Introduction to Natural Selection
A population of mice lived
in a desert with gray sand.
These drawings show how
the population changed
from time 1 to time 3.
1a. Describe how the population of mice was different at time 3 compared to time 1. Explain what happened
to cause this difference. ______________________________________________________________________
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1b. Suppose the mice in drawing 3 had babies. What color fur do you think most of the babies would have?
Explain your reasoning. ______________________________________________________________________
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2. Fitness is defined as the ability to survive and reproduce. For the mice in the figure, which characteristic
increased fitness? ___________________________________________________________________________
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3. The term fitness can have two different meanings, depending on what subject you are discussing.
What does the term fitness mean when biologists are discussing evolution?
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What does the term physical fitness mean?
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A characteristic that is influenced by genes and passed from parents to offspring is called a heritable trait. For
example, fur color is a heritable trait for mice.
A biologist released three pairs of adult mice in a gray sand desert. The first two rows of this chart show these
Generation 1 Adults and their Generation 2 Babies.
4. The last row of this chart shows the Generation 2 Adults, which are the Generation 2 Babies that survived to
become adults. Why was survival higher for the Generation 2 mice with gray fur and lower for the Generation
2 mice with white fur or black fur? _____________________________________________________________
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This table shows some characteristics of the Generation 1 adult females.
Running speed
Number of babies born
Age at death
White Fur
5 cm/sec.
5
3 months
Gray Fur
6 cm/sec.
14
6 months
Black Fur
7 cm/sec.
Tom 7
4 months
5. The Generation 1 adult female with gray fur had the most babies. What is a likely explanation?
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6a. An adaptation is a heritable trait that increases fitness. Which color fur is an adaptation for these mice?
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6b. What evidence supports your answer?
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6c. The percent of Generation 2 Adults that had this adaptation was higher than the percent of Generation 1
Adults that had this adaptation. What caused this increase?
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7a. Predict what would happen to this population of mice after many generations on the gray sand (choose one).
a. About two-thirds (67%) of the mice would have gray fur.
b. Almost all of the mice would have gray fur.
c. There would be equal numbers of mice with white fur, gray fur, and black fur.
7b. Explain your reasoning.
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8a. Suppose that six of the Generation 2 adults migrated to a nearby desert with white sand. Suppose that
these six mice included a pair of white mice, a pair of gray mice, and a pair of black mice. What do you think
the population of mice on the white sand would look like after many generations? Explain your reasoning.
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8b. Which color fur would be an adaptation for the mice on white sand?
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9. Complete this table.
True or
False?
Give an example.
If a characteristic like gray fur is an adaptation
in one environment, then this characteristic
will be an adaptation in all environments.
Over time, an adaptation becomes common in
a population.
10. Explain how a heritable trait that increases fitness becomes common in a population.
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The process that you have been learning about is called natural selection. A heritable trait that increases
fitness tends to become common in a population because individuals that have this trait are more likely to
survive and reproduce and their offspring generally have the same trait.
Evolution is defined as a change over time in the inherited characteristics of a population. Natural selection
plays a major role in evolution.
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