Evolution__3rd_Edition

..
(a) 1976–1977
(b) 1981–1982
(c) 1984–1985
. . . and in other studies
Figure 9.10
Frequency distribution
of selection differentials (S)
found in 63 studies of
directional selection in 62
species. Drawn from the
database of Kingsolver et al.
(2001).
Weight
Wing length
Tarsus size
Beak length
Beak depth
Beak width
Weight
Wing length
Tarsus size
Beak length
Beak depth
Beak width
Weight
Wing length
Tarsus size
Beak length
Beak depth
Beak width
–0.5 0 0.5
Selection differential
CHAPTER 9 / Quantitative Genetics 241
Figure 9.9
(a) In the drought of 1976–77, Geospiza fortis individuals with
larger beaks survived better and the average size of the finch
population increased. (b) In the normal years of 1981–82 there
was a slight advantage to having a larger beak, but this was much
smaller than during the drought. (c) After the 1983 El Niño, in
1984–85, finches with smaller beaks survived better and the
average size of the finch population decreased. The x-axis
expresses the selection differential (S) in standardized form:
the mean for the survivors after selection minus the population
mean before selection all divided by the standard deviation of
the character. (The standard deviation is the square root of the
variance; see Box 9.1 for the meaning of variance.) A value of S
of about 5% for beak depth in the text corresponds here to a
standardized S of about 0.6. Redrawn, by permission, from Gibbs
& Grant (1987). © 1987 Macmillan Magazines Ltd.
response. When we come to those more puzzling results, it is worth keeping in mind
the “successful” results for Darwin’s finches.
Kingsolver et al. (2001) compiled the results of 63 studies of directional selection, on
62 species, performed by many different biologists and published between 1984 and
1997. Figure 9.10 shows the distribution of selection differentials found in the studies.
For a survey of many characters, the selection differentials need to be “standardized.”
The selection differentials we have looked at so far have been “non-standardized.” They
were absolute measurements (0.4 lb or 0.18 kg) in salmon and percentages (5%) in
the finches. The equation R = h 2 S works in any one study with absolute numbers or
percentages. A standardized selection differential expresses the deviation from the
mean of the successful individuals as a fraction of the phenotypic standard deviation in
the population. (Box 9.1 formally explains standard deviation, but intuitively it is a
Frequency
0.5
0.4
0.3
0.2
0.1
0
–1.0 –0.5 0 0.5 1.0 1.5 2.0 2.5
Selection differential ( S )