Evolution__3rd_Edition

598 PART 5 / Macroevolution
(a)
5
Beak size
4
3
2
1
0
0 1 2 3 4
Time interval
Figure 21.4
The inverse relation between measured evolutionary rate and
time interval (Figure 21.3) will be found if the direction of
evolution fluctuates through time. (a) Simplified cycle of
evolutionary change. The rate of change measured for short
time units is higher than for the cycle as a whole, there is no
net change over the cycle, and the rate of evolution is zero. The
numbers under “measurement interval” in the table refer to
Other factors may contribute too
Some trends ...
(b)
Character
Time
Measurement interval
Rate of
evolution
change over a cycle, but if there are any fluctuations in evolutionary direction (Figure
21.4b) it will be true that a rate measured over a shorter interval will be higher. Likely
almost all evolutionary lineages show some reversals in the direction of change, and the
pattern illustrated by Darwin’s finches may be quite common. This would explain the
general relation in Figure 21.3.
Other factors may be contributing. For instance, the cases of rapid evolution over
short time intervals are for artificial selection experiments (dataset I) and natural ecological
colonizations (dataset II); it may be that these are extraordinary events and have
higher than average selection intensities. (Alternatively, however, it might be argued
that the rates are high only because the measurement interval is short enough to
catch evolution in its unidirectional phase, and not because the intensity of selection is
peculiar. Opinions differ about how representative the selection intensities in datasets
I and II are of those in the lineages making up datasets III and IV.)
This interpretation, if it is correct, matters for some kinds of generalizations about
evolutionary rates, but not others. It does not invalidate the measurements themselves.
In the 14 million years between Epihippus gracilis and Mesohippus bairdii, horse teeth
evolved at a rate of 0.023–0.037 darwins (Figure 21.1), and that is that. All questions
about individual measurements, and comparisons between them, remain valid. It is
for the more general patterns that Gingerich’s result should make us suspicious. The
generalization that mammals evolve faster than mollusks, for example, is reflected in
Gingerich’s data. He found that vertebrates as a whole tended to evolve faster than
invertebrates (compare the mean rates of evolution for the two groups in Table 21.1).
While it remains true that in the samples measured vertebrates did evolve 1.14 times
as fast as invertebrates, this might mainly be due to the shorter time intervals for the
1 unit
2 units
3 units
4 units
Within
1 or 2 or 3 or 4
1 + 2
2 + 3
3 + 4
1 + 2 + 3
2 + 3 + 4
1
0
1 av. 1/3
0
1/3
1/3
1 + 2 + 3 + 4 0
the time intervals in the x-axis of the graph (the arbitrary
beak size units can be thought of as logarithmic, to make
the rates properly comparable with the formula for
calculating rates in Section 21.1). (b) With a more
realistic pattern of evolution, the inverse relation
between rate and measurement interval will still
be found to some extent if there are any fluctuations
in the direction of change.
..