A population of deer mice lives in a snowy habitat. Mice with very light fur are easily spotted by predators; mice with very dark fur absorb less heat and suffer in winter cold. Intermediate gray coloration shows the highest survival rates. Which selection mode is operating, and what will happen to the population over generations?
ADirectional selection: the average color will shift toward darker gray as light mice are eliminated
BStabilizing selection: the average color will remain approximately constant, but variation will decrease as both extremes are removed
CDirectional selection: because both extremes are disadvantaged, the population will become more variable over time
DDisruptive selection: both light and dark mice are disadvantaged, simultaneously favoring both phenotypic extremes
When both extremes are penalized and intermediates are favored, stabilizing selection is operating. The mean coloration stays roughly constant (already at the intermediate optimum), while both tails of the distribution are trimmed each generation, reducing overall variation. Option A is wrong because directional selection favors ONE extreme and moves the mean toward it — here, both extremes are penalized equally. Option D confuses stabilizing with disruptive selection: disruptive selection FAVORS both extremes and penalizes the middle, the opposite of the scenario described.
Question 2 Multiple Choice
After a severe drought, a population of finches shows a significant increase in average beak depth over three generations, and beak depth variance also decreases. This pattern is most consistent with:
AStabilizing selection — variance decreases as the population clusters around the pre-existing optimal beak depth
BDirectional selection — the mean shifted as shallow-beaked individuals were culled from the disfavored tail, simultaneously reducing variance
CDisruptive selection — both modes reduce variance, so either could explain this pattern
DGenetic drift — small populations lose variation randomly, incidentally shifting the mean
Directional selection shifts the mean toward the favored extreme AND reduces variance, because it culls individuals from the disfavored tail. The mean shift (increased average beak depth) and variance reduction together are the characteristic signature of directional selection favoring deep beaks. Stabilizing selection (option A) would maintain the mean while reducing variance — inconsistent with the observed mean shift. Disruptive selection (option C) actually increases variance by favoring both extremes and cannot explain a unimodal mean shift.
Question 3 True / False
Stabilizing selection causes populations to evolve rapidly toward a new phenotypic optimum.
TTrue
FFalse
Answer: False
Stabilizing selection does not shift the phenotypic mean — it is a force for evolutionary stasis. By penalizing deviations from the current optimum in both directions, it maintains the status quo while reducing variation. Rapid evolution toward a new optimum is the signature of directional selection, which favors one extreme over others. Stabilizing selection explains why many traits appear static over long periods in the fossil record despite ample genetic variation — the trait is already near its local optimum, and any deviation is penalized.
Question 4 True / False
Both directional and stabilizing selection reduce phenotypic variance within a population, though through different mechanisms and with different effects on the mean.
TTrue
FFalse
Answer: True
This is correct and often overlooked. Directional selection reduces variance by culling the disfavored tail — the population loses that end of the distribution while the mean moves. Stabilizing selection reduces variance more symmetrically, trimming both tails each generation while keeping the mean constant. Both modes reduce variance, but only directional selection moves the mean. This shared effect on variance means variance measurements alone cannot distinguish the two modes — examining whether the mean changes is the key diagnostic.
Question 5 Short Answer
Why is stabilizing selection considered the most common mode of selection in nature, despite receiving less attention than directional selection?
Think about your answer, then reveal below.
Model answer: Most traits in most populations are already close to their local adaptive optimum — the phenotype that maximizes fitness in the current stable environment. Deviations in either direction are typically costly: growing too large exhausts resources, growing too small impairs competition or thermoregulation. Since organisms are generally well-adapted to their environments, selection acts primarily to maintain the current phenotype by removing deviants from both distribution tails. Directional selection requires either an environmental shift that makes the current optimum suboptimal, or colonization of a new environment — events that are less common than ongoing stabilizing pressure in stable environments.
Stabilizing selection explains a key observation in evolutionary biology: despite enormous genetic variation and constant mutation, many traits remain remarkably stable over geological time. This stability is not from lack of variation but from continuous removal of variants that deviate from the optimum. Evolutionary change requires a new selective regime; in its absence, stabilizing selection maintains the existing phenotype.