Download Chapter 17 Evolution of Populations

CHAPTER 17
EVOLUTION OF
POPULATIONS
17.1 GENES AND VARIATION
• Gene pool:
• Consists of all the genes, including all the different
alleles, for each gene, that are present in a
population.
• Allele frequency:
• Number of times an allele occurs in a gene pool,
compared to the total number of alleles in that pool
for the same gene.
• What is the allele frequency for brown hair in this
classroom?
• What is the allele frequency for blonde hair in this
classroom?
WHAT ARE THE SOURCES OF GENETIC
VARIATION?
• Mutations
• Genetic recombination
• Sexual Reproduction
• Lateral Gene Transfer:
• Swap genes or plasmids.
What determines the number of phenotypes for a
given trait?
The number of phenotypes produced for a trait
depends on how many genes control the trait.
•
•
•
•
Single gene trait:
A trait controlled by only one gene.
Example
Hair color
• Polygenic gene trait:
• Traits that are controlled by two or more genes.
• Human height varies from being tall to short.
17.2 EVOLUTION AS GENETIC CHANGE
IN POPULATIONS
• Natural selection on polygenic traits can affect the
relative fitness of phenotypes and thereby produce
one of three types of stabilization.
• Directional
• Stabilizing
• Disruptive
DIRECTIONAL SELECTION
• Directional selection:
• When individuals at one end of the curve have
higher fitness than individuals in the middle or at the
other end.
DIRECTIONAL SELECTION
• EX: Peppered moths, antibiotic resistance, insecticide
resistance
STABILIZING SELECTION
• Stabilizing selection:
• When individuals near the center of the curve have
higher fitness than individuals at either end.
DISRUPTIVE SELECTION
• Disruptive selection:
• When individuals at the outer ends of the curve
have higher fitness than individuals near the middle
of the curve.
• What is genetic drift?
• Genetic drift:
• Random change in allele frequency caused by a
series of chance occurrences that cause an allele
to become more or less common in a population.
• If I had marbles in a bottle, and shook it upside
down, what would happen?
• Bottleneck effect:
• Change in allele frequency following dramatic
reduction in the size of a population.
• Disasters such as earthquakes, floods, droughts, and
fires may drastically reduce the size of a population
and lower the genetic variation. Reducing the size
of a population.
• Just because some organisms are better suited for
the environment does not mean that can survive a
natural disaster. Due to chance.
• Founder effect:
• Allele frequencies change as a result of the
migration of a small subgroup of a population.
• When a few individuals colonize an isolated island,
the smaller the colony the less its genetic makeup
will represent the gene pool of the larger
population from which the colonists came. Due to
chance.
• Genetic equilibrium:
• Allele frequencies in its gene pool do not change.
• Hardy-Weinberg Principle:
• States the allele frequencies in a population should
remain constant unless one or more factors cause
those frequencies to change.
5 CONDITIONS CAN DISTURB GENETIC
EQUILIBRIUM
• 1. Random mating
• Sexual selection: Individuals select mates based on
heritable traits.
• 2. Small Population Size
• 3. Immigration or Emigration
• 4. Mutations
• 5. Natural Selection
SECTION 16–2: EVOLUTION AS GENETIC
CHANGE
D. Evolution Versus Genetic Equilibrium
1. Random Mating - ensures that each individual has an equal
chance of passing on its alleles to offspring
Mating must be
random.
Individuals tend
to choose mates
similar to
themselves
SECTION 16–2: EVOLUTION AS GENETIC
CHANGE
D. Evolution Versus Genetic Equilibrium
2. Large Population – allele frequencies of large
populations are less likely to change
A large breeding
population helps to
ensure that chance
alone does not
disrupt genetic
equilibrium.
In a small
population, if for
some chance
reason,
organisms with a
certain allele do
not reproduce,
then the allele
frequencies will
change.
SECTION 16–2: EVOLUTION AS GENETIC
CHANGE
D. Evolution Versus Genetic Equilibrium
3. No Movement Into or Out of the Population –
• Migration may bring new alleles into a population
• Population's gene pool must be kept together and
separate from the gene pools of other populations.
Population's gene
pool must be kept
together and kept
separate from the
gene pools of
other populations.
Because
individuals bring
new alleles into
a population, the
allele frequency
change when
migration
occurs.
SECTION 16–2: EVOLUTION AS GENETIC
CHANGE
D. Evolution Versus Genetic Equilibrium
4. No Mutations - If genes mutate from one form into
another, new alleles may be introduced into the
population, and allele frequencies will change.
SECTION 16–2: EVOLUTION AS GENETIC
CHANGE
D. Evolution Versus Genetic Equilibrium
5. No Natural Selection
• All genotypes in the population must have equal
probabilities of survival and reproduction.
• No phenotype can have a selective advantage over
another.
If selection occurs, those
alleles that are selected for will
become more common and the
allele frequency in the
population will increase.
SECTION 16–3: THE PROCESS OF SPECIATION
C. Speciation in Darwin’s Finches
1. Founders arrive - mainland
species flew or were blown to
islands
2. Separation of populations populations remained separate
due to water between islands and
mainland (geographic isolation)
3. Changes in gene pool - occur over
time as organisms became
adapted to their local habitats
4. Reproductive isolation – courtship
behavior of inspecting mate’s
beak
5. Ecological competition – compete
for same food source/seeds;
evolution of species to decrease
competition for food
6. Continued Evolution
17.3 THE PROCESS OF SPECIATION
• Species:
• Population or group of populations whose members
can interbreed and produce fertile offspring.
•
•
•
•
What does a lion and a tiger make?
Liger
Is this considered a specie?
This is a hybrid species.
• Speciation:
• Formation of a new species.
• Reproductive isolation:
• Separation of a species or population so that they
no longer interbreed and evolve into separate
species.
SECTION 16–3: THE PROCESS OF SPECIATION
• How does natural selection and changes in the relative frequencies
lead to the formation of new species, or speciation?
• Changes in gene pool= can cause speciation
• What must happen for a species to evolve into two new species?
• Isolation
TYPES OF BARRIERS
• Behavioral isolation:
• Two species have different courtship or mating
behaviors.
• What do birds to to attract mates?
• They sing or dance, but these birds have different
courtship behaviors.
TYPES OF BARRIERS
• Geographic isolation:
• Separation of populations as a result of geographic
change.
• Ex.
• Grand Canyon
TYPES OF BARRIERS
• Temporal (timing) isolation:
• Two different breeding seasons.
• Western spotted skunk and Eastern spotted skunk.
They both live in the Great plains region.
• The Western spotted skunk breeds in the fall, and
the Eastern spotted skunk breeds in the winter.
17.4 MOLECULAR EVOLUTION
• Molecular clocks:
• Compare stretches of DNA to mark the passage of
evolutionary time.
• Uses mutation rates in DNA to estimate the time that
two species have been evolving independently.
MOLECULAR CLOCKS
• Where do new genes come from?
• One way in which genes evolve is through the
duplication, and then modification, of existing new
genes.
• Sometimes crossing over (exchanging DNA)
involved unequal swapping of DNA. One
chromosome in the pair gets an extra DNA.
DEVELOPMENTAL GENES AND BODY
PLANS
• How may Hox genes be involved in evolutionary
change?
• Hox genes determine which parts of the embryo
develop arms, legs, or wings. Groups of Hox genes
also control the size and shape of those structures
and their bodies.
• Small changes in Hox gene activity during
embryological development can produce large
changes in adult animals.
HOX GENES