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Evolution__3rd_Edition

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600 PART 5 / Macroevolution<br />

Figure 21.5<br />

The crucial difference between<br />

punctuated equilibrium<br />

and phyletic gradualism<br />

concerns the observed rate<br />

of evolutionary change at,<br />

and between, splitting events.<br />

(a) Punctuated equilibrium.<br />

(b) Phyletic gradualism.<br />

(c) The theory of punctuated<br />

equilibrium also predicts that<br />

evolution will not occur except<br />

at times of speciation. Rapid<br />

change without splitting<br />

contradicts the theory.<br />

Punctuated equilibrium exists in<br />

more, and less, orthodox versions<br />

Time<br />

(a) Punctuated equilibrium (b) Phyletic gradualism (c) Punctuated change<br />

without split in lineage<br />

Character<br />

Time<br />

Character<br />

forms are absent is again the incompleteness of the fossil record a but not in the same<br />

way as the theory of phyletic gradualism suggested.<br />

Between speciation events, species may have a low rate of evolutionary change a a<br />

condition Eldredge and Gould called stasis. In theory, the absence of evolutionary<br />

change in a species can be explained by stabilizing selection (Section 4.4, p. 76) or constraint<br />

(Section 10.7, p. 272). Constraint means that the species does not change<br />

because it lacks genetic variation, or lacks expressed genetic variation. 2 As we saw in<br />

Section 10.7.3 (p. 280), the evidence does not suggest that species stay constant because<br />

they lack genetic variation. Stabilizing selection, by contrast, is a well documented<br />

fact and is highly plausible in theory. Stabilizing selection, therefore, is the most likely<br />

(if not universally accepted) explanation for stasis in the fossil record. The theory of<br />

punctuated equilibrium holds that stasis is the normal condition within a species. Stasis<br />

breaks down only when speciation occurs. <strong>Evolution</strong>ary change is concentrated in<br />

speciation events. Any observation of change without speciation (Figure 21.5c) would<br />

contradict the theory of punctuated equilibrium.<br />

In the account here, the theory of punctuated equilibrium is relatively “orthodox.”<br />

Eldredge and Gould took the (or a) standard theory of speciation, and pointed out that<br />

is implies that fossils will usually show sudden, rather than smooth, change. However,<br />

the theory of punctuated equilibrium has stimulated much controversy, as Gould<br />

(2002b) documents. There are two main reasons. One is that punctuated equilibrium<br />

was sometimes said to challenge the fundamental “gradualism” of Darwin’s theory of<br />

evolution. Box 21.1 distinguishes two meanings of the word “gradual.” When the two<br />

meanings are distinguished, this line of controversy is defused.<br />

The second source of controversy is that the theory of punctuated equilibrium has<br />

also drawn on and been associated with much less widely accepted ideas about speciation.<br />

The theory of punctuated equilibrium has been actively developed for about<br />

30 years and exists in many different versions. In particular, “valley crossing” theories<br />

of speciation (in terms of Section 14.4.4, p. 394) have often been used to predict<br />

punctuated equilibrium. Speciation requires valley crossing if two species have different<br />

adaptations, and the intermediate forms between them have lower fitness. The two<br />

Time<br />

Character<br />

2 Genetic variation may be present, but not expressed, if it is concealed by developmental canalization<br />

(Sections 9.9, p. 242, and 10.7.3, p. 276). One version of punctuated equilbrium suggests that canalization<br />

creates a developmental constraint. <strong>Evolution</strong> is only possible in revolutionary circumstances, such as in a<br />

stressed subpopulation at the edge of a species’ main range. See the further reading section of Chapter 20 for a<br />

reason why normally concealed genetic variation could be expressed in these conditions.<br />

..

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