CHAPTER 17
Evolution of Populations
17.1 Genes and Variations
Key Questions
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How is evolution defined in genetic terms?
What are the sources of genetic variation?
What determines the number of phenotypes for a given
trait?
Vocabulary you should learn:
Gene pool
Allele frequency
Single-gene trait
Polygenic trait
Natural selection
Quick review
A population is a group of individuals of the
same species that mate and produce
offspring.
 A gene pool consists of all the genes,
including all the different alleles for each gene
that are present in a population.
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Populations and Gene Pools
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Researchers study gene pools by examining the
relative frequency of an allele.
The relative frequency of an allele is the number of
times a particular allele occurs in a gene pool,
compared with the number of times other alleles
for the same gene occur.
mutations
mutations
mutations
mutations
mutations
A single-gene trait is a trait controlled
by only one gene.
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Single-gene traits may have just two or
three distinct phenotypes.
The most common form of the allele
can be dominant or recessive.
An example is the presence of dark
bands that appear on the shells of
snails.
Even though the allele for shells
without bands is dominant, a
population may show a greater
frequency of the “with bands”
phenotype
Polygenic traits are traits controlled by two or
more genes.
•Each gene has two or more alleles & many
possible genotypes and different phenotypes.
•Human height, which varies from very short to
very tall, is an example of a polygenic trait.
• The bell-shaped curve in the graph is typical of
polygenic traits.
17.2
Evolution as Genetic Change in Populations
Key Questions
 How does natural selection affect single-gene and polygenic traits?
 What is genetic drift?
 What conditions are required to maintain genetic equilibrium?
Vocabulary you need to learn:
 Directional selection
 Stabilizing selection
 Disruptive selection
 Genetic drift
 Bottleneck effect
 Founder effect
 Genetic equilibrium
 Hardy-Weinberg principle
 Sexual selection
Genetic Drift
•Genetic drift or allelic drift is the change in
the frequency of a gene variant (allele) in a
population due to random sampling.
When the beetles reproduced, just by random luck more brown genes
than green genes ended up in the offspring. In the diagram at right,
brown genes occur slightly more frequently in the offspring (29%)
than in the parent generation (25%).
The Founder Effect
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a particular allele or spontaneous mutation in a
small or isolated population may become very
common in just a few generations.
Founder Effect: Kentucky Blue People
These two are probably relatives of the original mutant.
http://www.rootsweb.ancestry.com/~kyperry3/Blue_Fugates_Troublesome_Creek.html
Activity