Evolution__3rd_Edition

78 PART 1 / Introduction
Figure 4.3
Directional selection by fishing
on pink salmon, Onchorhynchus
gorbuscha. The graph shows the
decrease in size of pink salmon
caught in two rivers in British
Columbia since 1950. The
decrease has been driven by
selective fishing for the large
individuals. Two lines are
drawn for each river: one for
the salmon caught in oddnumbered
years, the other for
even years. Salmon caught in
odd years are consistently
heavier, which is presumably
related to the 2-year life cycle
of the pink salmon.
(5 lb ≈ 2.2 kg.) From Ricker
(1981). Redrawn with
permission of the Minister of
Supply and Services Canada,
1995.
Percentage survival to age 4 weeks
Minimal mortality × 1,000
Mean weight (lb)
Mean weight (lb)
99
(a)
98
97
96
95
90
80
Mean survival rate (females)
Mean survival rate (males)
70
60
Females
50
40
30
20
10
Males
0
2 3 4 5 6 7 8 9 10
Birth weight (lb)
25
20
15
10
5
0
(b)
Italy (1954–74)
Japan (1969–84)
USA, whites (1950–76)
6.0
5.0
4.0
3.0
6.0
5.0
4.0
3.0
USA, non-whites (1950–76)
0 5 10 15 20 25 30 35
Average mortality × 1,000
Bella Coola
Upper Johnston Strait
Odd years
Odd years
Even years
Even years
’51 ’52 ’53 ’54 ’55 ’56 ’57 ’58 ’59 ’60 ’61 ’62 ’63 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 ’74
Year
Figure 4.4
(a) The classic pattern of stabilizing selection on human birth
weight. Infants weighing 8 lb (3.6 kg) at birth have a higher
survival rate than heavier or lighter infants. The graph is based
on 13,700 infants born in a hospital in London, UK, from 1935 to
1946. (b) Relaxation of stabilizing selection in wealthy countries
in the second half of the twentieth century. The x-axis is the
average mortality in a population; the y-axis is the mortality of
infants that have the optimal birth weight in the population (and
so the minimum mortality achieved in that population). In (a),
for example, females have a minimum mortality of about 1.5%
and an average mortality of about 4%. When the average equals
the minimum, selection has ceased: this corresponds to the 45°
line (the “no selection” case in Figure 4.2d would give a point on
the 45° line.) Note the way in Italy, Japan, and the USA, the data
approach the 45° line through time. By the late 1980s the Italian
population had reached a point not significantly different from
the absence of selection. From Karn & Penrose (1951) and Ulizzi
& Manzotti (1988). Redrawn with permission of Cambridge
University Press.
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