Evolution Exam 3 V2

What data is depicted in the mosquito graph (Slide 2), and what does it demonstrate about natural selection?

It shows the frequency of the Ester-1 insecticide-resistance allele decreasing with distance from the coast. It demonstrates natural selection driven by pesticide spraying near the coast, creating a geographic gradient of adaptation.

What types of questions do population geneticists ask, and how do they differ from organismal biologists?

Population geneticists study the distribution of alleles within populations and the mechanisms causing allele frequencies to change over time. Organismal biologists focus on individual organisms' traits, behavior, anatomy, and physiology.

What are the two Hardy-Weinberg equations, and what do their variables represent?

p + q = 1 and p^2 + 2pq + q^2 = 1. p = frequency of the dominant allele; q = frequency of the recessive allele; p^2 = frequency of homozygous dominant genotype; 2pq = frequency of heterozygous genotype; q^2 = frequency of homozygous recessive genotype.

Why is the Hardy-Weinberg theorem considered a "null hypothesis"?

It predicts allele frequencies when no evolution is occurring. If population frequencies deviate from HW expectations, evolution is actively taking place.

What are the 5 conditions required for Hardy-Weinberg equilibrium?

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

What is the primary purpose of the Hardy-Weinberg theorem in evolutionary biology?

It serves as a baseline model to detect whether evolutionary forces (selection, drift, gene flow, mutation, non-random mating) are acting on a population.

What is genetic drift, and what are two primary mechanisms/examples?

Genetic drift is random fluctuation in allele frequencies from generation to generation due to chance sampling events. Examples include the bottleneck effect and the founder effect.

Why is genetic drift more effective in small populations than in large ones?

Chance events have a proportionally larger impact on small gene pools, making imperfect sampling more likely to significantly alter or fix allele frequencies.

What is a deterministic model vs. a probabilistic model?

A deterministic model predicts one exact outcome given specific initial conditions. A probabilistic model incorporates randomness/chance to predict a range of possible outcomes with varying probabilities.

Define deme and metapopulation.

Deme: A local, randomly interbreeding subpopulation of a species. Metapopulation: A group of spatially separated demes of the same species that interact via occasional migration/gene flow.

What was the significance of Peter Buri’s 1956 Drosophila melanogaster experiment?

By tracking eye-color alleles (bw75) across small fruit fly populations for 19 generations, Buri demonstrated that genetic drift causes random fixation or loss of alleles and leads to genetic divergence among isolated populations.

Why did the Cornu aspersum snail populations on Slide 25 diverge genetically?

Restricted mobility and urban physical barriers isolated local snail colonies, allowing random genetic drift to alter allele frequencies independently in each subpopulation.

What are the 5 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 within a population, 4. Drift causes initially identical populations to diverge, 5. An allele can become fixed without natural selection.

What two factors determine the probability of an allele surviving a population bottleneck?

The size of the bottleneck population and the initial frequency of the allele prior to the bottleneck.

Why are small population sizes dangerous for endangered species?

Small populations experience high genetic drift, leading to a loss of overall genetic diversity, fixations of harmful recessive alleles, inbreeding depression, and reduced adaptive capacity.

How is biological fitness defined?

The reproductive success of an individual, measured as the relative number of viable, fertile offspring contributed to the next generation.

What is sexual selection vs. ecological selection?

Sexual selection: Differential reproductive success resulting from competition for mates. Ecological selection: Differential survival and reproduction driven by environmental pressures like predators, climate, or food availability.

Do sexual selection and ecological selection work together or against each other?

They can work together or in opposition. For instance, bright mating displays favored by sexual selection may increase visibility to predators, creating a trade-off with ecological selection.

What two conditions are required for evolution by natural selection to occur?

1. A correlation between the phenotypic trait and fitness, 2. The phenotypic trait must be heritable.

What is the equation for relative fitness (w) and how is it calculated?

w = Absolute fitness of genotype (W) / Absolute fitness of reference genotype (Wmax). It normalizes fitness on a scale where the most successful genotype equals 1.0.

How do life tables relate to absolute and relative fitness?

Life tables track age-specific survival (lx) and fecundity (mx), allowing researchers to calculate net reproductive rate (R0) and quantify individual lifetime fitness components (W = Ps * E).

What was the evolutionary significance of loggerhead sea turtle matrix models and Sinkey Boone's invention?

Stage-class matrix models showed that sea turtle populations were most sensitive to subadult/adult survival rather than hatchling numbers. Sinkey Boone invented the Turtle Excluder Device (TED), which reduced adult turtle mortality in shrimp nets and successfully boosted population growth.

Absolute Fitness vs. Relative Fitness: Which is more useful for predicting allele frequency changes?

Relative fitness (w) is more important because selection depends on the relative differential contribution of alternative genotypes to the next generation, not just raw offspring counts.

What is the formula for change in allele frequency (Delta p) due to selection, and what do the variables represent?

Delta p = p * (aA1 / w_bar). Delta p = change in allele frequency per generation; p = initial frequency of allele A1; aA1 = average excess of fitness for allele A1; w_bar = mean fitness of the population.

Why is natural selection more effective in large populations than in small ones?

In large populations, random genetic drift is minimal, allowing even small selective advantages (s) to drive allele frequency changes deterministically.

Is natural selection or genetic drift the more important mechanism of evolution?

Neither is universally more important; their relative strength depends on population size (N). Selection dominates in large populations (N*s >> 1), while drift dominates in small populations (N*s << 1).

Define pleiotropy, antagonistic pleiotropy, and epistasis.

Pleiotropy: One gene influences multiple phenotypic traits. Antagonistic pleiotropy: A single gene mutation has beneficial effects on one trait but detrimental effects on another. Epistasis: The phenotypic effect of an allele at one locus depends on the presence of alleles at another locus.

What is the "cost of adaptation"? Give an example.

The trade-off where an allele conferring resistance or survival in a harsh environment reduces fitness in the absence of that pressure. Example: Mosquitoes with Ester-4 resist pesticides but suffer higher predation and lower survival in untreated habitats.

What was Richard Lenski’s E. coli Long-Term Experimental Evolution project?

An experiment tracking 12 parallel populations of E. coli grown in glucose-limited media for >60,000 generations, recording evolutionary changes, fitness increases, novel mutations, and key adaptations like citrate metabolism.

Why is Lenski’s experiment important to evolutionary biology?

It directly demonstrates macro-evolutionary adaptation in real time, showing how complex traits evolve via step-by-step epistatic mutations and historical contingency.

What is an additive allele?

An allele whose phenotype yields twice the phenotypic effect when two copies are present compared to one copy (no dominance masking).

How do additive, dominant, and recessive alleles differ in response to positive selection over time?

Additive: Increases steadily and rapidly toward fixation. Dominant: Increases quickly at first, but slows near fixation because recessive alleles hide in heterozygotes. Recessive: Remains rare for many generations (hidden in heterozygotes) until homozygous pairs form, after which it rapidly approaches fixation.

If single point mutations are rare (1.2 x 10^-8 per base pair), why is human genetic variation so high?

The human genome is large (~3.5 billion bp), and the global human population is vast. Approximately 61 new point mutations arise per newborn, generating billions of new mutations worldwide every year.

How do negative frequency-dependent selection and heterozygote advantage maintain genetic variation?

Negative frequency-dependent selection: Rare phenotypes have a fitness advantage over common ones, preventing any single phenotype from reaching fixation. Heterozygote advantage: Heterozygotes have higher fitness than either homozygote, maintaining both alleles in the gene pool.

Negative frequency-dependent selection and heterozygote advantage are both categorized as what type of selection?

Balancing selection.

Give an example of positive frequency-dependent selection.

Warning coloration (aposematism) in poison dart frogs. Predators learn to avoid the most common color pattern, so individuals with common patterns have higher survival than rare mutants.

Why is inbreeding evolutionarily disadvantageous?

It increases homozygosity across the genome, revealing rare, harmful recessive alleles that were previously hidden in heterozygous genotypes.

What is the inbreeding coefficient (F)?

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

What is inbreeding depression?

A reduction in average fitness, survival, or reproductive output of inbred individuals compared to outbred individuals.

What is landscape genetics?

A discipline combining population genetics, landscape ecology, and spatial statistics to analyze how geographic features and environmental barriers structure gene flow and genetic variation.

What general conclusion is drawn from the bighorn sheep landscape genetics data (Slide 102)?

Topographic barriers like highways and steep canyons isolate subpopulations, restricting ram movement and significantly increasing genetic distance (FST) between isolated groups.

Define genetic distance.

A quantitative measure of the genetic divergence between species or subpopulations.

What is FST?

The proportion of total genetic variance contained within subpopulations relative to the total genetic variance across the entire population. It ranges from 0 (no differentiation) to 1 (complete isolation).

What is population genetic structure?

The spatial organization of genetic variation, characterized by subpopulation numbers, allele frequencies within subpopulations, and degrees of genetic isolation among demes.

What is gene flow, and does it make populations more or less similar over time?

The movement or migration of alleles between populations. Gene flow makes populations more similar over time and counteracts genetic drift.

Why might different species in the same landscape show different levels of population subdivision?

Differences in dispersal abilities, body size, mobility, habitat requirements, and behavioral responses to landscape structures shape how species interact with physical barriers.

What is restoration ecology?

The scientific practice of repairing and re-establishing degraded ecosystems and ecological processes back to their natural state.

Give an example of how humans attempt to restore gene flow in human-altered landscapes.

Constructing wildlife corridors, overpasses, and underpasses across highways (e.g., grizzly bear corridors, Everglades culverts) to reconnect fragmented populations.

What was the purpose of showing the HHMI short film on the rock pocket mouse?

To illustrate how natural selection acts on coat color mutations (Mc1r) in response to predatory pressures across lava flow environments, connecting molecular genetics to field ecology.

Give an example of polygenic inheritance, epistasis, and phenotypic plasticity.

Polygenic inheritance: Human skin color or height. Epistasis: Coat color in Labrador retrievers or beach mice (Agouti masking/interacting with Mc1r). Phenotypic plasticity: Leaf morphology changes in Hydrocotyle bonariensis across dune-to-shrub environments, or seasonal coat color changes in snowshoe hares.

What is quantitative genetics, and why is it called quantitative rather than qualitative?

Quantitative genetics is the study of continuously varying phenotypic traits and their underlying genetic/evolutionary mechanisms. It is termed "quantitative" because traits are measured on a numerical scale rather than grouped into discrete categories.

What do normal distribution, mean, and variance describe in quantitative traits?

Normal distribution: Bell-shaped curve of trait values in a population. Mean: The average phenotypic value. Variance: Statistical dispersion of trait values around the mean.

In the equation VP = VG + VE, what do the terms represent?

VP = Total phenotypic variance of a population; VG = Variance attributable to genetic differences; VE = Variance attributable to environmental differences.

How does Slide 10 (variance curves) illustrate the equation VP = VG + VE?

It demonstrates how narrower or wider bell curves reflect changes in phenotypic variance (VP) resulting from underlying genetic differences (VG) or environmental variability (VE).

What is broad-sense heritability (H^2)?

The proportion of total phenotypic variance attributable to overall genetic variance (VG).

What is the equation for broad-sense heritability?

H^2 = VG / VP = VG / (VG + VE)

In the equation VG = VA + VD + VI, what do the components represent?

VG = Total genetic variance; VA = Additive genetic variance; VD = Variance due to dominance effects; VI = Variance due to epistatic interactions.

Why is additive genetic variance (VA) so important to evolutionary biologists?

VA is the primary cause of phenotypic resemblance between parents and offspring, allowing populations to evolve predictably under selection.

What is narrow-sense heritability (h^2)?

The proportion of total phenotypic variance attributable specifically to the additive effects of alleles (VA).

What is the equation for narrow-sense heritability?

h^2 = VA / VP = VA / (VA + VD + VI + VE)

What does the slope of the midparent-offspring regression line represent in Figure 7.5 (Slide 18)?

The slope directly estimates narrow-sense heritability (h^2).

What are the three modes of natural selection on quantitative traits, and an example of each?

1. Directional selection: Shifts mean toward one extreme (e.g., selecting for larger breast size in farmed turkeys). 2. Stabilizing selection: Favors intermediate phenotypes and reduces variance (e.g., human birth weight). 3. Disruptive selection: Favors extreme phenotypes over intermediates (e.g., small vs. large coho salmon mating strategies).

Are natural selection and evolution the same thing?

No. Natural selection is a process where differential reproductive success occurs due to phenotypic traits. Evolution is the resulting change in population allele frequencies over generations. Selection only causes evolution if phenotypic differences are heritable.

How quickly a population evolves in response to selection depends on what two factors?

The strength of phenotypic selection (selection differential, S) and the narrow-sense heritability (h^2) of the trait.

What is the selection differential (S)?

The difference between the mean trait value of reproducing individuals (xB) and the mean trait value of the entire initial population (xP)

How do Figures 7.9 and 7.10 demonstrate response to selection?

They show that high heritability (h^2) combined with strong selection (S) yields a large shift in offspring trait values (R), whereas low heritability results in little to no evolutionary response regardless of selection strength.

What is the breeder’s equation, and what do its variables stand for?

R = h^2 * S. R = Evolutionary response to selection; h^2 = Narrow-sense heritability; S = Selection differential.

What is linkage equilibrium?

A state where an allele at one locus occurs independently of an allele at another locus across a population.

What is linkage disequilibrium?

A state where specific alleles at two or more loci are non-randomly associated more frequently than expected by chance.

How does meiosis affect linkage equilibrium and disequilibrium?

Independent assortment and genetic recombination during meiosis erode linkage disequilibrium over generations, driving loci toward linkage equilibrium unless counteracted by physical proximity or selection.

Were Gregor Mendel’s seven pea plant traits linked?

No, the genes governing those traits were located on separate chromosomes or far enough apart on the same chromosome that they sorted independently (linkage equilibrium).

What is a map unit (centimorgan), and why is it important?

A unit measuring genetic distance corresponding to a 1% recombination frequency. It helps map the physical position of genes along chromosomes.

How can natural selection and genetic recombination act antagonistically?

Selection may favor specific combinations of alleles across loci, while meiotic recombination breaks those co-adapted allele combinations apart.

What is a supergene?

A group of tightly linked, functionally related genes located close together on a chromosome that segregate as a single inherited unit (often protected from recombination by chromosomal inversions).

What is a Quantitative Trait Locus (QTL)?

A region of DNA (marker or gene) that correlates significantly with variation in a quantitative phenotypic trait.

What are the basic steps in performing a QTL analysis?

1. Cross two inbred strains that differ sharply in a quantitative trait. 2. Interbreed F1 hybrids to generate an F2 population with recombined chromosomes. 3. Genotype F2 individuals across genetic markers and measure their phenotypic trait values. 4. Use statistical analysis (LOD scores) to link specific marker regions to phenotypic variation.

What steps did Hopi Hoekstra follow to explain coat color differences in beach vs. mainland mice?

Hoekstra crossed light beach mice with dark mainland mice, generated F2 hybrids, scored phenotypic coat patterns, mapped the markers via QTL analysis, and identified point mutations in Agouti and Mc1r that alter signaling to regulate dark vs. light pigment expression.

What is a Genome-Wide Association Study (GWAS), and how does it differ from traditional QTL analysis?

GWAS scans millions of single nucleotide polymorphisms (SNPs) across diverse natural populations (rather than lab-generated crosses) to find non-random associations between markers and phenotypic traits.

What is a reaction norm?

The range or pattern of phenotypic expression produced by a single genotype across varying environmental conditions.

What does Figure 7.26 (Slide 63) show regarding reaction norms and phenotypic variance?

It demonstrates how environmental variation (VE) and genotype-by-environment interactions (VGxE) alter reaction norm slopes, creating complex phenotypic distributions across environments.

In the equation VP = VA + VD + VI + VE + VGxE, what does VGxE represent?

VGxE is the genotype-by-environment interaction variance, which measures how different genotypes respond differently to environmental variation.

Can phenotypic plasticity evolve?

Yes. When VGxE > 0, reaction norm slopes vary genetically across individuals, allowing natural selection to act on and alter the degree of plasticity over generations.