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Biology
Chapter 16
Evolution Unit:
Evolution of
Populations
16-1 Genes and Variation
A. As Darwin developed
his theory of
evolution, he was not
aware of how
heritable
traits
_____________
passed from one
generation to the next
and how variation
appeared in
organisms.
B. Evolutionary biologists connected
Darwin’s work and Mendel’s work
during the 1930’s.
genes
1. Changes in ________
produce
heritable variation on which
natural
selection can operate.
_______________
2. Discovery of DNA demonstrated
the molecular nature of mutation
and genetic variation.
II. How Common is Genetic Variation?
A. Individual fishes, reptiles, and
mammals are typically heterozygous
for between 4-8% of their genes.
B. Variation and Gene Pools
1. Genetic variation is ______________
studied in
populations.
________________
a group of individuals of
2. Population: ____________________
the same species that interbreed.
_______________________________
3. Gene pool: all genes, including all the
different alleles, that are present in a
population.
4. 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.
Example: Fur color in a population of
mice
40% B (black fur)
60% b (brown fur)
Relative Frequencies of Alleles
Figure 16–2
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
is any
5. MICROEVOLUTION: Evolution
______________
change in the relative frequency of
__________________________________
alleles
________
in a population.
Microevolution
refers to
small scale
___________
change in allele
frequency over
time.
III. Sources of Genetic Variation
A. Two sources of genetic variation
1. Mutation
a. Ultimate source of variation.
b. Any change in a sequence of DNA
c. Most mutations
are bad.
Example: UV,
radiation, toxins
d. Mutations that
produce changes
in an organism’s
phenotype and
increase an
organism’s fitness,
or its ability to
reproduce in its
environment, will
be passed on.
2. Genetic shuffling
that results from
sexual reproduction.
a. Independent
assortment during
meiosis produces
8.4 million possible
combinations.
b. Crossing-over.
IV. Single-Gene and Polygenic Traits
A. The number of phenotypes produced for
a given trait depends on how many
genes control the trait.
1. Single-gene trait: Single gene that has
two alleles. Example: Free earlobes
(FF, Ff) or attached earlobes (ff).
Free
Attached
Phenotypes for Single-Gene Trait
Frequency of Phenotype
(%)
100
80
60
40
20
0
Attached Earlobes
Free Earlobes
(ff)
(FF, Ff)
Phenotype
2. Polygenic traits: Traits
that are controlled by
two or more genes.
One polygenic trait can
have many possible
genotypes or
phenotypes.
Example: Height, eye
color, skin color.
16-2 Evolution as Genetic Change
I. Natural Selection on Single-Gene Traits
A. Reminder: Evolution is any change over
time in the relative frequencies of alleles
in a population. Populations, not
individual organisms, evolve over time.
B. Natural selection on single-gene traits
can lead to changes in allele
frequencies and thus to evolution.
Effect of Color Mutations on Lizard Survival
(Figure 16-5):
1. Organisms of one color may produce
fewer offspring than organisms of
other colors.
Example: Red lizards are more visible to
predators and therefore, may be more
likely to be eaten and not pass on that
red gene.
II. Natural Selection on Polygenic Traits
Natural selection can affect the distribution
of phenotypes in any of three ways:
(1) directional selection
(2) stabilizing selection
(3) disruptive selection.
A. Directional Selection
1. One of the two
possible
extremes is
favored.
Example: Darkcolored peppered
moths in regions
of England with
industrial
pollution.
Section 16-2
Directional Selection
Figure 16–6
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
B. Stabilizing Selection
1. Intermediate characteristics are favored.
Examples: Human babies with very high or
very low birth weights have lower survival
than babies with intermediate weights.
Stabilizing Selection
Figure 16–7
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.
C. Disruptive Selection
1. Natural selection moves characteristics
toward both extremes, and intermediate
phenotypes become rarest.
Example: Populations of West African
birds with either large or small, but not
intermediate size beaks.
Section 16-2
Disruptive Selection
Figure 16–8
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
III. Genetic Drift
A. In small populations, an allele can
become more or less common
simply by chance.
B. Genetic drift is a random change in
allele frequency.
C. Two types of genetic drift:
1. Genetic bottleneck:
If a population crashes,
then there will be a
loss of alleles from
the population.
Example: Northern
Elephant Seals,
Cheetahs.
Genetic Bottleneck
2. Founder effect: A population can
become limited in genetic variability if
it’s founded by a small number of
individuals.
Example: Polydactyly in Amish.
Figure 16-9: Founder Effect
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
IV. Hardy-Weinberg and Genetic Equilibrium
A. What would be necessary for no change
to take place?
1. Hardy-Weinberg principle states that
allele frequencies in a population
will remain constant unless one or
more factors cause those
frequencies to change.
2. If allele frequencies remained constant
then there would be genetic
equilibrium.
Conditions necessary for
Hardy-Weinberg Equilibrium
a. The population is very large.
b. The population is isolated (no migration of
individuals, or alleles, into or out of the
population).
c. Mutations do not alter the gene pool.
d. Mating is random.
e. All individuals are equal in reproductive
success (no natural selection).
16-3 The Process of Speciation
I. How do we get new species?
A. What is a Species?
1. Species:
a group of interbreeding
organisms that breed with one another
and produce fertile offspring.
This means that the individuals of the same
species share a common gene pool.
2. If a beneficial genetic change
occurs in one individual, then that
gene can be spread through the
population as that individual and its
offspring reproduce.
B. Isolating Mechanisms (Leads to a new
species!)
Reproductive Isolation – members of
two populations cannot interbreed and
produce fertile offspring.
1. Behavioral Isolation: Members of two
populations are capable of interbreeding
but have differences in mating displays
or courtship rituals.
a. specific scents (pheromones of insects).
b. color patterns/strutting.
c. specific sounds or calls.
Courtship Dance
Different Mating Songs
2. Geographic/Ecological Isolation: Two
populations are separated by geographic
barriers such as rivers, mountains, or
bodies of water.
3. Temporal Isolation: Two or more
species live in the same habitat but have
different mating/reproductive seasons.
a. Brown trout and Rainbow trout are
found in the same streams but
Rainbow trout spawn in the Spring and
Brown trout spawn in the Fall.
b. Three similar species of orchid living in
the same tropical habitat each release
pollen on different days; therefore, they
cannot pollinate one another.
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
NOTE: Several isolating mechanisms can
compound one another to insure mating
doesn’t occur.