Evolution, 4th Edition

PHEnoTyPiC EvoluTion 141

statistically significant, shows there was directional selection. Note

that this comparison is between individuals of the same genera-

tion, before and after the drought. At this point, no evolution has yet

occurred.

A fitness function can also act on the variance of a trait (FIGURES

6.6B and C). Stabilizing selection favors individuals whose trait values

are near the population’s mean. After stabilizing selection acts, the phe-

notypic variance is reduced. This is a common form of selection because

the means of many traits are near the values that have the highest fit-

ness (often referred to as the optimum phenotype). In that situation,

individuals that are much smaller or much larger than the mean have

lower fitness. The result is that the tails of the phenotypic distribution

are trimmed so the variance is smaller. Birth weight in humans is a

classic example. Babies that weight much less or much more than the

average at birth have a lower chance of surviving (FIGURE 6.8A).

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(A)

0.2

(B)

(C)

Before

selection

After

selection

Survival rate

0.1

0.3

0.4

0.6

0.5

50

Number of individuals

25

75

0

8 9

Bill depth (mm)

10 11

10

0

0

G. fortis

FIGURE 6.7 A severe drought in the Galápagos Islands in 1977 produced

very strong directonal selection on bills of the Galápagos finch Geospiza

fortis. Bill depth, which is the distance from the top to the bottom of the bill

at its base, has a strong effect on the size of seed the finches are able to eat.

(A) The fitness function of survival as a function of bill depth shows there

was strong directional selection for deeper bills. The fitness function was

estimated from the data in (B) and (C). (B) Distribution in bill depth of 751 G.

fortis before the drought. (C) Distribution in bill depth of the 90 individuals

that survived the drought. The selection differential S, which is the change in

mean beak size (red triangles) from before to after selection (shown by the

arrow), is highly statistically significant. (A after [42]; B and C after [8].)

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7 8 9 10

(A)

Survival

1

0.8

0.6

0.4

0.2

0

2 4 6

Weight (lbs)

8 10

(B)

Number of individuals

0.5

0.25

0

Lower mandible length (mm)

FIGURE 6.8 A) Birth weight in humans is under stabilizing selec-(

tion. Infants with birth weights much smaller or much larger than

average have a lower probability of surviving to 28 days. Each cir-

cle shows the survival rate for a group of infants that had the birth

weight shown and the same gestation time. (B) The probability of

survival to adulthood in the black-bellied seedcracker (Pyrenestes

ostrinus) depends on an individual’s lower mandible length, a

measure of bill size. The fitness function is shown by the curve.

Green portions of the histogram show birds that did not survive;

blue portions show birds that did. Birds with intermediate-sized

bills have the lowest survival, showing that disruptive selection is

acting. (A courtesy of Dolph Schluter after data from [42];

B after [47], with photo courtesy of Thomas B. Smith.)

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