Evolution, 4th Edition

THE EvoluTion of BiologiCAl DivERsiTy 509

competition, a process called competitive displacement (FIG-

U R E 19. 20A). By contrast, an incumbent taxon may have pre-

vented an ecologically similar taxon from diversifying because

it already occupied resources. Extinction of the incumbent

taxon may then have vacated ecological niche space, permitting

the second taxon to radiate (FIGURE 19.20B). This process has

been called incumbent replacement [62].

Jack Sepkoski and colleagues developed a mathematical

model in which two clades increase in diversity, but in which

the increase in each clade is inhibited by both its own diversity

and that of the other clade [66]. They applied the model to data

on the number of genera of two groups of bryozoans (“moss

animals”), the cyclostomes and the cheilostomes. These sessile

colonial animals spread over rocks or other surfaces by bud-

ding. When colonies of these two groups meet, cheilostomes

generally overgrow cyclostomes (FIGURE 19.21A). Especially

since the end-Cretaceous extinction, the diversity of cheilo-

stomes has increased, whereas cyclostomes have not recovered

(FIGURE 19.21B). When Sepkoski and colleagues simulated the end-Cretaceous

drop in the diversity of both clades, their model rendered a profile of subsequent

diversity change that closely matches the data (FIGURE 19.21C). This result does

not prove that competition determined the history of bryozoan diversity, but it is

consistent with that hypothesis.

In general, a pattern of replacement is consistent with competitive displacement

if the earlier and later taxa lived in the same place at the same time, if they used the

same resources, if the earlier taxon was not decimated by a mass extinction event,

and if the diversity and abundance of the later taxon increased as the earlier taxon

Futuyma Kirkpatrick Evolution, 4e

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Evolution4e_19.20.ai Date 12-09-2016

Time

Number of species

Time

Number of species

(A) (B)

Clade

2

Clade

1

Clade

1

Clade

2

FIGURE 19.20 Models of competitive displacement and in-

cumbent replacement. In each diagram, the width of a “spindle”

represents the number of species. (A) Competitive displacement,

in which the increasing diversity of clade 2 causes a decline in

clade 1 by direct competitive exclusion. Compare with Figure

19.19B. (B) Incumbent replacement, in which the extinction of

clade 1 enables clade 2 to diversify later. (After [65].)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_19.21.ai Date 02-02-2017

Q: Should x-axis be changed to (My) for consistency with other gures?

Number of genera

100

200

300

0

Diversity

Time (Mya)

100

200

300

0

(B)

(C)

Cheilostomata

Cyclostomata

Clade 2

Clade 1

J K Pg Ng

150 100 50 0

150 100 50 0

(A)

Cyclostome

Cheilostome

FIGURE 19.21 Competitive displacement among bryozo-

ans, sessile colonial animals that spread over hard surfaces.

(A) Colonies of cheilostome bryozoans (left) can overgrow

colonies of cyclostome bryozoans (right), causing their death.

(B) Cheilostomes appeared in the late Jurassic and soon in-

creased greatly in diversity, whereas the increase of cyclostomes

was reversed in the Cenozoic. (C) The changes in diversity in a

model of competition among species in two clades, assuming

that clade 2 species are competitively superior and that the diver-

sity of both clades was reduced by an external perturbation 60

Mya. (A from [42], courtesy of Frank K. McKinney; B, C after [66].)

19_EVOL4E_CH19.indd 509 3/22/17 1:42 PM