For a polygenic trait like body size, a population forms a bell-shaped curve. Natural selection can push on that curve in three ways, and each leaves its own mark on the graph.
Key idea
Directional selection favors one extreme, so the whole curve shifts that way. Stabilizing selection favors the middle, so the curve gets narrower and taller. Disruptive selection favors both extremes over the middle, so the curve spreads and can split into two peaks.
Why it works
Ask which individuals leave the most offspring. If the largest do best, the next generation's average moves up: directional. If the middle does best and both extremes do poorly, the spread shrinks: stabilizing. If both extremes do well and the middle does poorly, the middle thins out: disruptive.
| Pattern | Who does best | Curve after | Example |
|---|---|---|---|
| Directional | one extreme | shifts | deeper beaks in ground finches after a drought |
| Stabilizing | the middle | narrower | goldenrod galls: small galls are attacked by wasps, large ones by birds |
| Disruptive | both extremes | two peaks | seedcracker finches in Africa: small and large bills crack the two common seed types |
Worked example: Reading a graph
Before selection, a lizard population's body lengths center on 10 cm with a wide spread. Later the center is still 10 cm, but the spread is much narrower. Which pattern?
- The average did not move, so not directional.
- The curve did not split.
- The extremes became rarer.
Answer: Stabilizing selection.
Worked example: Another graph
A trait's average moves from 4 mm to 6 mm over many generations, with the same spread. Which pattern?
- The whole curve moved toward larger values.
Answer: Directional selection.
Common mistake
Stabilizing and disruptive are easy to mix up because both keep the average in about the same place. Look at the middle of the curve. Stabilizing makes the middle taller and the sides thinner; disruptive makes the middle lower and the sides taller.