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Variety Is the Spice of Life
Imagine a field full of mice. Some mice are brown, and
some are white. Which mice would most likely be killed
by a hungry hawk flying overhead? The white mice would
be easier to see, and would therefore be more likely to be
caught and eaten. If this scenario repeated itself many
times, and the white mice were killed before they even
had a chance to reproduce, nearly the entire population
of mice would likely be brown after just a few
generations. This is an example of what biologists call
"natural selection."
Natural Selection and Evolution
English biologist Charles Darwin was the first to describe
natural selection. In his book, The Origin of Species,
Darwin proposed that natural selection results in
evolution.
Charles Darwin is widely regarded
Darwin spent 4 years traveling around the world on a
as the father of evolution.
small sailing ship, the HMS Beagle. Over the course of
his voyage, he saw numerous unusual plants and animals. He
also saw seashells in rocks from high mountain peaks and
documented numerous other sights and events. Although he
was religious, Darwin could not reconcile his spiritual beliefs
with his experience of nature. He sought a scientific explanation
for his observations.
Darwin had several influences when as he developed his ideas
and thoughts. Sir Charles Lyell greatly influenced Darwin’s
ideas. Lyell and Darwin shared the same idea about geological
processes. Lyell’s theories helped Darwin validate his own ideas.
Over many years and through painstaking research, Darwin
developed a revolutionary new theory. According to his idea,
species of plants and animals evolved, or changed, over time,
and with enough time such changes could result in new species.
While Darwin privately worked on his ideas, he consulted with
Alfred Wallace. Wallace shared his thoughts with Darwin and
aided Darwin by contributing species for study. Wallace sought
Darwin’s help in publishing his own theories. To Darwin’s
surprise, the theories that he had developed were very much like
those of Wallace. It was through work with Lyell and Wallace
that Darwin was encouraged to publish his works.
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Variety Is the Spice of Life
Darwin was not the first to propose such a thing. Other
scientists had suggested similar ideas. In fact, thinkers as far
back as the ancient Romans and Greeks had proposed that
species changed from one form into another. Jean Lamarck also
had his own ideas about how organisms evolved. Lamarck felt
that organisms evolved to better themselves or to better adapt to
their lifestyles. They developed the traits that they needed and
they lost the ones that they did not use. But before Darwin, no
one had proposed how organisms evolved. Darwin’s process of
natural selection provided a mechanism for evolution.
Darwin had three key insights into how natural selection occurs.
First, he recognized that not all individuals within the same
species are identical. Such variation is easily observable in
plants and animals.
Darwin’s second insight was that plants and animals pass on
their specific features or traits. If two parents both have brown
hair, the chances are that their children have brown hair. Hair
color is an example of a heritable trait—a trait that can be
inherited. Not all traits are heritable. For example, when people
get a tan, they do not pass the tan on to their children. The
change in skin color resulting from a tan is not heritable.
Modern biologists know that genes are responsible for passing
on traits. Darwin did not know about genes, but he recognized
that some traits were heritable.
Darwin’s third insight was that natural selection affects survival.
An individual can only pass on its traits if it survives to
reproduce. If it does not survive, it will not reproduce and its
traits will not be passed on. A colleague of Darwin’s coined the
phrase “survival of the fittest” to describe this principle.
Darwin’s insights explain how natural selection results in
evolution. Trait variation causes some individuals to reproduce
more successfully. Since the variation is heritable, individuals
that reproduce successfully pass on their traits to offspring.
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Variety Is the Spice of Life
Man-Made Evolution?
Almost all scientists today accept Darwin’s theory of
evolution. It is the simplest way to explain Earth’s
variety of life. Evolution also explains why life is
changing as humans alter the environment. For
example, many insects and weeds are now resistant
to formerly effective pesticides. Antibiotics that used
to quickly clear up an infection sometimes fail to
cure the disease.
Aphids, such as these infesting the
Pesticides prevent reproduction of most target
underside of a leaf, are among the
organisms, but some individuals survive. These
many pests resistant to pesticides.
individuals pass on their ability to resist the toxins.
Over time, the resistance genes spread through the population
and the pest becomes resistant due to selection imposed by the
pesticide. The same principle is at work when bacteria develop
resistance to antibiotics.
Evolution may take place gradually over long periods of time, or
it can occur relatively quickly. In the southwestern United States
deserts and in Mexico, such evolution can be seen in the rock
pocket mouse (chaetodipus intermedius). The mouse comes in
two color variations: light and dark. The rock pocket mouse that
lives in the lighter colored desert is lighter colored. The rock
pocket mouse that lives in rocky outcroppings will usually be a
darker color. The darker mice that live in the lighter desert are
easier for predators to locate visually so they can be eaten. The
lighter mice that live near the darker rocks are also easier for
predators to locate visually so they can be eaten. As a result of
this, the lighter-colored mice are plentiful and likely to
reproduce in the lighter-colored desert. The darker-colored mice
are more plentiful in the rockier and darker–colored areas of the
desert. Because of this, scientists can observe evolution over a
shorter period of time.
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Variety Is the Spice of Life
This process might seem obvious today, but in
Darwin’s time many did not accept his ideas. He
was overturning hundreds of years of doctrine
about how life originated. From the outset, his
ideas were controversial. Publication of The
Origin of Species stimulated a flurry of research
activity. Scientists across the world worked to
gather evidence to support or refute Darwin’s
theory.
Over time, researchers accumulated facts and
Natural selection is the best way to explain
evolution of species such as the horses and
observations. Scientists have documented
their relatives.
thousands of examples of evolution. The fossil
record shows how horses evolved from small dog-sized
ancestors into the beautiful creatures we see today. Researchers
in the Galapagos Islands have documented changes in the size of
bird beaks. The observed changes only make sense if they have
been brought about by natural selection.
Humans deliberately change animals and plants through
selection. Darwin used the artificial selection by breeders as an
analogy for natural selection. Humans created hundreds of dog
breeds in a few thousand years. What could natural selection do
over millions of years?
The Root of Ongoing Controversy
Why is it that even though most scientists accept
the reality of evolution, Darwin’s ideas remain
controversial today? The controversy results
partly from confusion over the meaning of the
word “theory.” It might mean a tentative
explanation, or it could mean a well-supported
body of knowledge that explains some feature of
the natural world, such as the theory of gravity.
No one seriously doubts that gravity exists. In
the same way, when scientists talk about the
theory of evolution they are using the word in the latter sense.
Controversy over evolution outside the scientific community
also exists partly because life is so extraordinarily complex.
Scientists do not have all the answers about how a particular
trait evolved, and they do not fully understand all the genetic
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Variety Is the Spice of Life
mechanisms. Nor do they have a time machine to go back and
investigate the unique circumstances that result in new species
arising. However, Darwin’s idea of natural selection is the only
scientific way to make sense of the variety and beauty of life.
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Variety Is the Spice of Life
Comprehension Questions
1. Identify a trait that is heritable and one that is not
heritable.
2. Cite one example over evolution over a long period of time
and once example of evolution over a short period of time.
3. How did Darwin’s ideas contribute to the history of
science?
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