Evolution, 4th Edition

MACROEvOLuTiON: EvOLuTiON AbOvE THE SPECiES LEvEL 533

Gradualism and punctuated equilibria

Paleontologists have held two rather different views of evolutionary rates, which

were expressed during a controversy that is not entirely over. Gradual transitions

through intermediate states have been described

for many morphological transitions, but the fos-

sil record of many other groups is marked by gaps

rather than continuous change (FIGURE 20.16A).

Most paleontologists have followed Darwin in

supposing that evolution was actually gradual

but that the fossil record is incomplete. In 1972,

however, Niles Eldredge and Stephen Jay Gould

proposed a more complicated, and much more

controversial, explanation, which they called

punctuated equilibria [20].

Eldredge and Gould said that species in the

fossil record often show long periods of little or

no detectable phenotypic change, interrupted by

rapid shifts from one such “equilibrium” state to

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Difference between observed and

predicted phenotype

0 5 10 15 20

Elapsed time (My)

0.8

1.0

1.2

1.4

1.6

250 μm

Ancestor-descendant

populations

Sister

populations

FIGURE 20.15 The relationship between evolutionary changes

in shape in fossilized lineages of the marine ostracod crustacean

Poseidonamicus and the elapsed time during which evolution has

occurred. The figure shows the ostracod’s shell; the colored circles

are landmarks used for measurments that were used to character-

ize the shape. In the graph, blue dots show changes between vari-

ous ancestral and descendant populations in individual lineages

and red dots show differences between sister populations that

diverged from a common ancestor. The vertical axis shows the

difference between the shape change that actually occurred (“ob-

served” change) and how the shape would have changed if it had

evolved strictly along the direction of greatest phenotypic (and

presumably genetic) variation within species (i.e., evolution along

lines of least resistance, as described in Chapter 6). Especially for

pairs of sister populations, changes that deviate more greatly from

the presumed line of least genetic resistance occur over longer

time spans. (After [45]; photo courtesy of Gene Hunt.)

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Time

Character value

Time

(B) Punctuated equilibria

Character value

(e.g., size)

Time

(A) Hypothetical data

Character value

(C) Phyletic gradualism

FIGURE 20.16 Two models of evolution, as applied to a hypothetical set of

fossils. (A) Hypothetical values for a character in fossils from different time periods.

These data might correspond to either of the models shown in panels B and C.

(B) The punctuated equilibria model of Eldredge and Gould, in which morpho-

logical change occurs in new species. Morphological evolution, although rapid, is

still gradual, as shown in the inset by the shift in the frequency distribution of the

phenotype. (C) The traditional gradualism model. The character mean changes

gradually within a single species, faster at some times than others.

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