Which is the best definition of directional selection?
Directional selection is a process by which one extreme of a trait is favored, shifting the population's trait distribution toward that extreme over successive generations. The average value of the trait moves in one direction as the favored phenotype becomes more common.
The answer
Directional selection is the mode of natural selection in which one of the two extremes of a trait is favored, causing the entire distribution of that trait to shift in that direction over generations. On a bell-curve graph of the trait, the peak slides toward one tail. Individuals at the favored extreme survive and reproduce more, so their version of the trait becomes increasingly common while the opposite extreme is selected against.
A textbook example is the peppered moth during the Industrial Revolution: as soot darkened tree bark, dark moths were better camouflaged, so the population shifted toward darker coloration. Other examples include bacteria evolving toward antibiotic resistance and predators driving prey toward larger body size.
Why the other options are wrong
The three modes of natural selection are easy to confuse, so distractors usually describe one of the other two:
- Stabilizing selection favors the intermediate trait and selects against both extremes. The distribution gets narrower and taller but the average stays put — human birth weight is the classic case. A definition mentioning "the average or middle value is favored" describes stabilizing selection, not directional.
- Disruptive (diversifying) selection favors both extremes and selects against the middle, splitting one distribution into two peaks. A definition about "both extremes increasing while intermediates decline" is disruptive selection.
- A distractor saying selection "has no effect on the trait distribution" describes genetic drift or no selection at all — not directional selection.
The defining feature that separates directional selection from the others is that it favors only one extreme and moves the mean, rather than tightening it (stabilizing) or splitting it (disruptive).
The bigger picture
Directional selection requires that a trait has heritable variation and that one extreme confers higher fitness in the current environment. It is often triggered by an environmental change — a new predator, a climate shift, a new food source, or human activity like antibiotic use. Because it steadily pushes a population toward one phenotype, it is the mode most associated with rapid, observable evolutionary change and with adaptation to new conditions. If the environment later stabilizes, directional selection can give way to stabilizing selection, which then fine-tunes the population around the new optimum. Recognizing which way the curve moves — one tail, the middle, or both tails — is the single skill this question rewards.
| Directional | One extreme | Peak shifts toward that extreme; mean moves | Peppered moth darkening; antibiotic resistance |
| Stabilizing | The intermediate | Curve narrows; mean stays the same | Human birth weight |
| Disruptive | Both extremes | Curve splits into two peaks | Beak sizes splitting into two forms |
Frequently asked
What is an example of directional selection?
The peppered moth is the classic example: pollution darkened tree bark, so darker moths survived better and the population shifted toward dark coloration. Antibiotic resistance in bacteria and increasing prey body size under predation are other examples.
How is directional selection different from stabilizing selection?
Directional selection favors one extreme and shifts the population's average toward it. Stabilizing selection favors the intermediate value and selects against both extremes, keeping the average the same while narrowing the range of variation.
What causes directional selection?
It is usually driven by an environmental change that gives one extreme of a trait a fitness advantage — a new predator, climate shift, new food source, or human pressures like antibiotics. The advantaged phenotype then becomes more common each generation.
Is the peppered moth an example of directional selection?
Yes. During the Industrial Revolution, soot darkened tree trunks, making dark moths better camouflaged from birds. Selection favored the dark extreme, shifting the population toward darker coloration — a textbook case of directional selection.
What does a directional selection graph look like?
On a bell curve of the trait, the entire peak slides toward one tail. One extreme grows more common while the opposite extreme shrinks, so the mean of the distribution moves in the favored direction over generations.