, as it is used in biology,

does not only refer to the human population. In

the study of biology, a consists of all

the individuals of a species that live in a specific

geographical area and that can interbreed.

population

Fruit eater

Insect

eaters

Insect eater

Seed

eater

Vegetarian

tree finch

Small

insectivorous

tree finch

Cactus

ground finch

Large

ground

finch

South American

warbler finch

Figure 3 Darwin’s finches.
Darwin discovered that these
finches closely resembled
South American finches.

Time

Geometric

progression

Arithmetic

progression

Two Rates of Progression

278 CHAPTER 13The Theory of Evolution

Figure 4 Geometric and
arithmetic progressions.
The blue graph line shows
uncontrolled population
growth, in which the numbers
increase by a multiplied con-
stant. The red graph line
shows increased food supply,
in which the numbers increase
by an added constant.