Evolution Exam 3

Chapter 6 - Slide #2 Graph Description & Selection

The graph depicts the frequency of the Ester^1 allele relative to distance in kilometers from the coast across 1973, 1975, and 1978. Ester^1 frequency is highest near the coast due to natural selection from pesticide/insecticide spraying, favoring resistance-conferring alleles.

Population Geneticists vs. Organismal Biologists

Population genetics focuses on the distribution of alleles within gene pools and the mechanisms causing allele frequencies to change over time. Organismal biologists focus on individual organisms, phenotypic traits, and individual survival/reproduction.

Hardy-Weinberg Equations & Variables

Allele Frequencies: p + q = 1. Genotype Frequencies: p^2 + 2pq + q^2 = 1. p = dominant allele frequency, q = recessive allele frequency, p^2 = homozygous dominant (AA), 2pq = heterozygous (Aa), q^2 = homozygous recessive (aa).

Hardy-Weinberg as a Null Hypothesis

It predicts that allele frequencies will remain constant across generations if no evolutionary forces act on a population. Deviations from equilibrium indicate that evolution is actively occurring.

Five Conditions of Hardy-Weinberg Equilibrium

1. No mutation 2. Random mating 3. No natural selection 4. Infinitely large population size (no genetic drift) 5. No gene flow (no migration).

Purpose of the Hardy-Weinberg Theorem

It provides a baseline expectations baseline to test whether a population is evolving and helps identify which evolutionary mechanisms (selection, drift, migration) cause allele frequency shifts.

Genetic Drift & Examples

Genetic drift is the process where chance events cause unpredictable fluctuations in allele frequencies from generation to generation. Examples: Population bottlenecks (northern elephant seals), founder effects (Pitcairn/Norfolk Islands, Ellis-van Creveld syndrome), and random sampling of beads.

Genetic Drift in Small vs. Large Populations

Random sampling errors have a proportionately larger impact on smaller sample sizes, causing random allele fixation or loss to occur much faster in small populations.

Deterministic Model & Example

A model that predicts one exact outcome given a set of initial conditions without random variation. Example: Direct mathematical population growth curve.

Probabilistic (Stochastic) Model & Example

A model that incorporates probability, yielding a range of possible outcomes rather than a single fixed value. Example: Stochastic population growth or random walk of allele frequencies under genetic drift.

Deme vs. Metapopulation

Deme: A local, interbreeding subpopulation. Metapopulation: A broader population divided into several local subpopulations (demes) among which individuals disperse.

Peter Buri’s 1956 D. melanogaster Experiment

Buri tracked bw and bw^75 eye-color alleles across 19 generations in 107 replicate populations. He showed that random genetic drift in finite populations causes allele frequencies to fluctuate unpredictably until fixation (100%) or loss (0%) occurs in individual demes.

Divergence in Snail Populations (Slide 25)

Snail populations (Cornu aspersum) diverged due to random genetic drift (random walks of allele frequencies) across spatial demes with restricted mobility.

Five Fundamental Features of Genetic Drift

1. Drift is unbiased. 2. Random fluctuations are larger in smaller populations. 3. Drift causes genetic variation to be lost. 4. Drift causes initially identical populations to become different. 5. An allele can become fixed without natural selection.

Factors Affecting Bottleneck Survival Probability

1. Population size during the bottleneck. 2. Initial frequency of the allele (p).

Why Small Population Size Threatens Endangered Species

Small population size leads to rapid loss of genetic variation via drift, fixation of deleterious mutations, and increased inbreeding, which reduces survival and fitness.

Definition of Fitness

The number of offspring an organism produces that survive to reproduce themselves (lifetime reproductive success).

Sexual Selection & Examples

Variation in reproductive success resulting directly from competition for mates. Examples: Peacock tail displays, male cardinal coloration, fiddler crab claw size, Hercules beetle horns.

Ecological Selection & Examples

Selection arising from environmental interactions (predators, climate, food supply, disease) affecting survival and resource acquisition. Examples: Sickle cell allele selection in malaria regions, body size survival under environmental stress.

Sexual vs. Ecological Selection Interactions

They can act in opposition (trade-offs). Bright sexual displays increase mating success (sexual selection) but make individuals more visible to predators (ecological selection).

Requirements for Evolution by Selection

1. Correlation between the trait and fitness. 2. The trait must be heritable.

Slide 43 Fitness Equation & Variables

W = P_s * E. W = Absolute fitness, P_s = Probability of survival, E = Expected number of offspring.

Life Tables and the Fitness Equation (W = P_s * E)

Life tables track age-specific survivorship (P_s) and fecundity (E), supplying the parameters needed to calculate lifetime fitness (W = P_s * E).

Sea Turtles, Matrix Algebra, and Sinkey Boone

Matrix models showed adult/subadult survival was far more critical to loggerhead growth than egg/hatchling survival. Sinkey Boone invented the Turtle Excluder Device (TED), allowing adult/subadult turtles to escape shrimp nets and reversing population declines.

Relative vs. Absolute Fitness

Absolute fitness (W) is total offspring produced in a lifetime. Relative fitness (w) is absolute fitness standardized relative to the reference genotype (w = W / W_max). Relative fitness determines allele frequency shifts because natural selection depends on fitness differences.

Slide 69 Selection Equation & Variables

Delta p = p * (a_A1 / w_bar). Delta p = change in allele frequency due to selection, p = frequency of allele A1, a_A1 = average excess of fitness for allele A1, w_bar = average fitness of population.

Why Selection is More Effective in Large Populations

Random fluctuations from genetic drift are minimal in large populations, allowing even small average excesses of fitness (a_A1) to predictably drive allele frequency changes without being swamped by chance events.

Natural Selection vs. Genetic Drift Importance

Both are essential evolutionary mechanisms. Natural selection causes adaptive evolution, whereas genetic drift causes non-adaptive random fluctuations. Drift dominates in small populations; selection dominates in large populations.

Pleiotropy, Antagonistic Pleiotropy, and Epistasis

Pleiotropy: One gene affects multiple phenotypic traits. Antagonistic Pleiotropy: A gene/mutation has beneficial effects on one trait but detrimental effects on another. Epistasis: Interaction between genes where the phenotypic effect of one gene depends on another gene.

Cost of Adaptation & Example

A trade-off where a beneficial allele under specific selection carries a fitness penalty when that selector is absent. Examples: Insecticide-resistant mosquitoes carrying Ester^1/Ester^4 suffer higher mortality inland without insecticide; Warfarin resistance in rats.

Richard Lenski’s E. coli Experiment

Lenski tracked 12 replicate E. coli populations across >50,000 generations, sequencing bacterial genomes every 500 generations to record how mutations arise, interact, fix, or are lost over time.

Importance of Lenski’s Experiment

Provides direct, real-time empirical evidence of evolution, showing how beneficial mutations and epistatic interactions consistently increase population fitness over long timescales.

Additive Allele

An allele that yields twice the phenotypic effect when two copies are present in a genotype compared to a single copy.

Selection Dynamics: Additive, Dominant, Recessive Alleles

Additive: Increases steadily to fixation because every copy exposed contributes to fitness. Dominant: Increases rapidly initially, then slows near high frequency as recessive alleles hide in heterozygotes. Recessive: Increases slowly at low frequency while hidden in heterozygotes, but rapidly reaches fixation once common enough to form homozygotes.

High Human Genetic Variation Despite Low Point Mutation Rate

Although point mutation rate is low (1.2 x 10^-8), human genomes are massive (~3.5 billion bp), producing ~61 new point mutations per infant, totaling ~7.9 billion new mutations annually worldwide (~130 million newborns).

Mechanisms Maintaining Genetic Variation

1. Negative Frequency-Dependent Selection: Rare genotypes have higher fitness than common ones (e.g., elderflower orchids). 2. Heterozygote Advantage: Heterozygotes (AS) have higher fitness than either homozygote (AA or SS) (e.g., sickle cell protection against malaria).

Balancing Selection

Both negative frequency-dependent selection and heterozygote advantage are forms of balancing selection that maintain genetic diversity within populations.

Positive Frequency-Dependent Selection Example

Occurs when common phenotypes have higher fitness than rare ones. Example: Warning coloration (aposematism) in poison dart frogs, where predators recognize and avoid common color patterns.

Why Inbreeding is Evolutionarily Harmful

Inbreeding increases overall homozygosity, exposing rare, deleterious recessive alleles that were previously hidden in heterozygotes, thereby reducing fitness.

Inbreeding Coefficient (F)

The probability that two alleles at any locus in an individual are identical by descent from a common ancestor.

Inbreeding Depression & Examples

Reduction in average fitness of inbred individuals compared to outbred individuals. Examples: Decreased infant survival in the Habsburg dynasty; reduced germination success in white campion (Silene latifolia).

Landscape Genetics & Importance

Integrates population genetics, landscape ecology, and spatial statistics to evaluate how geographic features influence gene flow and mating structure across real landscapes.

Bighorn Sheep Landscape Figure (Slide 102)

Shows that bighorn sheep (Ovis canadensis) populations are subdivided into discrete local subpopulations structured around geographic landscape features and obstacles.

Genetic Distance

A quantitative measure of how genetically different or divergent two populations or subpopulations are from one another.

F_ST Definition

A measure of genetic differentiation representing the proportion of total genetic variance contained in a subpopulation (S) relative to the total genetic variance across all subpopulations combined (T).

Genetic Structure (Population Structure)

Characterized by the number of subpopulations, the frequencies of alleles within each subpopulation, and the degree of genetic isolation between them.

Gene Flow & Population Similarity

Gene flow is the movement/migration of alleles from one subpopulation to another. Gene flow makes populations more genetically similar over time and increases local genetic diversity.

Species Differences in Population Subdivision

Species differ in subdivision across the same landscape due to differences in mobility, dispersal ability, behavioral sensitivity to obstacles, and ecological requirements (e.g., bighorn sheep vs. brown bears vs. coyotes vs. lynx).

Restoration Ecology

The science and practice of restoring, renewing, and reconnecting degraded, damaged, or fragmented habitats and ecosystems.

Human Efforts to Restore Gene Flow

1. Construction of wildlife overpasses/corridors across highways (e.g., Banff National Park). 2. Regional conservation initiatives (e.g., Yellowstone to Yukon Conservation Initiative). 3. Removal of dams and canals to restore aquatic connectivity (e.g., Comprehensive Everglades Restoration Plan / CERP).

What is the purpose of the HHMI Rock Pocket Mouse film?

It demonstrates how natural selection acts on coat color adaptation through specific genetic mutations in Mc1r and Agouti in response to environmental background matching.

What is an example of a polygenic trait?

Human skin color or height.

What is an example of epistasis?

Labrador retriever coat color, where the E locus masks the B locus.

What is an example of phenotypic plasticity?

Snowshoe hare seasonal coat color changes.

What is quantitative genetics?

The study of continuous phenotypic traits measured on a numerical scale and the evolutionary mechanisms underlying them.

What is a normal distribution?

A bell-shaped curve showing the frequency of phenotypic values for a polygenic trait.

What is the mean?

The average phenotypic value of a population.

What is variance?

The statistical spread of trait values around the mean.

What is the equation for phenotypic variance?

VP = VG + VE.

What does VP represent?

Total phenotypic variance.

What does VG represent?

Genetic variance.

What does VE represent?

Environmental variance.

What does Figure 7.4 illustrate?

Multiple polygenic loci contribute genetic variance while environmental variation smooths discrete genotypes into a continuous bell-shaped phenotypic distribution.

What is broad-sense heritability (H²)?

The proportion of total phenotypic variance attributable to total genetic variance.

What is the equation for broad-sense heritability?

H² = VG / VP = VG / (VG + VE).

What is the equation for partitioning genetic variance?

VG = VA + VD + VI.

What does VA represent?

Additive genetic variance.

What does VD represent?

Dominance variance.

What does VI represent?

Epistatic interaction variance.

Why is additive genetic variance (VA) important?

It represents allele effects inherited predictably from parents, causes parent-offspring resemblance, and allows predictable evolutionary responses to selection.

What is narrow-sense heritability (h²)?

The proportion of total phenotypic variance attributable specifically to additive genetic variance.

What is the equation for narrow-sense heritability?

h² = VA / VP = VA / (VA + VD + VI + VE).

What does the slope of a midparent-offspring regression estimate?

Narrow-sense heritability (h²).

What is directional selection?

Selection that shifts the population mean toward one extreme phenotype.

What is an example of directional selection?

Turkey breast size.

What is stabilizing selection?

Selection that favors intermediate phenotypes and decreases phenotypic variance.

What is an example of stabilizing selection?

Human birth weight.

What is disruptive selection?

Selection that favors both extreme phenotypes over intermediate phenotypes.

What is an example of disruptive selection?

Male coho salmon.

What is natural selection?

Differential reproductive success among individuals in a population.

What is evolution?

A change in allele frequencies in a population over time.

When does natural selection cause evolution?

When trait differences have a heritable component, particularly when additive genetic variance (VA) is greater than zero.

What two factors determine the rate of evolution?

Selection differential (S) and narrow-sense heritability (h²).

What is the selection differential (S)?

The difference between the mean phenotype of reproducing individuals and the mean phenotype of the entire population.

What is the equation for the selection differential?

S = X̄B − X̄P.

What does X̄B represent in the selection differential equation?

The mean phenotype of reproducing individuals.

What does X̄P represent in the selection differential equation?

The mean phenotype of the entire population.

What does Figure 7.9 show?

Stronger selection produces a larger selection differential (S).

What does Figure 7.10 show?

Evolutionary response increases with narrow-sense heritability.

What is the breeder's equation?

R = h² × S.

What does R represent in the breeder's equation?

Evolutionary response.

What does h² represent in the breeder's equation?

Narrow-sense heritability.

What does S represent in the breeder's equation?

Selection differential.

What is linkage equilibrium?

A state in which alleles at two different loci are associated randomly and occur independently of one another.

What is linkage disequilibrium?

A state in which alleles at two different loci are associated non-randomly and occur together more or less frequently than expected by chance.

How does meiosis affect linkage disequilibrium?

Independent assortment and meiotic recombination break apart non-random allele combinations, causing linkage disequilibrium to decrease over time.

What are Mendel's seven pea traits in terms of linkage?

The genes were unlinked, meaning they were on separate chromosomes or far apart on the same chromosome, allowing independent assortment and the expected 9

What is a map unit or centimorgan (cM)?

A unit of genetic distance representing a 1% recombination frequency between two loci.

What does a centimorgan measure?

The relative genetic distance between loci based on recombination frequency.

What is the antagonism between selection and recombination?

Natural selection favors specific combinations of alleles while meiotic recombination breaks apart those favorable combinations.