A beneficial allele conferring antibiotic resistance starts at 1% frequency in a bacterial population. Researchers observe that its frequency increases slowly at first, then accelerates, then slows again near fixation. What explains this S-shaped trajectory?
AMutation rate fluctuates across the experiment, causing variable rates of allele introduction
BSelection pressure decreases as resistance becomes common, reducing the advantage
CWhen rare, most copies are hidden in heterozygotes and selection is inefficient; near fixation, few disfavored copies remain to replace
DGenetic drift dominates when the allele is rare or near fixation, overriding directional selection
The S-shaped (logistic) trajectory is an inherent feature of directional selection, not a sign of changing selection pressure. When the beneficial allele is rare, most individuals are homozygous for the ancestral allele and selection acts on relatively few favorable copies. As the allele reaches intermediate frequencies, selection is maximally efficient — many favorable vs. unfavorable comparisons are made each generation. Near fixation, there are few disfavored alleles left to replace, so the rate slows again. This is the expected mathematical trajectory, not an artifact of changing conditions.
Question 2 Multiple Choice
In a population of mice exposed to consistent cold winters, larger mice survive better and reproduce more. Which statement best characterizes directional selection in this scenario?
AThe population's mean body size will fluctuate around a stable optimum as allele frequencies reach equilibrium
BBoth large and small mice will be favored simultaneously, splitting the distribution
CThe mean body size will shift upward monotonically over generations, with variation at the small tail decreasing
DBody size will not evolve because it is a polygenic trait not subject to simple directional selection
Directional selection consistently favors one extreme (large body size here), causing the population mean to shift in that direction generation after generation. As large-body alleles increase in frequency, small-body alleles are progressively eliminated from the population — reducing variance at the disfavored tail. This contrasts with stabilizing selection (which maintains the mean while reducing variation at both extremes) and disruptive selection (which favors both extremes, increasing variance and potentially splitting the distribution).
Question 3 True / False
Under directional selection, variation in the population decreases at the disfavored end of the trait distribution as alleles favoring that extreme are gradually eliminated.
TTrue
FFalse
Answer: True
As directional selection consistently favors one extreme, the alleles producing the disfavored extreme become progressively rarer. This asymmetrically reduces variation: the disfavored tail shrinks while the favored tail may persist or even expand as the mean shifts. If selection continues to fixation, all variation in that trait is eliminated — the population becomes monomorphic for the favored allele. This is distinct from stabilizing selection, which reduces variation at both extremes symmetrically.
Question 4 True / False
Directional selection produces a constant, linear increase in allele frequency each generation until the favored allele reaches fixation.
TTrue
FFalse
Answer: False
The trajectory is S-shaped (logistic), not linear. Rate of change is slowest when the allele is rare (few favorable copies to select among), fastest at intermediate frequencies (maximum contrast between favored and disfavored), and slowest again near fixation (few disfavored copies left to replace). A constant linear increase would require that the same number of alleles switch each generation regardless of current frequency — this ignores that selection efficiency depends on the frequency of both alleles in the population.
Question 5 Short Answer
Why does the rate of allele frequency change under directional selection follow an S-shaped curve rather than increasing at a constant rate, and what does the steepness of the curve reflect?
Think about your answer, then reveal below.
Model answer: The S-shape arises because selection efficiency depends on the current frequencies of both alleles. When the favored allele is rare, few favorable vs. unfavorable comparisons occur per generation, so change is slow. At intermediate frequencies, both alleles are common and selection acts with maximum efficiency, producing rapid change. Near fixation, few disfavored alleles remain to be replaced, so change slows again. The steepness of the S is controlled by the selection coefficient (s): a strongly favored allele (large s) sweeps rapidly; a weakly favored allele (small s) follows a shallower, longer S-curve and is more vulnerable to genetic drift before it can reach fixation.
This S-shaped dynamic has practical implications for predicting evolutionary change. A beneficial allele can lurk at low frequency for many generations before becoming visible — a period when it is vulnerable to loss by drift. Once it reaches intermediate frequency, the sweep can be rapid. This helps explain why antibiotic resistance can appear to emerge 'suddenly' in clinical settings: the resistance allele may have been present at undetectable low frequency for a long time before selection drove it to dominance.