Three patterns of natural selection

0:00How does natural selection change a population?

0:04Each dot is one individual, placed along the bottom by its value of one trait.

0:10Their outline is the trait distribution: how common each value is.

0:15For many traits, most are near the average and few are at the extremes, the far ends.

0:22Natural selection acts on individuals: those with helpful inherited traits survive and reproduce more.

0:30Over many generations the population changes, so we compare its curve before and after.

0:37There are three common patterns: directional, stabilizing and disruptive selection.

0:44In directional selection, one extreme is favoured: those individuals survive and reproduce best.

0:52On soot-darkened bark, birds ate more light peppered moths, so light moths were selected against.

0:59Over many generations, the whole curve shifted toward dark wings.

1:04In stabilizing selection, the middle is favoured, and both extremes are selected against.

1:11For human birth weight, very light and very heavy babies are less likely to survive.

1:18Over generations, the average stays the same, but the curve gets narrower.

1:23In disruptive selection, both extremes are favoured, and the middle is selected against.

1:30In seedcracker finches, small bills suit soft seeds, large bills suit hard seeds, and medium bills do worse on both.

1:39Over many generations, the middle shrinks and the curve can split into two peaks.

1:46So directional selection shifts the curve, stabilizing narrows it, and disruptive splits it.