VARIATION - We can measure variation
in lots of different ways. Humans can be tall or short, skinny or fat, dark
or light skinned, etc. It is best to think of variation as a distribution
with an average and extremes.
Individuals vary on either side of
the average. Change operates on this variation. Change can either stabilize
around an average or can favor an extreme.
Stabilizing selection, such as that seen in the baseball example, works on the extremes.
Directional selection favors one or the other of the extremes. Individuals
with a particular trait will reproduce more offspring. Future generations
will have more individuals with an extreme. Eventually the extreme becomes
the norm.
