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Chapter 23
The Evolution of Populations
Western Historical Context
Gregor Mendel (1822-1884)
Austrian monk whose
breeding experiments
with peas shed
light on the rules of
inheritance
Mendel was a contemporary of Darwin, but
his work was
overlooked until
the 20th century
Western Historical Context
The Modern Synthesis (early 1940s)
A conceptual synthesis of Darwinian
evolution, Mendelian inheritance, and
modern population genetics
Potential for rapid population
growth when resources
are not limiting
Resource availability
generally limits
population size
Competition for resources
(“struggle for existence”)
Phenotypic variability
(morphology, physiology,
behavior, etc.)
Natural Selection: Survival
and reproduction
of the “fittest” individuals
Some variability
results from heritable
genotypic differences
Phenotype vs. Genotype
Phenotype vs. Genotype
Phenotype: all expressed traits of an
organism
Phenotype vs. Genotype
Phenotype: all expressed traits of an
organism
Genotype: the entire genetic makeup of an
individual (i.e., its genome – it’s full
complement of genes and the two alleles that
comprise each locus), or a subset of an
individual’s genes
Evolution
A change in allele frequency
in a population (a change in the
gene pool)
Population = all of the individuals
of a species in a given area
Potential for rapid population
growth when resources
are not limiting
Resource availability
generally limits
population size
Competition for resources
(“struggle for existence”)
Phenotypic variability
(morphology, physiology,
behavior, etc.)
Natural Selection: Survival
and reproduction of the
“fittest” individuals
Some variability
results from heritable
genotypic differences
Adaptive evolution: A change in the phenotypic constitution
of a population owing to selection on heritable variation
among phenotypes that changes the genotypic
constitution of the population
Population Genetics
Examines the frequency,
distribution, and inheritance of
alleles within a population
Hardy-Weinberg Equilibrium
The population genetics theorem
that states that the frequencies
of alleles and genotypes in a
population will remain constant
unless acted upon by nonMendelian processes (i.e.,
mechanisms of evolution)
See Figs. 23.4 & 23.5 – An example
See Figs. 23.4 & 23.5 – An example
See Figs. 23.4 & 23.5 – An example
See Figs. 23.4 & 23.5 – An example
This means that 80% of sperm & eggs will carry R,
and 20% of sperm & eggs will carry r
Allele Frequencies
Under strict Mendelian inheritance, allele
frequencies would remain constant from one
generation to the next (Hardy-Weinberg Equilibrium)
Sperm
R
80% (p=0.8)
20% (q=0.2)
80% (p=0.8)
RR
p2=0.64
rR
qp=0.16
r
Eggs
20% (q=0.2)
Rr
pq=0.16
rr
q2=0.04
Genotype frequencies: p2=0.64 (RR) 2pq=0.32 (Rr) q2=0.04 (rr)
Allele frequencies: p=0.8 (R) q=0.2 (r)
Allele Frequencies
At a later date, you determine the genotypes
of 500 individuals, and find the following:
280 RR
165 Rr
55 rr
Frequency of R (a.k.a. “p”):
280 + 280 + 165 = 725 R alleles in the pop.
725 / 1000 = 0.725
Frequency of r (a.k.a. “q”):
165 + 55 + 55 = 275 r alleles in the pop.
275 / 1000 = 0.275
Allele Frequencies
The frequencies of alleles R and r have
changed:
T1:
320 RR
160 Rr
20 rr
p=0.8, q=0.2
T2:
280 RR
165 Rr
55 rr
p=0.725, q=0.275
The population has
EVOLVED!
Hardy-Weinberg Equation
For a two-allele locus:
Let p = the frequency of one allele in the
population (usually the dominant)
Let q = the frequency of the other allele
Notice that:
p+q=1
p=1–q
q=1–p
Genotypes should occur in the population
according to:
p2 + 2pq + q2 = 1
Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
p2 = proportion of population that is
homozygous for the first allele
(e.g., RR)
2pq = proportion of population that is
heterozygous (e.g., Rr)
q2 = proportion of population that is
homozygous for the second
allele (e.g., rr)
Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
Given either p or q, one can solve for the
rest of the above equation
What would q be if p = 0.6?
What would 2pq be if p = 0.5?
Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
Given the frequency of either homozygous
genotype, the rest of the equation can be
solved
What would q be if p2 = 0.49?
Hint: q = q2
Hardy-Weinberg Equilibrium
Is a null model…
like Newton’s first law of motion:
Every object tends to remain in a state
of uniform motion (or stasis),
assuming no external
force is applied to it
The Hardy-Weinberg Equation will be
satisfied, as long as all the assumptions are
met…
Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size
Because genetic drift affects smaller
populations more than larger pops.
Genetic drift = allele frequency
change due to chance
Genetic drift reduces genetic
variability
See Fig. 23.7 Genetic drift in a small
population of wildflowers
See Fig. 23.7 Genetic drift in a small
population of wildflowers
See Fig. 23.7 Genetic drift in a small
population of wildflowers
Genetic drift often results from
populations passing through a
population bottleneck
Genetic drift often results from
populations passing through a
population bottleneck
The founder effect is an example of a
population bottle neck
Mainland
population
The founder effect is an example of a
population bottle neck
Mainland
population
Colonists from the
mainland colonize
an island
The founder effect is an example of a
population bottle neck
Mainland
population
Colonists from the
mainland colonize
an island
Island gene pool
is not as variable
as the mainland’s
Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size (no genetic drift)
2) No gene flow among populations
Gene flow = transfer of alleles among
populations
Emigration transfers alleles out of a
population and immigration transfers
them in
Gene flow connects populations
time
Population
at t1
Population Island gene po
at t2 is not as variable
(after immigration)
as the mainland
Gene flow connects populations
Population
at t1
Island gene po
is not as variable
as the mainland
Gene flow connects populations
time
Population
at t1
Population Island gene po
at t2 is not as variable
(after immigration)
as the mainland
Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size (no genetic drift)
2) No gene flow among populations
3) No mutations
Mutations generally boost genetic
diversity
time
Population
at t1
Population Island gene po
at t2 is not as variable
(after immigration)
as the mainland
Mutations generally boost genetic
diversity
time
Population
at t1
Population Island gene po
at t2 is not as variable
(after a mutationas
event)
the mainland
Hardy-Weinberg Equilibrium
Assumptions:
1)
2)
3)
4)
Infinite population size (no genetic drift)
No gene flow among populations
No mutations
Random mating with respect to
genotypes
E.g., imagine what would happen if RR
males mated only with rr females
Those particular matings would result
in no RR or rr offspring, thereby
altering population-wide genotype
frequencies
Hardy-Weinberg Equilibrium
Assumptions:
1)
2)
3)
4)
Infinite population size (no genetic drift)
No gene flow among populations
No mutations
Random mating with respect to
genotypes
5) No natural selection
E.g., imagine what would happen if rr
flowers were the only ones that ever
attracted pollinators (even though the
population contains RR and Rr
individuals as well)
Hardy-Weinberg Equilibrium
Assumptions:
1)
2)
3)
4)
Infinite population size (no genetic drift)
No gene flow among populations
No mutations
Random mating with respect to
genotypes
5) No natural selection
Variation within Populations
Let’s briefly review…
Adaptive evolution: A change in the phenotypic constitution
of a population owing to selection on heritable variation
among phenotypes that changes the genotypic
constitution of the population
Variation within Populations
Since selection acts on phenotypes, yet
evolution requires population-level
genotypic change, it is important to
understand intraspecific variation
Note: If all individuals were phenotypically
identical, there would be no opportunity for
selection
Note: If all individuals were genotypically
identical, there would be no opportunity for
evolution
Variation within Populations
Phenotypic variation results from both
environmental and genetic influences
Consider identical vs. fraternal twins
Variation within Populations
Phenotypic variation results from both
environmental and genetic influences
Phenotypic variation within
populations is either discrete or
quantitative/continuous
Discrete variation: polymorphism
= mutiple phenotypes that are readily
placed in distinct categories co-occur
(e.g., our red and white flowers
result from a polymorphic locus)
E.g., a “bar graph” trait like ABO
blood type
Variation within Populations
Phenotypic variation results from both
environmental and genetic influences
Phenotypic variation within
populations is either discrete or
quantitative/continuous
Continuous variation: quantitative
characters
= multiple loci produce a trait
(e.g., flower size), and the trait varies
continuously in the population
E.g., a “bell curve” trait like human
height
Variation within Populations
Phenotypic variation results from both
environmental and genetic influences
Phenotypic variation within
populations is either discrete or
quantitative/continuous
Phenotypic variation also exists among
populations
E.g., geographic variation
Heliconius species A
Heliconius species B
Variation within Populations
How is genetic variation maintained?
1) Diploidy provides heterozygote
protection
2) Balanced polymorphism
Heterozygote advantage
E.g., A locus for one chain of
hemoglobin in humans has a
recessive allele that causes sicklecell anemia in homozygotes, but
provides resistance to malaria in
heterozygotes
Variation within Populations
How is genetic variation maintained?
1) Diploidy provides heterozygote
protection
2) Balanced polymorphism
Heterozygote advantage
Frequency-dependent selection
3) Neutrality
Fitness
Darwinian fitness = an individual’s
reproductive success (genetic contribution
to subsequent generations)
Relative fitness = a genotype’s contribution
to subsequent generations compared to the
contributions of alternative genotypes at
the same locus
Effects of Selection
See Fig. 23.12
Coat color
Effects of Selection
Directional selection consistently favors
phenotypes at one extreme
See Fig. 23.12
Coat color
Coat color
Effects of Selection
Stabilizing selection favors
intermediate phenotypes
See Fig. 23.12
Coat color
Coat color
Effects of Selection
Diversifying (disruptive) selection
simultaneously favors both phenotypic
extremes
See Fig. 23.12
Coat color
Coat color
Effects of Selection
Directional, diversifying (disruptive),
and stabilizing selection
See Fig. 23.12
Coat color
Coat color
Coat color
Coat color
Sexual Selection
Intrasexual selection,
usually male-male competition
Sexual Selection
Intrasexual selection,
usually male-male competition
Dynastes tityus
Often leads to sexual dimorphism &
exaggerated traits
Sexual Selection
Intrasexual selection,
usually male-male competition
Dynastes hercules
Often leads to sexual dimorphism &
exaggerated traits
Sexual Selection
Intrasexual selection,
usually male-male competition
Lucanus elaphus
Often leads to sexual dimorphism &
exaggerated traits
Sexual Selection
Intersexual selection,
usually female mate choice
Sexual Selection
Intersexual selection,
usually female mate choice
Often leads to sexual dimorphism &
exaggerated traits
Sexual Selection
Intersexual selection,
usually female mate choice
Often leads to sexual dimorphism &
exaggerated traits
Sexual Selection
Intersexual selection,
usually female mate choice
Often leads to sexual dimorphism &
exaggerated traits