Selection can move the mean or the spread
- The idea
- Predict
- See it
- Why
- Check
The idea
“I can tell whether selection happened from one number, like whether the average changed.”
Famous examples show a trait shifting, like finch beaks getting deeper after a drought. So a moving average looks like selection. Stabilizing selection leaves the average alone.
The short version
The mistake: I can tell whether selection happened from one number, like whether the average changed.
What's actually true: Directional selection shifts the mean. Stabilizing and disruptive selection change the spread. You have to check both.
- The mean tells you where a trait sits. The spread, often given as a standard deviation, tells you how much it varies.
- Directional selection favors one extreme, so the mean moves toward it. The spread can stay about the same.
- Stabilizing selection favors the middle, so the spread narrows around a steady mean. Disruptive selection favors both extremes, so the spread widens.
On the exam: Check the mean and the spread. A change in either one can be a sign of selection.