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Ch 16: Evolution of Populations
16.1 Genes and Variation
A. Introduction
1. Darwin’s theory of evolution by natural selection explained how life on Earth changed, or evolved, over
many generations.
2. What Darwin did not know was how heritable traits were passed down through each generation
3. The study of genetics helps scientists understand the relationship between inheritance and evolution
B. Variation and Gene Pools
1. Genetics supports Darwin’s ideas
a. Scientists know that genes control traits and that many genes have at least two forms, or alleles.
b. Also know that members of all species are heterozygous for many genes
2.
In genetic terms, evolution is any change in the relative frequency of alleles in a population
3. Genetic variation is studied in populations.
4. Population- a group of individuals of the same species that interbreed.
5. Members of a population share a gene pool
6. Gene pool- all the genes, and their alleles, in the population.
7. Relative frequency of an allele- the number of times the allele occurs in a gene pool, compared with the
number of times other alleles for the same gene occur.
8. Relative frequency is often expressed as a percentage.
C. Sources of Genetic Variation
1.
The two main sources of genetic variation are mutations and the genetic shuffling that results from
sexual reproduction.
a. Mutation- any change in a sequence of DNA.
i. Mutations occur because of mistakes in DNA replication or as a result of radiation or
chemicals in the environment.
ii. Mutations do not always affect an organism’s phenotype.
b. Gene Shuffling- occurs during gamete formation
i. Most heritable differences are due to gene shuffling.
ii. Crossing-over increases the number of genotypes that can appear in offspring.
iii. Sexual reproduction produces different phenotypes, but it does not change the relative
frequency of alleles in a population
D. Single-Gene and Polygenic Traits
1.
The number of phenotypes produced for a given trait depends on how many genes control the trait.
2. Single-gene trait: is a trait controlled by only one gene
a. If there are two alleles for the gene, two or three genotypes are possible.
b. Example: the allele for a widow’s peak is dominant over the allele for a hairline with no peak.
i. So there are only 2 phenotypes- having a widow’s peak or not having one
ii. However, the presence of a widow’s peak may be less common in a population.
iii. In real populations, phenotypic ratios are determined by the frequency of alleles as well
as by whether the alleles are dominant or recessive
3. Polygenic trait: is controlled by two or more genes.
a. One polygenic trait can have many possible genotypes and phenotypes.
b. Example: Height in humans is a polygenic trait.
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c. Variation in a polygenic trait in a population often forms a bell-shaped curve with most members
in the middle.
d. A bell-shaped curve is also called normal distribution
Polygenic Traits 
Follow the prompts to
make a graph showing
the frequency of different
heights in a group of
students
 Draw 1 bar for
each height range.
The bar should
show how many
students have
heights in that
range.
 Draw a curve
connecting the
tops of the bars
1. What shape is the
curve you drew?
16-2 Evolution as Genetic Change
A. Introduction
1. Natural selection affects which individuals survive and reproduce and which do not.
2. If an individual dies without reproducing, it does not contribute its alleles to the population’s gene pool.
3. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency.
4. Evolution is any change over time in the relative frequencies of alleles in a population.
5. Populations, not individual organisms, can evolve over time.
B. Natural Selection on Single-Gene Traits
1.
Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution.
2. Organisms of one color may produce fewer offspring than organisms of other colors.
3. Example: a lizard population is normally brown, but has mutations that produce red and black forms.
a. Red lizards are more visible to predators, so they will be less likely to survive and reproduce.
Therefore, the allele for red color will become rare.
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b. Black lizards may warm up faster on cold days. This may give them energy to avoid predators.
In turn, they may produce more offspring.
c. The allele for black color will increase in relative frequency.
4. Natural selection can affect the distributions of phenotypes in any of three ways:
• directional selection
• stabilizing selection
• disruptive selection
Natural Selection on a Single-gene Trait 
A colorful mutation occurred in a brown
mouse population, causing barker fur. The
table shows how the population changed over
the next 30 generations.
**What is happening to the relative frequency
of the:
a. lighter fur color allele?
b. darker fur color allele?
**Is the darker mutation favorable or unfavorable?
C. Directional Selection
1. When individuals at one end of the curve have higher fitness than
individuals in the middle or at the other end, directional selection takes
place.
2. The range of phenotypes shifts as some individuals survive and reproduce
while others do not.
3. Directional selection causes a shift in the curve toward the higher fitness
end
4. In this case, birds with larger beaks have higher fitness. Therefore, the
average beak size increases
D. Stabilizing Selection
1. When individuals near the center of the curve have higher fitness than
individuals at either end of the curve, stabilizing selection takes place.
2. This keeps the center of the curve at its current position, but it narrows the
overall graph.
3. Human babies born at an average mass are more likely to survive than
babies born either much smaller or much larger than average
E. Disruptive Selection (sometimes called diversifying selection)
1. When individuals at the upper and lower ends of the curve have higher
fitness than individuals near the middle, disruptive selection takes place.
2. If the pressure of natural selection is strong enough and long enough, the
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Directional selection graph
Stabilizing selection graph
Disruptive selection graph
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curve will split, creating two distinct phenotypes.
3. If average-sized seeds become scarce, a bird population will split into two groups: one that eats small
seeds and one that eats large seeds.
F. Genetic Drift
1. A random change in allele frequency is called genetic drift
2.
In small populations, individuals that carry a particular allele may leave more descendants than other
individuals do, just by chance.
3.
Over time, a series of chance occurrences of this type can cause an allele to become common in a
population.
4. Genetic drift may occur when a small group of individuals colonizes a new habitat.
5. Individuals may carry alleles in different relative frequencies than did the larger population from which
they came.
6. The new population will be genetically different from the parent population.
7. When allele frequencies change due to migration of a small subgroup of a population it is known as the
founder effect.
G. Evolution Versus Genetic Equilibrium
1. To understand how evolution occurs, scientists first ask, “Under what conditions does evolution not
occur?” …the Hardy-Weinberg principle answers this question.
2. The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless
one or more factors cause those frequencies to change.
3. Genetic equilibrium- the condition where allele frequencies remain constant
4. Five conditions are required to maintain genetic equilibrium from generation to generation:
a. Random Mating
i. Random mating ensures that each individual has an equal chance of passing on its alleles
to offspring.
ii. In natural populations, mating is rarely completely random.
iii. Many species select mates based on particular heritable traits.
b. Large Population
i. Genetic drift has less effect on large populations than on small ones.
ii. Allele frequencies of large populations are less likely to be changed through the process
of genetic drift
c. No Movement Into or Out of the Population
i. Because individuals may bring new alleles into a population, there must be no movement
of individuals into or out of a population.
ii. The population's gene pool must be kept together and kept separate from the gene pools
of other populations.
d. No Mutations
i. If genes mutate, new alleles may be introduced into the population, and allele frequencies
will change.
e. No Natural Selection
i. All genotypes in the population must have equal probabilities of survival and
reproduction.
ii. No phenotype can have a selective advantage over another.
iii. There can be no natural selection operating on the population.
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5. If all five conditions are met, relative allele frequencies will not change and evolution will not occur.
16-3 The Process of Speciation
A. Introduction
1. Natural selection and chance events can change the relative frequencies of alleles in a population and
lead to speciation.
2. Speciation- the formation of new species.
3. Species- a group of organisms that breed with one another and produce fertile offspring
*What factors are involved in the formation of new species?
4. The gene pools of two populations must become separated for them to become new species.
B. Isolating Mechanisms
1. As new species evolve, populations become reproductively isolated from each other.
2. When the members of two populations cannot interbreed and produce fertile offspring, reproductive
isolation has occurred.
3. Reproductive isolation can develop in a variety of ways, including:
a. Behavioral Isolation- occurs when two populations are capable of interbreeding but have
differences in courtship rituals or other reproductive strategies that involve behavior.
b. Geographic isolation- occurs when two populations are separated by geographic barriers such as
rivers or mountains.
i. Geographic barriers do not guarantee the formation of new species.
ii. If two formerly separated populations can still interbreed, they remain a single species.
iii. Potential geographic barriers may separate certain types of organisms but not others.
c. Temporal isolation- occurs when two or more species reproduce at different times
C. Testing Natural Selection in Nature
1. Studies showing natural selection in action involve descendants of the finches that Darwin observed in
the Galápagos Islands.
2. The finches Darwin saw were different, but he hypothesized that they had descended from a common
ancestor.
3. Peter and Rosemary Grant tested Darwin’s hypothesis, which relied on two testable assumptions:
• For beak size and shape to evolve, there must be enough heritable variation in those traits
to provide raw material for natural selection.
• Differences in beak size and shape must produce differences in fitness, causing natural
selection to occur.
4. The Grants tested these hypotheses on the medium ground finch on Daphne Major, one of the Galápagos
Islands.
5. During the rainy season, there is plenty of food.
6. During droughts, food becomes scarce.
7. Individual birds with different-sized beaks had different chances of survival during a drought.
8. When food was scarce, individuals with large beaks were more likely to survive.
9. The Grants provided evidence of the process of evolution.
10. Beak size can be changed by natural selection
11. Describe the process of speciation in the Galápagos finches.
D. Speciation in Darwin's Finches
1. Speciation in the Galápagos finches occurred by:
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a. Founding of a new population- A few finches may have traveled from the mainland to one of the
islands. There, survived and reproduced.
b. Geographic isolation- some birds then moved to a second island. The two populations were
geographically isolated. They no longer shared a gene pool.
c. Changes in new population's gene pool- seed sizes on the second island favored birds with larger
beaks. So this bird population evolved into a population with larger beaks.
d. Reproductive isolation- in time, the large beaked birds were reproductively isolated fro birds on
other islands and evolved into a new species
e. Ecological competition- if birds from the second island cross back to the first, they live in
competition. Individuals that are most different from one another compete less and are most able
to reproduce. In time, this may lead to the evolution of yet another species.
E. Studying Evolution Since Darwin
1. Scientific evidence supports the theory that living species descended with modification from common
ancestors that lived in the ancient past.
2. Scientists predict that as new fossils are found, they will continue to expand our understanding of how
species evolved.
3. Evolution continues today
a. Example: some bacteria are evolving resistance to certain drugs
Analyzing Data: Textbook p. 408 Complete #1-4 below….
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