Over 10 generations, the frequency of a dark-coloration allele in a moth population rises from 0.10 to 0.35. A biologist says 'this population has not evolved because no new mutations occurred.' What is wrong with this claim?
AThe biologist is correct — evolution requires new mutations to introduce new alleles
BThe biologist is wrong — evolution is defined as allele frequency change, and the frequency of the dark allele clearly changed
CThe biologist is wrong — evolution requires selection, not mutation, and selection has occurred here
DThe biologist is correct — 10 generations is too short a timeframe for evolution to be detectable
Evolution is precisely defined as change in allele frequencies in a population over time. The dark allele's frequency changed from 0.10 to 0.35 — that is evolution, by definition, regardless of whether new mutations occurred. Mutations introduce new variants, but evolution happens whenever existing allele frequencies change through any mechanism: selection, drift, gene flow, or mutation. The Modern Synthesis redefined evolution in these precise population-genetic terms to unify Darwin's theory with Mendelian genetics.
Question 2 Multiple Choice
In a small isolated population of 20 individuals, a slightly deleterious allele (conferring a modest fitness cost) reaches fixation (frequency = 1.0) over 50 generations. Which evolutionary force most likely drove this outcome?
APositive selection — the allele must confer some undetected fitness advantage
BMutation pressure — the allele kept appearing faster than selection could remove it
CGenetic drift — random sampling effects dominate in small populations and can fix even deleterious alleles by chance
DGene flow — the allele was introduced from a neighboring population at high frequency
In small populations, genetic drift — random changes in allele frequency due to sampling — is strong relative to selection. The effective population size determines the balance: when 4Nes << 1 (where Ne is effective population size and s is selection coefficient), drift dominates and alleles can be fixed or lost by chance regardless of their fitness effects. With only 20 individuals, a slightly deleterious allele can easily drift to fixation before selection has time to remove it. This is why small populations accumulate deleterious mutations and why conservation genetics focuses on maintaining adequate population size.
Question 3 True / False
Natural selection is the primary evolutionary force capable of producing lasting genetic change in a population over time.
TTrue
FFalse
Answer: False
Genetic drift can also produce lasting, permanent genetic change — including fixation of alleles — without any selection pressure. In finite populations, random sampling changes allele frequencies every generation, and once an allele reaches fixation (frequency = 1.0) or is lost (frequency = 0), that change is permanent (barring new mutation). Neutral theory, developed by Motoo Kimura, demonstrated that much of molecular evolution is driven by drift acting on neutral variants, not by selection. Mutation, gene flow, and drift all produce lasting genetic change.
Question 4 True / False
Hardy-Weinberg equilibrium describes the null condition for evolutionary genetics: a population at equilibrium is not evolving, and deviations from Hardy-Weinberg proportions indicate that at least one evolutionary force is operating.
TTrue
FFalse
Answer: True
Hardy-Weinberg equilibrium is maintained only when all four forces are absent: no mutation, no selection (random mating with no fitness differences), no drift (infinite population size), and no gene flow. Real populations almost never meet all these conditions, but H-W serves as the null hypothesis. When observed genotype frequencies deviate significantly from H-W predictions, it signals that at least one evolutionary force is acting. This is why H-W tests are used to detect selection, inbreeding, population structure, and recent admixture in empirical datasets.
Question 5 Short Answer
Why is it more precise to define evolution as 'allele frequency change' rather than as 'change in a species over time'? What does the more precise definition reveal about evolutionary mechanisms?
Think about your answer, then reveal below.
Model answer: The allele frequency definition makes evolution measurable and mechanistically tractable. 'Change in a species over time' is too vague to study rigorously — it could refer to anything from morphological shifts to geographic range changes. By defining evolution as allele frequency change, the Modern Synthesis reframed every evolutionary question as a population-genetic question: which forces are acting, how strong are they, and what do they predict about future frequencies? This means evolution can be described mathematically and the contributions of mutation, selection, drift, and gene flow can be quantified and disentangled. The definition also clarifies that individuals do not evolve — only populations do, as the statistical composition of their alleles shifts across generations.
The precision of the allele frequency definition transformed evolutionary biology from a descriptive historical science into a predictive quantitative science. It also resolved the apparent paradox between Mendelian genetics (discrete inheritance, no blending) and Darwinian evolution (gradual change), showing that gradual population-level change emerges from discrete allele frequency shifts across many generations.