16-1 Genes and Variation
• In genetic terms, evolution is any
change in the relative frequency of
alleles in a population.
– Gene pool is the combined genetic
information of all members of a particular
population.
– Relative frequency is the number of times
an allele occurs in a gene pool compared
with the number of times other alleles for
the same gene occur.
Sources of Genetic Variation
• A mutation is any change in a sequence of
DNA. Mutations increase genetic variation,
which is needed for natural selection to
bring about evolutionary change.
– Mutations do not always affect an organism’s
phenotype or fitness.
– Some can affect an organism’s fitness, or its
ability to survive and reproduce in its
environment.
Sources of Genetic Variation
• Many heritable differences are due to gene
shuffling that occurs during the production of
gametes.
– The separation of homologous chromosomes
during meiosis is random with a possibility of 8.4
million different combinations of genes.
– Crossing over during meiosis increases the number
of different genotypes that can appear in offspring.
– Sexual reproduction produces many different
combination of genotypes.
Single-Gene and Polygenic Traits
The number of phenotypes produced for a
given trait depends on how many genes
control the trait.
- single-gene traits produce only two
phenotypes.
- polygenic traits have many possible
genotypes and phenotypes.
Generic Bell Curve for Polygenic Trait
The graph shows the
distribution of phenotypes
that would be expected for a
polygenic trait. The
symmetrical bell-like shape
of this curve is typical of
polygenic traits. This is
called a normal distribution.
Frequency of Phenotype
Section 16-1
Phenotype (height)
Figure 16–3 Phenotypes for Single-Gene Trait
Section 16-1
Frequency of Phenotype
(%)
100
80
60
40
20
0
Widow’s peak
Phenotype
No widow’s peak
Figure 16–2 Relative Frequencies of Alleles
Section 16-1
Sample Population
48%
heterozygous
black
16%
homozygous
black
36%
homozygous
brown
Frequency of Alleles
allele for
brown fur
allele for
black fur
Hardy Weinberg Equilibrium
Genetic Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Figure 16–6 Graph of Directional Selection
Section 16-2
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Figure 16–7 Graph of Stabilizing Selection
Section 16-2
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Figure 16–8 Graph of Disruptive Selection
Section 16-2
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
16-3 The Process of Speciation
• A species is a group of similar organisms
that can breed and produce fertile offspring.
Individuals in the same species share a
common gene pool.
– A genetic change that occurs in one individual
can spread through the population as that
individual and its offspring reproduce.
– If a genetic change increases fitness, that allele
will eventually be found in many individuals of
that population.
Isolation Mechanisms
• Speciation is the formation of new species.
– As new species evolve, populations become
reproductively isolated from each other.
– In reproductive isolation, the members of two
populations cannot interbreed and produce
fertile offspring.
– Can be due to behavioral isolation, geographic
isolation, and temporal isolation.
Reproductive Isolation
• Behavioral isolation occurs when two populations
are capable of interbreeding but have differences
in courtship rituals or other reproductive
stratagies.
• Geographic isolation – two populations are
separated by geographic barriers (rivers,
mountains, or bodies of water).
• Temporal isolation – two or more species
reproduce at different times.
Concept Map
Section 16-3
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species
Speciation in Darwin’s Finches
 In finches on the Galapagos Islands, a few finches
traveled from S. America to one of the islands. They
survived and reproduced.
 Some birds crossed to other islands, and the different
populations no longer shared a gene pool.
 Differences in food sources on the islands favored birds
with different types of beaks.
 This led to the populations becoming reproductively
isolated and evolving into separate species.
 The reproductive isolation was reinforced by ecological
competition between members of the different species
whenever they did live in the same environment.
Go Online
Links from the authors on restricting use of antibiotics
Interactive test
For links on population genetics, go to www.SciLinks.org and enter the
Web Code as follows: cbn-5161.
For links on speciation, go to www.SciLinks.org and enter the Web
Code as follows: cbn-5163.
Interest Grabber Answers
1. Make a list of physical traits that you think are influenced by genes.
Then, write next to each trait whether you have the trait or not
(e.g., a widow’s peak) or whether there are many variations of the
trait (e.g., hair color).
Students’ answers will include dimples and detached earlobes.
2. Are most of the traits you listed clear-cut or are they mostly traits
that have many variations? Which traits in your list are difficult to
categorize?
Most traits listed likely have many variations.
3. Compare your list with that of another student. Did he or she think of
any traits that you missed? Why do you think some traits are clear-cut,
while others are not?
Some students may suggest that patterns of inheritance for traits with
many variations (polygenic) are more complex than for clear-cut
(single-gene) ones.
Interest Grabber Answers
1. Choose an animal that you know something about, such as a deer, and
write its name at the top of a sheet of paper. Then, divide your paper into
two columns, and write the heading Trait in one column and Advantage in
the other.
Animal choices should be sufficiently familiar that students can describe
several traits.
2. Under Trait, write in several of the animal’s traits.
Students should list traits such as size, color, and specialized behavior.
3. Under Advantage, write in how you think the trait would be helpful to the
animal.
Students should indicate that adaptive value is clearer for some traits than
for others. For example, white-tailed deer raise their tails upon sensing a
predator. This may be an alarm signal for other deer, or it may induce the
predator to chase the now-conspicuous deer.
Interest Grabber Answers
1. Make a list of everyday things that this person encounters that you don’t.
For example, does he or she eat different kinds of food? Does he or she
live in a climate different from yours?
Students’ lists should include several social/environmental factors.
2. All humans are the same species. What might happen if groups of humans
were separated for millions of years in very different environments, such as
those you have just described?
Students may understand that humans would evolve separately in
response to different environmental pressures.
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