Evolution, 4th Edition

510 CHAPTER 19

declined [40]. By these criteria, the rodents seem to have displaced the multituber-

culates (see Figure 19.19B). Vascular plants, which certainly compete for space and

light, showed this pattern during the Cretaceous, when flowering plants increased in

diversity and abundance at the expense of nonflowering plants.

Incumbent replacement has probably been more common than competitive dis-

placement [7, 8, 33]. The best evidence of incumbency and release is supplied by

repeated replacements. For instance, amphichelydians, the “stem group” of turtles,

could not retract their head and neck into their shell (FIGURE 19.22). Two groups of

modern turtles, which protect themselves by bending the neck within the shell or

under its edge, replaced the amphichelydians in different parts of the world four or

five times, especially after the K/Pg extinction event. The modern groups evidently

could not radiate until the amphichelydians had become extinct. That this replace-

ment occurred in parallel in different places and times makes it a likely example of

release from competition [62].

What can we conclude? Is diversity of species constrained by ecological limits?

Probably both sides in the debate are partly right. Most clades, viewed individually,

seem to have approached a diversity limit. But new clades, with new ways of living,

have arisen throughout life’s history, seeming to push the diversity ceiling higher and

higher—which, to advocates of boundless diversity, looks like no ceiling at all [29].

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_1922.ai Date 11-02-2016

(B)

(C)

(A)

FIGURE 19.22 Pleurodiran and cryptodiran turtles replaced

incumbent amphichelydian turtles, which became extinct.

(A) Amphichelydians, represented here by the reconstructed

skeleton of an early turtle (Proganochelys quenstedti, Trias-

sic), could not retract their head for protection. (B) Snakeneck

turtles such as Chelodina longicollis are pleurodiran turtles,

which flex the neck sideways beneath the edge of the cara-

pace. (C) Cryptodiran turtles, represented here by an eastern

box turtle (Terrapene carolina), fully retract the head into the

shell by flexing the neck vertically. (A courtesy of E. Gaffney,

American Museum of Natural History.)

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