Evolution__3rd_Edition

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Frequency
Fitness
(number of offspring produced)
Average size (in population)
CHAPTER 4 / Natural Selection and Variation 77
(a) Directional selection (b) Stabilizing selection (c) Disruptive selection (d) No selection
Body size Body size Body size Body size
Body size Body size Body size Body size
Time Time Time
Time
Figure 4.2
Three kinds of selection. The top line shows the frequency
distribution of the character (body size). For many characters
in nature, this distribution has a peak in the middle, near
the average, and is lower at the extremes. (The normal
distribution, or “bell curve,” is a particular example of
this kind of distribution.) The second line shows the relation
between body size and fitness, within one generation, and
the third the expected change in the average for the character
over many generations (if body size is inherited).
(a) Directional selection. Smaller individuals have higher
fitness, and the species will decrease in average body size
through time. Figure 4.3 is an example. (b) Stabilizing
selection. Intermediate-sized individuals have higher
fitness. Figure 4.4a is an example. (c) Disruptive selection.
Both extremes are favored and if selection is strong enough,
the population splits into two. Figure 4.5 is an example.
(d) No selection. If there is no relation between the
character and fitness, natural selection is not operating
on it.
lighter than average did not survive as well as babies of average weight. Stabilizing selection
has probably operated on birth weight in human populations from the time of the
evolutionary expansion of our brains about 1–2 million years ago until the twentieth
century. In most of the world it still does. However, in the 50 years since Karn and
Penrose’s (1951) study, the force of stabilizing selection on birth weight has relaxed in
wealthy countries (Figure 4.4b), and by the late 1980s it had almost disappeared. The
pattern has approached that of Figure 4.2d: percent survival has become almost the
same for all birth weights. Selection has relaxed because of improved care for premature