Natural Selection

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Transcript Natural Selection

Chapter 16
Evolution Unit:
Evolution of
16-1 Genes and Variation
A. As Darwin developed
his theory of
evolution, he was not
aware of how
passed from one
generation to the next
and how variation
appeared in
B. Evolutionary biologists connected
Darwin’s work and Mendel’s work
during the 1930’s.
1. Changes in ________
heritable variation on which
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
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
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
40% B (black fur)
60% b (brown fur)
Relative Frequencies of Alleles
Figure 16–2
Section 16-1
Sample Population
Frequency of Alleles
allele for
brown fur
allele for
black fur
is any
change in the relative frequency of
in a population.
refers to
small scale
change in allele
frequency over
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
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).
Phenotypes for Single-Gene Trait
Frequency of Phenotype
Attached Earlobes
Free Earlobes
(FF, Ff)
2. Polygenic traits: Traits
that are controlled by
two or more genes.
One polygenic trait can
have many possible
genotypes or
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.
Directional, Stabilizing,
Disruptive video
A. Directional Selection
1. One of the two
extremes is
Example: Darkcolored peppered
moths in regions
of England with
Section 16-2
Directional Selection
Figure 16–6
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
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
against both
extremes keep
curve narrow
and in same
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
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.
Genetic Drift
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,
Genetic Bottleneck
2. Founder effect: A population can
become limited in genetic variability if
it’s founded by a small number of
Example: Polydactyly in Amish.
Launch Internet Explorer Browser.lnk
Figure 16-9: Founder Effect
Sample of
Original Population
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 it there would be genetic
3. If allele frequencies do not change,
THEN the population will not evolve.
4. Hardy-Weinberg Equation:
p2 + 2pq + q2 = 1
a. The population is made of: homozygous
dominant genotypes (p2) + heterozygous
genotypes (2pq) + homozygous recessive
genotypes (q2).
b. The sum of the frequencies must always
equal the entire population (100%).
c. Example: If 10% of the population
exhibits attached earlobes (homozygous
recessive phenotype: ff), then 90%
___ of the
population is FF or Ff and exhibits the
free earlobes).
________ phenotype (____
+ (attached earlobes) = 1
= 100%
p = 100% - 10%
p = 90%
5. Five conditions necessary for HardyWeinberg Equilibrium
NOTE: Hardy-Weinberg equilibrium
rarely exists in natural populations but
understanding the assumptions behind it
gives us a basis for understanding how
populations evolve.
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
c. Mutations do not alter the gene pool.
d. Mating is random.
e. All individuals are equal in reproductive
success (no natural selection).
IV. Agents of Change
1. Mutation
Alteration in an organism’s
Ultimate source of variation.
Sickle Cell Mutation
IV. Agents of Change
2. Gene Flow
The movement of alleles
from one population to
Occurs when individuals
move between populations.
IV. Agents of Change
3. Genetic Drift
The CHANCE alteration of
gene frequencies in a small
Can occur when populations
are reduced in size (genetic
bottleneck) or when a few
individuals start a new
population (founder effect).
IV. Agents of Change
4. Nonrandom
Occurs when one member of
a population is not equally
likely to mate with any other
Queen Victoria & Hemophilia
IV. Agents of Change
5. Natural
Some individuals will be more
successful than others in
surviving and reproducing.
Certain traits give them a better
“fit” with the environment.
Large Ground Finch
Small Tree Finch
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.
Diversity in Humans
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
Reproductive Isolation – members of
two populations cannot interbreed and
produce fertile offspring.
Reproductive Barriers
PRE-Mating Reproductive Isolation –
involves mechanisms which do not allow
mating to occur in the first place.
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.
When has speciation occurred?
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
evolving populations
which result in
Formation of
new species
NOTE: Several isolating mechanisms can
compound one another to insure mating
doesn’t occur. This permits two species to
occupy the same valuable habitat and
prevents wastage of valuable gametes.
POST-Mating Reproductive Isolation –
(fertilization occurred and zygote formed)
1. Hybrid inviability: hybrid zygotes fail
to develop or fail to reach sexual
2. Hybrid sterility: Hybrids fail to
produce functional gametes.
Example: horse x donkey => mule (sterile).
Hybrid Sterility
Mule (sterile)
II. Testing Natural Selection in Nature
A. Peter and Rosemary Grant from
Princeton University worked to band and
measure finches on the Galapagos Islands
for over twenty years. By documenting
natural selection in the wild, the Grants
provided evidence of the process of
B. Grants realized Darwin’s hypothesis
relied on two TESTABLE assumptions:
1. Variation must occur in beak size and
2. Natural Selection takes place.
a. Different finches compete and eat
different food.
b. During the rainy season, there is
plenty of food.
c. During dry-season drought, some foods
become scarce forcing birds to become
feeding specialists.
d. Each species selects the type of food its
beak handles best. Example: Birds with
big, heavy beaks can crack open big, thick
seeds that no other birds can open.
e. Grants observed that average beak size
in that population increased dramatically
over time. This is an example of
directional selection.
III. Speciation in Darwin’s Finches
Speciation in the Galapagos finches
occurred by the following events:
A. Founders Arrive: A few finches from the
South American mainland (species A) flew
to the Galapagos Islands.
B. Geographic Isolation: Some birds from
species A crossed to another island in the
Galapagos group.
C. Changes in the Gene Pool: Over time,
populations on each island became adapted to
their local environments causing a separate
species to form. On the second island, the
larger seeds would favor individuals with
larger, heavier beaks thus forming species B.
D. Reproductive Isolation: If birds from the
second island cross back to the first island and
mating does not occur between the two
populations, then reproductive isolation has
occurred. The two populations have become
separate species.
E. Ecological Competition: As these two
species compete on the first island, the
more specialized birds have less
competition. During a dry season,
individuals that are most different from
each other have the highest fitness.
Over time, species evolve in a way that
increases the differences between them.
The species-B birds on the first island
may evolve into a new species, C.
F. Continued Evolution: Isolation on
different islands, genetic change, and
reproductive isolation led to 13 different
finch species found there today.
Finch Species
IV. Studying Evolution Since Darwin
A. Data from genetics, physics, biochemistry,
geology and biology supports the theory
that living species descended with
modification from common ancestors that
lived in the ancient past.
B. Unanswered Questions
1. No scientist suggests
that all evolutionary
processes are fully
2. Why is understanding
evolution important?
a. Drug resistance in
bacteria and viruses.
b. Pesticide resistance
in insects.
c. Evolutionary theory
helps us respond to
these changes to
improve human life.