Evolution, 4th Edition

534 CHAPTER 20

another; that is, stasis that is “punctuated” by rapid change (FIGURE 20.16B).

They contrasted this pattern with what they called phyletic gradualism, the tra-

ditional notion of slow, incremental change (FIGURE 20.16C). Paleontologists

agree that stasis is a common pattern (FIGURE 20.17). For example, eight liv-

ing lineages of bivalves with a fossil record all show as much or more variation

among geographic populations as they do over the course of 4 My [103]. Most

fossil lineages fit a model of either stasis or random fluctuations [44, 47].

Eldredge and Gould proposed that the rapid shifts (“punctuations”) represent

speciation (especially founder effect speciation), whereby a reproductively isolated

new species that originated “offstage” expands its range and replaces the ancestral

species. One of the few examples of morphological change associ-

ated with speciation is in a Miocene genus of bryozoans, or “moss

animals,” that persisted with little change for several million years,

while new species appeared abruptly, without evident intermedi-

ates (FIGURE 20.18) [11]. Eldredge and Gould’s hypothesis that

evolutionary change requires speciation is not widely accepted [46],

but it stimulated interest in rates of evolution and posed the ques-

tion of whether or not speciation might facilitate, or be correlated

with, phenotypic evolution.

Speciation and phenotypic evolution

Might speciation enhance evolutionary change? The fossil record

can provide evidence only when it is exceptionally complete. Ceno-

zoic planktonic Foraminifera (shelled protists) have an outstand-

ing fossil record, and in these, morphological evolution is almost

always accompanied by speciation [106]. Evidence has also been

sought in phylogenies of living species, using statistical tests to

determine if the amount of morphological or DNA sequence dif-

ference among species is attributable mostly to evolution within

lineages, or is enhanced by the number of branching (speciation)

FIGURE 20.17 An example of stasis: specimens of

the bivalve Macrocallista maculata from a living

population and from fossil deposits dated at 1, 2,

4, and 17 Mya. All are from Florida. Scale bars = 1

cm. (Photos courtesy of Steven M. Stanley.)

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Evolution4e_20.17.ai Date 12-16-2016

Living

organism

1 Mya

2 Mya

4 Mya

17 Mya

Futuyma Kirkpatrick Evolution, 4e

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Troutt Visual Services

Evolution4e_20.17.ai Date 12-16-2016

Living

organism

1 Mya

2 Mya

4 Mya

17 Mya

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_20.17.ai Date 12-16-2016

Living

organism

1 Mya

2 Mya

4 Mya

17 Mya

Futuyma Kirkpatrick Evolution, 4e

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Troutt Visual Services

Evolution4e_20.18.ai Date 12-16-2016

Present

Pleistocene

Pliocene

Time (Mya)

Late Miocene

5

3

7

10

M. tenue

M. colligatum

M. auriculatum

New

species

#10

New

species

#9

Morphological difference

FIGURE 20.18 Punctuated equilibria: the phylogeny and temporal

distribution of a lineage of bryozoans (Metrarabdotos). The horizontal

distance between points represents the amount of morphological dif-

ference between samples. The general pattern is one of abrupt shifts

to new, rather stable morphologies. Only a part of the full phylogeny of

the genus, which has many more species, is shown. (After [11]; photos

courtesy of A. Cheetham.)

20_EVOL4E_CH20.indd 534 3/22/17 1:44 PM