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Forces of Evolution
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: December 17, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
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Chapter 1. Forces of Evolution
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Forces of Evolution
List processes that change gene frequencies of a population.
Describe how mutations alter allele frequencies.
Define gene flow.
Explain genetic drift.
Compare the bottleneck effect to the founder effect.
How do a population’s genes change?
Remember, without change, there cannot be evolution. Together, the forces that change a population’s gene frequencies are the driving mechanisms behind evolution.
Forces of Evolution
The conditions for Hardy-Weinberg equilibrium are unlikely to be met in real populations. The Hardy-Weinberg
theorem also describes populations in which allele frequencies are not changing. By definition, such populations are
not evolving. How does the theorem help us understand evolution in the real world?
From the theorem, we can infer factors that cause allele frequencies to change. These factors are the "forces of
evolution." There are four such forces: mutation, gene flow, genetic drift, and natural selection. Natural selection
will be discussed in the "Natural Selection" concept.
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Mutation
Mutation creates new genetic variation in a gene pool. It is how all new alleles first arise. In sexually reproducing
species, the mutations that matter for evolution are those that occur in gametes. Only these mutations can be passed
to offspring. For any given gene, the chance of a mutation occurring in a given gamete is very low. Thus, mutations
alone do not have much effect on allele frequencies. However, mutations provide the genetic variation needed for
other forces of evolution to act.
Gene Flow
Gene flow occurs when individuals move into or out of a population. If the rate of migration is high, this can have
a significant effect on allele frequencies. The allele frequencies of both the population they leave and the population
they enter may change.
During the Vietnam War in the 1960s and 1970s, many American servicemen had children with Vietnamese women.
Most of the servicemen returned to the United States after the war. However, they left copies of their genes behind
in their offspring. In this way, they changed the allele frequencies in the Vietnamese gene pool. Was the gene pool
of the American population also affected? Why or why not?
Genetic Drift
Genetic drift is a random change in allele frequencies that occurs in a small population. When a small number of
parents produce just a few offspring, allele frequencies in the offspring may differ, by chance, from allele frequencies
in the parents.
This is like tossing a coin. If you toss a coin just a few times, you may, by chance, get more or less than the expected
50 percent heads or tails. In a small population, you may also, by chance, get different allele frequencies than
expected in the next generation. In this way, allele frequencies may drift over time.
There are two special conditions under which genetic drift occurs. They are called bottleneck effect and founder
effect.
1. Bottleneck effect occurs when a population suddenly gets much smaller. This might happen because of a
natural disaster such as a forest fire. By chance, allele frequencies of the survivors may be different from those
of the original population.
2. Founder effect occurs when a few individuals start, or found, a new population. By chance, allele frequencies
of the founders may be different from allele frequencies of the population they left. An example is described
in the Figure 1.1.
Summary
• There are four forces of evolution: mutation, gene flow, genetic drift, and natural selection.
• Mutation creates new genetic variation in a gene pool.
• Gene flow and genetic drift alter allele frequencies in a gene pool.
Explore More
Use this resource to answer the questions that follow.
• Mechanisms of Evolution at http://www.cliffsnotes.com/sciences/biology/biology/principles-of-evolution/mech
anisms-of-evolution .
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Chapter 1. Forces of Evolution
FIGURE 1.1
Founder Effect in the Amish Population. The Amish population in the U.S. and Canada had a small number of
founders. How has this affected the Amish gene pool?
1. Define each of the following:
a.
b.
c.
d.
mutation,
gene flow,
genetic drift,
natural selection.
2. Which mechanism(s) is/are the source of variation within a population?
3. Which mechanism(s) is/are the most important influences on evolution?
Review
1.
2.
3.
4.
Identify the four forces of evolution.
Why is mutation needed for evolution to occur, even though it usually has little effect on allele frequencies?
What is founder effect? Give an example.
Explain why genetic drift is most likely to occur in a small population.
References
1. Horse and Buggy: Derek Jensen; Hands: G Baujat and M Le Merrer; Composite created by CK-12 Foundation. Horse and Buggy: http://commons.wikimedia.org/wiki/File:Shipshewana-indiana-amish-buggy.jpg;
Hands: http://commons.wikimedia.org/wiki/File:Polydactyly_ECS.jpg . Horse and Buggy: Public Domain;
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Hands: CC BY 2.0
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