Gregor Mendel 1822-1884 Purple & White Flowers on Pea Plants Peas in a Pod.
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Transcript of Chapter 23 The Evolution of Populations. Western Historical Context Gregor Mendel (1822-1884)...
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 contem-porary of Darwin, but his work wasoverlooked until the 20th century
Western Historical Context
A conceptual synthesis of Darwinian evolution, Mendelian inheritance, and modern population genetics
The Modern Synthesis (early 1940s)
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
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 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
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 non-Mendelian 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
This means that 80% of sperm & eggs will carry R, and 20% of sperm & eggs will carry r
See Figs. 23.4 & 23.5 – An example
Under strict Mendelian inheritance, allele frequencies would remain constant from one generation to the next
(Hardy-Weinberg Equilibrium)
Allele Frequencies
RRp2=0.64
Rrpq=0.16
rRqp=0.16
rrq2=0.04
RSperm Eggs
Genotype frequencies: p2=0.64 (RR) 2pq=0.32 (Rr) q2=0.04 (rr)
Allele frequencies: p=0.8 (R) q=0.2 (r)
R
rr
80% (p=0.8)80% (p=0.8)
20% (q=0.2)20% (q=0.2)
At a later date, you determine the genotypes of 500 individuals, and find the following:
Allele Frequencies
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
The frequencies of alleles R and r have changed:
Allele Frequencies
320 RR160 Rr 20 rr
T1:
p=0.8, q=0.2
280 RR165 Rr 55 rr
T2:
p=0.725, q=0.275
The population has
EVOLVED!
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
Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
Notice that: p + q = 1 p = 1 – q q = 1 – p
Genotypes should occur in the population according to:
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 stateof 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
Mainlandpopulation
Mainlandpopulation
Colonists from themainland colonize
an island
The founder effect is an example of a population bottle neck
Mainlandpopulation
Colonists from themainland colonize
an island
Island gene poolis not as variable
as the mainland’s
The founder effect is an example of a population bottle neck
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
Populationat t1
Island gene poolis not as variable
as the mainland’s
Population at t2
(after immigration)
time
Gene flow connects populations
Populationat t1
Island gene poolis not as variable
as the mainland’s
Gene flow connects populations
Populationat t1
Island gene poolis not as variable
as the mainland’s
Population at t2
(after immigration)
time
Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size (no genetic drift) 2) No gene flow among populations 3) No mutations
Populationat t1
Island gene poolis not as variable
as the mainland’s
Population at t2
(after immigration)
time
Mutations generally boost genetic diversity
Populationat t1
Island gene poolis not as variable
as the mainland’s
Population at t2
(after a mutation event)
time
Mutations generally boost genetic diversity
Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size (no genetic drift) 2) No gene flow among populations 3) No mutations4) 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) 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)
Hardy-Weinberg Equilibrium
Assumptions:
1) Infinite population size (no genetic drift) 2) No gene flow among populations 3) No mutations4) Random mating with respect to genotypes5) No natural selection
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
Let’s briefly review…
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
How is genetic variation maintained?
Variation within Populations
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
How is genetic variation maintained?
Variation within Populations
1) Diploidy provides heterozygote protection
2) Balanced polymorphism Heterozygote advantageFrequency-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
Directional selection consistently favors phenotypes at one extreme
Effects of Selection
See Fig. 23.12
Coat color
Coat color
Stabilizing selection favorsintermediate phenotypes
Effects of Selection
See Fig. 23.12
Coat color
Coat color
Diversifying (disruptive) selection simultaneously favors both phenotypic extremes
Effects of Selection
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
Dynastes tityus
Often leads to sexual dimorphism & exaggerated traits
Intrasexual selection, usually male-male competition
Sexual Selection
Dynastes hercules
Intrasexual selection, usually male-male competition
Often leads to sexual dimorphism & exaggerated traits
Sexual Selection
Lucanus elaphus
Intrasexual selection, usually male-male competition
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