The Evolution of Populations

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Chapter 23

• The Evolution of Populations

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Western Historical Context
Gregor Mendel (1822-1884)
Austrian monk whose breeding experiments with
peas shed light on the rules of inheritance
Mendel was a contem-porary of Darwin, but his
work wasoverlooked until the 20th century
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Western Historical Context
The Modern Synthesis (early 1940s)
A conceptual synthesis of Darwinian evolution,
Mendelian inheritance, and modern population
genetics
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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 variabilityresults from heritable
genotypic differences
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Phenotype vs. Genotype
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Phenotype vs. Genotype
Phenotype all expressed traits of an organism
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Phenotype vs. Genotype
Phenotype all expressed traits of an organism
Genotype the entire genetic makeup of an
individual (i.e., its genome its full
complement of genes and the two alleles that
comprise each locus), or a subset of an
individuals genes
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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
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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 variabilityresults 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
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Population Genetics
Examines the frequency, distribution, and
inheritance of alleles within a population
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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 non-Mendelian processes (i.e., mechanisms of
evolution)
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See Figs. 23.4 23.5 An example
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See Figs. 23.4 23.5 An example
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See Figs. 23.4 23.5 An example
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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
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Allele Frequencies
Under strict Mendelian inheritance, allele
frequencies would remain constant from one
generation to the next (Hardy-Weinberg
Equilibrium)
R
80 (p0.8)
80 (p0.8)
R
Sperm
Eggs
RR p20.64
r
20 (q0.2)
20 (q0.2)
r
Rr pq0.16
rR qp0.16
rr q20.04
Genotype frequencies p20.64 (RR) 2pq0.32
(Rr) q20.04 (rr)
Allele frequencies p0.8 (R) q0.2 (r)
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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
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Allele Frequencies
The frequencies of alleles R and r have changed
320 RR 160 Rr 20 rr
280 RR 165 Rr 55 rr
T1
T2
p0.8, q0.2
p0.725, q0.275
The population has EVOLVED!
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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
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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)
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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?
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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
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Hardy-Weinberg Equilibrium
Is a null model like Newtons 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
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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
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See Fig. 23.7 Genetic drift in a small
population of wildflowers
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See Fig. 23.7 Genetic drift in a small
population of wildflowers
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See Fig. 23.7 Genetic drift in a small
population of wildflowers
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Genetic drift often results from populations
passing through a population bottleneck
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Genetic drift often results from populations
passing through a population bottleneck
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The founder effect is an example of a population
bottle neck
Mainland population
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The founder effect is an example of a population
bottle neck
Colonists from themainland colonize an island
Mainland population
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The founder effect is an example of a population
bottle neck
Colonists from themainland colonize an island
Island gene poolis not as variable as the
mainlands
Mainland population
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(No Transcript)
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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
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Gene flow connects populations
time
Island gene poolis not as variable as the
mainlands
Population at t2 (after immigration)
Population at t1
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Gene flow connects populations
Island gene poolis not as variable as the
mainlands
Population at t1
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Gene flow connects populations
time
Island gene poolis not as variable as the
mainlands
Population at t2 (after immigration)
Population at t1
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Hardy-Weinberg Equilibrium
Assumptions
1) Infinite population size (no genetic drift)
2) No gene flow among populations
3) No mutations
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Mutations generally boost genetic diversity
time
Island gene poolis not as variable as the
mainlands
Population at t2 (after immigration)
Population at t1
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Mutations generally boost genetic diversity
time
Island gene poolis not as variable as the
mainlands
Population at t2 (after a mutation event)
Population at t1
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Hardy-Weinberg Equilibrium
Assumptions
1) Infinite population size (no genetic drift)
2) No gene flow among populations
3) No mutations
4) 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
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Hardy-Weinberg Equilibrium
Assumptions
1) Infinite population size (no genetic drift)
2) No gene flow among populations
3) No mutations
4) 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)
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Hardy-Weinberg Equilibrium
Assumptions
1) Infinite population size (no genetic drift)
2) No gene flow among populations
3) No mutations
4) Random mating with respect to genotypes
5) No natural selection
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Variation within Populations
Lets 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
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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
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Variation within Populations
Phenotypic variation results from both
environmental and genetic influences
Consider identical vs. fraternal twins
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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
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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
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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
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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
sickle- cell anemia in homozygotes, but
provides resistance to malaria in
heterozygotes
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Variation within Populations
How is genetic variation maintained?
1) Diploidy provides heterozygote protection
2) Balanced polymorphism
Heterozygote advantage
Frequency-dependent selection
3) Neutrality
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Fitness
Darwinian fitness an individuals reproductive
success (genetic contribution to subsequent
generations)
Relative fitness a genotypes contribution to
subsequent generations compared to the
contributions of alternative genotypes at the
same locus
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Effects of Selection
See Fig. 23.12
Coat color
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Effects of Selection
Directional selection consistently favors
phenotypes at one extreme
See Fig. 23.12
Coat color
Coat color
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Effects of Selection
Stabilizing selection favorsintermediate
phenotypes
See Fig. 23.12
Coat color
Coat color
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Effects of Selection
Diversifying (disruptive) selection
simultaneously favors both phenotypic extremes
See Fig. 23.12
Coat color
Coat color
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Effects of Selection
Directional, diversifying (disruptive), and
stabilizing selection
See Fig. 23.12
Coat color
Coat color
Coat color
Coat color
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Sexual Selection
Intrasexual selection, usually male-male
competition
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Sexual Selection
Intrasexual selection, usually male-male
competition
Dynastes tityus
Often leads to sexual dimorphism exaggerated
traits
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Sexual Selection
Intrasexual selection, usually male-male
competition
Dynastes hercules
Often leads to sexual dimorphism exaggerated
traits
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Sexual Selection
Intrasexual selection, usually male-male
competition
Lucanus elaphus
Often leads to sexual dimorphism exaggerated
traits
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Sexual Selection
Intersexual selection, usually female mate choice
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Sexual Selection
Intersexual selection, usually female mate choice
Often leads to sexual dimorphism exaggerated
traits
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Sexual Selection
Intersexual selection, usually female mate choice
Often leads to sexual dimorphism exaggerated
traits
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Sexual Selection
Intersexual selection, usually female mate choice
Often leads to sexual dimorphism exaggerated
traits