Types of selection
Normal distribution:

Advantageous trait in the middle with higher percentage of population and the other traits on the outside with the less percentage of population due to natural selection
Stabilizing selection

Natural selection that causes a decrease in extremes, kills off the undesirable (extreme) trait
Directional Selection

The extreme variant does better than the other, causing killing off of the other extreme
Disruptive selection

extreme traits do better than middle ground trait
cause decrease in middle trait with increase in the two extremes cause the double hill effect