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Natural Selection
What do you know about evolution? Evolution is a population’s
change in inheritable traits over time. One of the most common
examples of evolution is an ape walking and evolving into an animal
that stands in an upright position. One mechanism of evolution is
natural selection. Natural selection is a popular theory proposed
by Charles Darwin which suggests that species change over time
because organisms with traits well suited to their environments
have greater success surviving and reproducing.
natural selection:
theory that organisms
with traits that are well
suited to their
environment survive
and reproduce more
successfully
Charles Darwin’s
theory of evolution
by natural selection
changed the way
we understood the
diversity of life.
Factors that Contribute to Natural Selection
Genetic variation: There are differences among individuals within
the same species. Traits that are more favorable to a species are
more likely to be passed on from generation to generation,
increasing the species’ chances of survival.
Overproduction: If a species produces more offspring than the
environment can support, many of the offspring will not survive into
adulthood.
Competition: Since resources such as food, water, and space are limited, offspring must compete
for the resources for survival. Individuals with a trait that gives them a competitive advantage are
more likely to reproduce.
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Natural Selection
Adaptation: An adaptation is an inherited trait that improves the
survival of a species.
For example, the giraffe underwent mutations that caused some
offspring to have longer necks. The giraffes with longer necks
ultimately had a better rate of survival because they could stretch
their necks to reach leaves on tall trees. Over time, more and more
giraffes inherited the longer necks, increasing the survival of the
species.
Adaptation: an
inherited trait that
improves the
survival of a
species.
Many birds migrate
in the winter.
How does a species change over time?
An environment meets the needs of the organisms that live there.
Environments are always changing. Adaptations can be behavioral
traits that are inherited over generations as well as physical traits.
Think about the seasons; it is warm in the summer and cold in the
winter.
Organisms must find ways to deal with these changes. Some plants
lose their leaves and stop growing when it is cold.
Some animals grow thick fur coats. Animals like birds migrate to
warmer areas to make it easier to find food during the winter season.
Plants also have adaptations that help them survive. For example,
water lilies have large thin leaves. The structure of the leaves help
water lilies float so they can get enough sunlight for photosynthesis.
Floating is beneficial because the leaves can produce more food.
More food produces more seeds, and naturally the trait is passed to
the next generation.
Genetic variation:
differences in physical
traits as well as
behavioral traits
among individuals
competition: contest
between individuals or
species for resources
and space
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Natural Selection
Galapagos Finches’ Natural Selection
Prime examples of adaptation are seen in the Galapagos Medium
Ground Finches. A study in the 1970s revealed that variation in
beak size changed over time. During times of drought, finches with
large beaks had a higher survival rate because they were able to
eat the seeds available during the drought that those with smaller
beaks had a harder time eating. Therefore, the generations of
finches after the drought had more birds with larger beaks. After the
drought, more seeds were available to the the finches with smaller
beaks. The larger beak was no longer a beneficial trait, so the
number of birds with larger beaks stopped increasing.
Artificial Selection
Artificial selection is another mechanism by which evolution can
occur. Artificial selection is the intentional breeding of plants or
animals to produce desirable traits. It can increase diversity by
creating new variants and also decrease diversity by eliminating
other variants.
Artificial selection is also called selective breeding. Selective
breeding of domestic animals, thoroughbred horses, and meats we
eat such as cows, pigs, and chickens. Common examples of
artificial selection include cabbage, broccoli, cauliflower, collards,
and kale, which are all members of the same species, Brassica
oleracea. These common vegetables were cultivated from the wild
mustard plant.
There is a downside to artificial selection because it removes
genetic variation from a species, making it susceptible to disease
and environmental changes.
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Natural Selection
The chart below list examples of adaptations of different organisms. For each example, state
whether the adaptation is the result of natural selection or artificial selection. Describe at least one
advantage of the adaptation.
Example of Adaptation
Natural Selection or
Selective Breeding?
Advantage(s) of
Adaptation
Apples with large, sweet fruit
Cacti with sharp spines
Bats that can fly
Crabs that can hide in the sand
Tomato plants that can grow in
salty environments
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Natural Selection
Artificial Selection
Find samples of fruits and vegetables in your refrigerator, or search for pictures on the Internet.
Selective breeding is common in food crops; these are often called genetically modified crops.
Breeders can select for a variety of traits, such as root size, leaf size in lettuce, fruit size in
tomatoes, vitamin content in carrots, and flower size in cauliflower. Choose at least six different
fruits and/or vegetables to discuss.
Here are some questions to discuss with your child:
•  Which traits do you think were artificially selected for in each fruit or vegetable? Why do you
think these traits were selected?
•  Would similar adaptations be beneficial or harmful in the wild?
•  Which type of environmental conditions would be most favorable for these plants?
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