Evolution__3rd_Edition

600 PART 5 / Macroevolution
Figure 21.5
The crucial difference between
punctuated equilibrium
and phyletic gradualism
concerns the observed rate
of evolutionary change at,
and between, splitting events.
(a) Punctuated equilibrium.
(b) Phyletic gradualism.
(c) The theory of punctuated
equilibrium also predicts that
evolution will not occur except
at times of speciation. Rapid
change without splitting
contradicts the theory.
Punctuated equilibrium exists in
more, and less, orthodox versions
Time
(a) Punctuated equilibrium (b) Phyletic gradualism (c) Punctuated change
without split in lineage
Character
Time
Character
forms are absent is again the incompleteness of the fossil record a but not in the same
way as the theory of phyletic gradualism suggested.
Between speciation events, species may have a low rate of evolutionary change a a
condition Eldredge and Gould called stasis. In theory, the absence of evolutionary
change in a species can be explained by stabilizing selection (Section 4.4, p. 76) or constraint
(Section 10.7, p. 272). Constraint means that the species does not change
because it lacks genetic variation, or lacks expressed genetic variation. 2 As we saw in
Section 10.7.3 (p. 280), the evidence does not suggest that species stay constant because
they lack genetic variation. Stabilizing selection, by contrast, is a well documented
fact and is highly plausible in theory. Stabilizing selection, therefore, is the most likely
(if not universally accepted) explanation for stasis in the fossil record. The theory of
punctuated equilibrium holds that stasis is the normal condition within a species. Stasis
breaks down only when speciation occurs. Evolutionary change is concentrated in
speciation events. Any observation of change without speciation (Figure 21.5c) would
contradict the theory of punctuated equilibrium.
In the account here, the theory of punctuated equilibrium is relatively “orthodox.”
Eldredge and Gould took the (or a) standard theory of speciation, and pointed out that
is implies that fossils will usually show sudden, rather than smooth, change. However,
the theory of punctuated equilibrium has stimulated much controversy, as Gould
(2002b) documents. There are two main reasons. One is that punctuated equilibrium
was sometimes said to challenge the fundamental “gradualism” of Darwin’s theory of
evolution. Box 21.1 distinguishes two meanings of the word “gradual.” When the two
meanings are distinguished, this line of controversy is defused.
The second source of controversy is that the theory of punctuated equilibrium has
also drawn on and been associated with much less widely accepted ideas about speciation.
The theory of punctuated equilibrium has been actively developed for about
30 years and exists in many different versions. In particular, “valley crossing” theories
of speciation (in terms of Section 14.4.4, p. 394) have often been used to predict
punctuated equilibrium. Speciation requires valley crossing if two species have different
adaptations, and the intermediate forms between them have lower fitness. The two
Time
Character
2 Genetic variation may be present, but not expressed, if it is concealed by developmental canalization
(Sections 9.9, p. 242, and 10.7.3, p. 276). One version of punctuated equilbrium suggests that canalization
creates a developmental constraint. Evolution is only possible in revolutionary circumstances, such as in a
stressed subpopulation at the edge of a species’ main range. See the further reading section of Chapter 20 for a
reason why normally concealed genetic variation could be expressed in these conditions.
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