1. Mechanisms of Evolution
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2. Distribution Curves: Normal Distribution
The animals with the mean or average variation is selected for, the animals with extreme variations are at a disadvantage. There are more average animals than extreme.
Mean/ Average
Extreme

3. Distribution Curves: Directional Selection
Animals with one side of the extreme have a better advantage, so the curve shifts.

4. Distribution Curves: Stabilizing Selection
Animals with the average variation surge in population number, while animals with the extreme variation decrease rapidly.

5. Distribution Curves: Disruptive Selection
Animals with both extremes (both sides of the bell) are favored, and animals with the average variation die off.