Questions: Allele Frequency Change and Evolutionary Dynamics

Question 1

A slightly beneficial allele (selection coefficient s = 0.01) appears independently in two wildflower populations: one with N = 10,000 individuals and one with N = 50. In which population is the allele more likely to increase to fixation, and why?

• A. The small population — genetic bottlenecks accelerate fixation of any allele regardless of fitness
• B. Both equally — the selection coefficient is the same in both populations, so the outcome should be the same
• C. The large population — Ns = 100 >> 1, so selection is effective; in the small population Ns = 0.5, so drift dominates and may eliminate the allele despite its advantage
• D. The small population — fewer competing alleles means the beneficial allele faces less competition

Answer: (C)

The key is the product Ns. In the large population, Ns = 10,000 × 0.01 = 100 >> 1, meaning selection is far stronger than drift and reliably drives the beneficial allele upward. In the small population, Ns = 50 × 0.01 = 0.5, meaning drift (random sampling error) is roughly as strong as selection — the allele's slight advantage becomes statistically invisible against the noise of random sampling. The allele may be lost by chance despite its fitness benefit. Population size, not just selection coefficient, determines evolutionary outcomes.

Question 2

Of the four evolutionary forces (natural selection, mutation, gene flow, genetic drift), which is generally the weakest at shifting allele frequencies at a single locus per generation?

• A. Genetic drift — it only affects small populations and has no directional tendency
• B. Natural selection — it requires many generations to produce noticeable frequency changes
• C. Mutation — typical per-locus mutation rates are only 10⁻⁵ to 10⁻⁹ per generation
• D. Gene flow — most populations are geographically isolated and receive little migration

Answer: (C)

Mutation rates per locus per generation are typically 10⁻⁵ to 10⁻⁹ — meaning most generations see no new mutation at a given locus at all. While mutation is the ultimate source of all genetic variation, it changes allele frequencies at any single locus glacially slowly. Selection, gene flow, or drift can shift frequencies orders of magnitude faster. Mutation's evolutionary role is to supply the raw variation that other forces then act on, not to drive frequency change directly.

Question 3

A 'favorable' allele — one that increases reproductive success — will typically increase in frequency over time, because natural selection is a systematic, directional force.

• T. True
• F. False

Answer: False

In small populations, genetic drift can overpower selection. When Ns (population size × selection coefficient) is near or below 1, allele trajectories become essentially random regardless of fitness. A slightly favorable allele (s = 0.01) in a population of N = 50 has Ns = 0.5 — drift dominates, and the allele may be lost by chance. In large populations, the statement approaches truth (Ns >> 1 makes selection effective), but the blanket claim 'favorable alleles always increase' ignores the critical role of population size.

Question 4

Evolution at the molecular level is formally defined as a change in allele frequencies in a population — not as adaptation, morphological change, or the appearance of new species.

• T. True
• F. False

Answer: True

This is the population genetics definition of evolution: a change in the frequency of alleles in a gene pool over time. If allele A₁ makes up 40% of the gene pool this generation and 42% the next, evolution has occurred at that locus — even if no organism looks or behaves differently. This definition is powerful because it allows precise mathematical treatment of evolutionary processes. Adaptation, speciation, and morphological change are downstream consequences of allele frequency change, not the definition itself.

Question 5

Explain how a slightly beneficial allele could be permanently lost from a population despite natural selection favoring it.

Model answer: In small populations, genetic drift — random sampling error in which alleles happen to reproduce — can overpower weak selection. Each generation, the alleles that are transmitted to offspring are a random sample from the current generation. If the beneficial allele is rare and the population is small, random sampling may result in zero copies being transmitted, eliminating it permanently. The rough criterion is Ns: when population size (N) times selection coefficient (s) is much less than 1, drift dominates and allele trajectories become essentially random. The allele's slight advantage cannot overcome the noise of random sampling.

This is one of the most counterintuitive results in evolutionary biology. It means that 'survival of the fittest' is not guaranteed at the genetic level — beneficial alleles can be lost by chance, and mildly harmful alleles can become fixed. The outcome depends not just on fitness but on population size. This is why conservation genetics cares so deeply about population size: small populations lose beneficial variation and accumulate deleterious alleles by drift, regardless of selection pressures.