What is biological evolution?- In biology, evolution is genetic change in a line of descent
through successive generations. - Combining observations of the natural world with ideas about
interactions of populations with their environment, Darwin
forged his theory of evolution by natural selection.
taKe-Home messageevolution Genetic change
in a line of descent over
time.
n In Latin, the word evolutio means “unrolling.” That image is
an apt way to begin thinking of biological evolution.In biology, evolution is genetic
change in a line of descent over
time. The evolution of life on Earth
has resulted in many hundreds of
thousands of species. Of those living
today, some are closely related, others more distantly. As
described later in this chapter, genetic evidence reveals that
humans share a common ancestor with life forms as differ-
ent from us as bacteria and corn plants. Shared ancestry is
a basic idea in evolution.
In the early 1800s, the source of Earth’s remarkable diver-
sity of life forms was hotly disputed. Many people believed
that all species had come into existence at the same time
in the distant past, at the same center of creation, and had
not changed since. By the mid-1800s, however, centuries of
exploration and advances in the sciences of geology and
comparative anatomy had raised questions. Why weresome species found only in particular isolated regions?
Why, on the other hand, were similar (but not identical)
species found in widely separated parts of the world?
Why did species as different as humans, whales, and bats
have some strikingly similar body features? What was the
significance of discov eries of similar fossil organisms in
similar layers of Earth’s sedimentary rocks—regardless of
where in the world the layers were?
In 1831 Charles Darwin was a 22-year-old with a degree
in theology—though he wasn’t interested in being a clergy-
man. He enjoyed hunting, fishing, collecting shells, or sim-
ply watching wildlife. A botanist friend arranged for him to
work as a naturalist aboard the HMS Beagle during a 5-year
voyage around the world (Figure 23.1). The Beagle sailed first
to South America, and during the long Atlantic crossing
Darwin studied geology and collected marine life. During
stops along the coast and at various islands, he observed
other species of organisms in environments ranging from
sandy shores to high mountains. After returning to England
in 1836, Darwin began talking with other naturalists about a
topic that was on many scholars’ minds—the growing body
of evidence that life forms evolve, changing over time.
A clue as to how this might happen came from Thomas
Malthus, a British clergyman and economist, who proposed
that a population will tend to outgrow its resources, and so
in time its members must compete for what is available.
Darwin’s observations also suggested that any population
can produce more individuals than the environment can
support, yet populations tend to be stable over time. For
instance, a starfish can release 2.5 million eggs a year, but
the oceans are not filled with starfish. What determines
who lives and who dies as predators, starvation, and envi-
ronmental events take their toll? Chance could be a factor,
but Darwin and others came up with a second major clue—
that even members of the same species vary in their traits.
Darwin’s melding of his observations of the natural
world with the ideas of others led him to propose that evo-
lution could occur by way of a process called natural selec-
tion. Wide acceptance of this theory would not come until
nearly 70 years later, when the then-new field of genetics
provided insights into how traits could vary. Now let’s con-
sider some current views of the mechanisms of evolution.a Little evolutionary History
Figure 23.1 Darwin’s voyage on the Beagle spurred his
thinking about evolution. A The Beagle’s route from England to
South America, where the ship called at B the Galápagos Islands.
C Charles Darwin. (A: © Cengage Learning; B: Dieter & Mary Plage/Survival Anglia/
Oxford Scientific; C: Charles Darwin (w/c on paper), English School, (19th century)/Down
House, Downe, Kent, UK/© English Heritage Photo Library/ The Bridgeman Art Library)Galápagos
IslandsEQUATORroute of
BeagleAB C23.1
440 Chapter 23Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).