Evolution__3rd_Edition

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Figure 21.3
The relation between the
estimated logarithmically
transformed evolutionary rate
and the time interval used, for
the 521 studies summarized in
Table 21.1. The relation is
negative. For the meaning of
samples I, II, III, and IV see
Table 21.1. The digits higher
than 1 on the graph mean that
number of cases fell on that spot
(x for numbers higher than 9).
From Gingerich (1983). © 1983
American Association for the
Advancement of Science.
. . . which may be due to
fluctuations in evolutionary
direction
Ln evolutionary rate (darwin)
120
100
80
60
40
20
I
0
0
0
2
0
0
3
0
2
7
2
3
2
0
2
0 4
0
0
0
II
0
2
0
4
X
X
9
X
32
0
0
2
1
1
1 1
1
CHAPTER 21 / Rates of Evolution 597
2
11
33 2
13
17
0
–20
–40
–60
III
19
212
1
2
2 1
1 1 1
1
1
11
1 1
1
1
1 1
1
11
IV
1
1
1 13
3 2 1
1 1 21
1 2
2 21 2541 2 1 1
2 33 5 1 2 1
11 1 23 5 2 1 1 24 2 1
1 1 8 5 1 215
3 1 1
1 1 121534 31
2 4 1 1 1323
1 9 2 1 13
3 13133 3 1
1 2 3 4 2 2 1 11
1
1 33 2 3 1 1 2 4 1
1 1111522
6 1 1 2 7 4 13 2 11
1 5 6 4 1 2 1 1
1 8 6 1 1
1 1 1211 4 1 2 1
2 1 2
1
1 1 1 1 1
1
–140 –120 –100 –80 –60 –40 –20 0 20 40 60
Ln measurement interval (Myr)
the molecular level, by way of comparison, the rates of evolution seem to be fairly
constant over all time periods (e.g., Figure 7.3, p. 165).
Gingerich’s basic observation can now be supplemented. Hendry & Kinnison (1999)
reviewed 20 studies of microevolutionary change within a species (that is, studies like
that by the Grants on the Galápagos finch a Section 9.1, p. 223). The timescales and
measured rates of evolution fall between, and partly overlap with, categories I and II in
Figure 21.3. The results also show a negative relation between measurement interval
(from 2 to 125 years) and the observed rate of evolution.
Hendry and Kinnison point out several factors that can produce a negative relation
as in Figure 21.3 and their data. The most important factor can be explained in terms of
the Darwin’s finch example. However, we need to concentrate on a slightly different
feature. In the previous section, we considered the rate of evolution during a single
burst of evolutionary change. Now we need to turn to the fluctuating selection pressure
over several consecutive years (Section 9.1, p. 223).
The finches’ beaks evolved to be larger in times of food shortage and smaller in times
of abundance, and the food supply fluctuated through time according to the weather,
particularly the periodic El Niño disturbance. Imagine measuring the rate of evolution
both within one of these cycles and over the cycle as a whole (Figure 21.4a). If the direction
of evolution fluctuates, the rate of evolution measured over a short interval is
inevitable higher than the rate measured over a longer time interval because the shortterm
changes cancel out. The pattern in Figure 21.4a is simplified, giving zero net