Chapter 23 The Evolution of Populations

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 20 th 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 variability results from heritable genotypic differences

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 non- Mendelian processes (i.e., mechanisms of evolution)

See Figs & 23.5 – An example

This means that 80% of sperm & eggs will carry R, and 20% of sperm & eggs will carry r See Figs & 23.5 – An example

Under strict Mendelian inheritance, allele frequencies would remain constant from one generation to the next (Hardy-Weinberg Equilibrium) Allele Frequencies RR p 2 =0.64 Rr pq=0.16 rR qp=0.16 rr q 2 =0.04 R SpermEggs Genotype frequencies: p 2 =0.64 (RR) 2pq=0.32 (Rr) q 2 =0.04 (rr) Allele frequencies: p=0.8 (R) q=0.2 (r) R r r 80% (p=0.8) 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”): = 725 R alleles in the pop. 725 / 1000 = Frequency of r (a.k.a. “q”): = 275 r alleles in the pop. 275 / 1000 = 0.275

The frequencies of alleles R and r have changed: Allele Frequencies 320 RR 160 Rr 20 rr T1:T1: p=0.8, q= RR 165 Rr 55 rr T2: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 p 2 + 2pq + q 2 = 1 Notice that: p + q = 1 p = 1 – q q = 1 – p Genotypes should occur in the population according to:

Hardy-Weinberg Equation p 2 + 2pq + q 2 = 1 p 2 = proportion of population that is homozygous for the first allele (e.g., RR) 2pq = proportion of population that is heterozygous (e.g., Rr) q 2 = proportion of population that is homozygous for the second allele (e.g., rr)

Hardy-Weinberg Equation p 2 + 2pq + q 2 = 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 p 2 + 2pq + q 2 = 1 Given the frequency of either homozygous genotype, the rest of the equation can be solved What would q be if p 2 = 0.49? Hint: q =  q 2

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 Genetic drift in a small population of wildflowers

Genetic drift often results from populations passing through a population bottleneck

The founder effect is an example of a population bottle neck Mainland population

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 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 Population at t 1 Island gene pool is not as variable as the mainland’s Population at t 2 (after immigration) time

Gene flow connects populations Population at t 1 Island gene pool is not as variable as the mainland’s

Gene flow connects populations Population at t 1 Island gene pool is not as variable as the mainland’s Population at t 2 (after immigration) time

Hardy-Weinberg Equilibrium Assumptions: 1) Infinite population size (no genetic drift) 2) No gene flow among populations 3) No mutations

Population at t 1 Island gene pool is not as variable as the mainland’s Population at t 2 (after immigration) time Mutations generally boost genetic diversity

Population at t 1 Island gene pool is not as variable as the mainland’s Population at t 2 (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 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

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 mutations 4) Random mating with respect to genotypes 5) 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), andthe 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 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 Coat color

Directional selection consistently favors phenotypes at one extreme Effects of Selection See Fig Coat color

Stabilizing selection favors intermediate phenotypes Effects of Selection See Fig Coat color

Diversifying (disruptive) selection simultaneously favors both phenotypic extremes Effects of Selection See Fig Coat color

Effects of Selection Directional, diversifying (disruptive), and stabilizing selection See Fig 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