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10.3 Theory
Selection
Key Concept Darwin proposed natural selection as a
mechanism for evolution.
Several key insights* led to Darwin’s idea for
natural selection.
The variation of similar species among islands, fossil
evidence, and geologic events convinced Darwin that
evolution occurs. But he still wondered how evolution
occurs. Here, you will read about some of Darwin’s
reasoning that led him to his idea for natural selection.
Artificial Selection Darwin noticed that plants and
animals that are raised by humans had variations in
traits that were not seen in their wild relatives. Think of
all the different breeds of dogs that you have seen. In a
process called artificial selection, humans select individuals with the traits they desire, and then breed them
to produce more individuals with those traits.
Heritability In order for artificial selection to occur, traits
must be heritable. Heritability (her-ih-tuh-BIHL-uhtee) is the ability of a trait to be passed down from one
generation to the next. Things that are acquired in an
organism’s life, like a broken bone, are not heritable.
Natural Section Darwin reasoned that a process similar
to artificial selection could happen in nature. In artificial
selection, humans are the source of selection. In natural
selection, the environment is the source of selection.
Natural selection is a process in which individuals that
have inherited beneficial* adaptations produce more offspring than do other individuals.
Humans have changed animal species,
such as these different dog breeds,
through artificial selection.
* Academic Vocabulary
insight a clear realization about a topic
beneficial resulting in good; helpful
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Struggle for Survival Darwin was influenced by the
work of an economist named Thomas Malthus. Malthus
proposed that resources like food, water, and shelter were
limits to human population growth. Darwin reasoned
that a similar struggle happened in nature.
Darwin saw great variation within populations of
organisms. A population is all the individuals of a
species that live in an area. He saw individuals with
adaptations that matched their environment. Darwin
proposed that these adaptations arose over many generations in a process he called “descent with modification.”
What is the difference between artificial and
natural selection?
Natural selection explains how
evolution can occur.
The white fur of this rabbit allows it to
blend in with its environment.
Darwin was not the only scientist studying evolution during this time.
Another scientist named Alfred Wallace independently developed an
explanation of how evolution occurs. Wallace’s explanation was very
similar to Darwin’s. In the late 1850s, the ideas of Darwin and Wallace
were presented to the scientific community.
There are four main principles to the theory of natural selection:
variation, overproduction, adaptation, and descent with modification.
• Variation Individuals of a species differ due to genetic variation.
Heritable differences are the basis for natural selection.
• Overproduction Organisms have more offspring than can survive.
This results in competition among offspring for resources.
• Adaptation Some individuals have certain variations that allow
them to survive better than other individuals in their environment.
These individuals are “naturally selected” to live longer and produce
more offspring that also have those adaptations.
• Descent with modification Over time, natural selection will result
in species with adaptations that are beneficial for survival and reproduction in a particular environment. More individuals will have the
trait in every following generation, as long as the environmental
conditions stay the same.
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VOCABULARY
The term descent is used
in evolution to mean the
passing of genetic information from generation
to generation.
Let’s apply these four principles to an example of natural selection.
About 11,000 years ago, jaguars faced a shortage of food due to a changing climate. There were fewer mammals to eat, and jaguars had to eat
reptiles to survive. Variations in jaw and tooth size allowed some individuals to more easily eat shelled reptiles.
jaguar 2
jaguar 1
overproduction
A jaguar may produce many offspring, but not all of
young will survive due to competition for resources.
adaptation
Jaguars with larger jaws and teeth are able to eat shelled
reptiles. These jaguars are likely to survive longer and
leave more offspring than jaguars that can eat
only mammals.
variation
Some jaguars, such as jaguar 1
shown here, may be born with slightly
larger jaws and teeth due to natural
variation in the population. Some
variations are heritable.
jaguar skull 1
jaguar skull 2
descent with modification
Because large teeth and jaws are heritable traits,
they become more common
characteristics in
the population.
In biology, the term fitness is a measure of the
ability to survive and reproduce. In the example
above, jaguars with larger teeth and jaws had higher
fitness after the change in climate.
What is the difference between the biological term fitness and
the common meaning of the word?
Natural selection acts on existing variation.
Natural selection cannot make new alleles. It can only work with variation that already exists. In other words, natural selection acts on phenotypes, or physical traits, and not on genetic material itself.
Changing environments As an environment changes, different traits
become beneficial. Think about the jaguars. When mammals were their
main food source, small teeth and jaws were beneficial. But when the
environment changed, larger teeth and jaws became beneficial to better
eat reptiles. Because the environment constantly changes, a trait that is
an advantage today may be a disadvantage in the future.
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Adaptations as compromises Adaptations can sometimes be thought of
as compromises. One example of an adaptive compromise is the panda
bear’s “thumb.” Modern pandas have five digits that are like your fingers
and a sixth digit that functions like a thumb. This digit is actually a wrist
bone. Ancestral pandas with bigger wrist bones had an advantage in hold­ing and eating their food. Over time, this adaptation became typical of
the species.
Why might a trait that is an advantage today become a disadvantage in the future?
10.3 Vocabulary Check
artificial selection
heritability
natural selection
population
fitness
Mark It Up
Go back and highlight
each sentence that
has a vocabulary
word in bold.
1. In the list above, draw an arrow pointing to the term that describes
the process by which humans breed animals or plants for certain traits.
2. Circle the term that describes a mechanism for evolution in nature.
3. Underline the term that means the ability of a trait to be passed from
one generation to the next.
4. Box the term that is a measure of the ability to survive and reproduce.
10.3 The Big Picture
5. What is the difference between the meanings of the terms evolution
and natural selection?
6. What are four main principles to the theory of natural selection?
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