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The Evolution of Populations
Chapter 23 The Evolution of Populations
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Western Historical Context
Gregor Mendel ( ) Austrian monk whose breeding experiments with peas shed light on the rules of inheritance Mendel was a contem- porary of Darwin, but his work was overlooked until the 20th century With hindsight we can say that Darwin’s theory of evolution by natural selection was correct, but he didn’t know the mechanism of inheritance.
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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 variability results from heritable genotypic differences The Modern Synthesis goes like this…
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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 – it’s full complement of genes and the two alleles that comprise each locus), or a subset of an individual’s genes
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Population = all of the individuals of a species in a given area
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 The Modern Synthesis also identified other mechanisms of evolution besides natural selection…
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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 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
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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 Let’s dissect an example…
See Figs & 23.5 for a textbook example in which the heterozygotes have a distinct phenotype.
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See Figs & 23.5 – An example Let’s dissect an example…
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See Figs & 23.5 – An example Let’s dissect an example…
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See Figs & 23.5 – An example Let’s dissect 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% (p=0.8) 80% (p=0.8) R Sperm Eggs RR p2=0.64 r 20% (q=0.2) 20% (q=0.2) r rR qp=0.16 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)
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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”): = 725 R alleles in the pop / 1000 = 0.725 Frequency of r (a.k.a. “q”): = 275 r alleles in the pop / 1000 = 0.275
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The population has EVOLVED!
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: p=0.8, q=0.2 p=0.725, q=0.275 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 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…
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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 the mainland colonize an island Mainland population
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The founder effect is an example of a population bottle neck
Colonists from the mainland colonize an island Island gene pool is not as variable as the mainland’s Mainland population
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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 Population at t2 (after immigration) Island gene pool is not as variable as the mainland’s Population at t1
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Gene flow connects populations
Island gene pool is not as variable as the mainland’s Population at t1
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Gene flow connects populations
time Population at t2 (after immigration) Island gene pool is not as variable as the mainland’s 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 Population at t2 (after immigration) Island gene pool is not as variable as the mainland’s Population at t1
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Mutations generally boost genetic diversity
time Population at t2 (after a mutation event) Island gene pool is not as variable as the mainland’s 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
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
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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 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
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Effects of Selection See Fig. 23.12 Coat color
This could be how the long neck of the giraffe evolved.
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Directional selection consistently favors phenotypes at one extreme
Effects of Selection Directional selection consistently favors phenotypes at one extreme See Fig Coat color This could be how the long neck of the giraffe evolved. Coat color
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Stabilizing selection favors intermediate phenotypes
Effects of Selection Stabilizing selection favors intermediate phenotypes See Fig Coat color Coat color
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Effects of Selection Diversifying (disruptive) selection simultaneously favors both phenotypic extremes See Fig Coat color Coat color
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Directional, diversifying (disruptive), and stabilizing selection
Effects of Selection Directional, diversifying (disruptive), and stabilizing selection See Fig Coat color Coat color Coat color Coat color
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Intrasexual selection, usually male-male competition
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Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intrasexual selection, usually male-male competition Dynastes tityus Often leads to sexual dimorphism & exaggerated traits
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Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intrasexual selection, usually male-male competition Dynastes hercules Often leads to sexual dimorphism & exaggerated traits
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Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intrasexual selection, usually male-male competition Lucanus elaphus Often leads to sexual dimorphism & exaggerated traits
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Intersexual selection, usually female mate choice
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Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intersexual selection, usually female mate choice Often leads to sexual dimorphism & exaggerated traits
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Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intersexual selection, usually female mate choice Often leads to sexual dimorphism & exaggerated traits
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Sexual Selection Often leads to sexual dimorphism & exaggerated traits
Intersexual selection, usually female mate choice Often leads to sexual dimorphism & exaggerated traits
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