Three four modes of natural selection:

1
Three (four) modes of natural selection
Directional selection
Stabilizing selection
Disruptive selection
2
Directional selection
mean
frequency
(Modified from Freeman and Heron, Evolutionary
Analysis, 1998)
flower size
From C. Galen. 1996. Evolution 50120-125.
3
Directional selection
new mean
mean
frequency
(Modified from Freeman and Heron, Evolutionary
Analysis, 1998)
Plant that make smaller flowers fail to get
visited, and thus pollinated, by bumblebees.
flower size
So the average flower size increases
From C. Galen. 1996. Evolution 50120-125.
4
Stabilizing selection
frequency
(Modified from Freeman and Heron, Evolutionary
Analysis, 1998)
birth weight
5
Stabilizing selection
variance decreases
frequency
(Modified from Freeman and Heron, Evolutionary
Analysis, 1998)
birth weight
Because low and high birth weight babies have
lower survival rates, the variance decreases.
6
Disruptive selection
frequency
(Modified from Freeman and Heron, Evolutionary
Analysis, 1998)
beak size
7
Disruptive selection
Mean beak size
variance increases
frequency
(Modified from Freeman and Heron, Evolutionary
Analysis, 1998)
after
before
beak size
Because intermediate seed sizes may be less
abundant, birds with intermediate beak sizes
survive poorly. The result is that most of the
birds are more different from the population mean
than they were before selection.
8
SoThree modes of natural selection
Directional selection -- changes mean
Stabilizing selection -- decreases variance
Disruptive selection -- increases variance