Evolution, 4th Edition

520 CHAPTER 20

Gradualism and Saltation

Darwin proposed that evolution proceeds gradually, by small steps. His ardent sup-

porter Thomas Henry Huxley, however, cautioned that Darwin’s theory of evolution

would be just as valid even if evolution proceeded by leaps (sometimes called salta-

tions). Some later biologists proposed just this. The geneticist Richard Goldschmidt

argued in The Material Basis of Evolution [34] that species and higher taxa arise not

from the genetic variation that resides within species, but instead “in single evolu-

tionary steps as completely new genetic systems.” He postulated that major changes

of the chromosomal material, or “systemic mutations,” would give rise to highly

altered creatures. Most would have little chance of survival, but some few would

be “hopeful monsters” adapted to new ways of life. Goldschmidt’s genetic system

hypothesis has been completely repudiated, but the possibility of evolution by more

modest jumps remains one of the most enduring controversies in evolutionary the-

ory. Quite different species are often connected by intermediate forms, so that it

becomes arbitrary whether the complex is classified as two genera (or subfamilies, or

families) or as one (see Chapter 2). Nonetheless, there exist many conspicuous gaps,

especially among higher taxa such as orders and classes. No living species bridge

the gap between cetaceans (dolphins and whales) and other mammals, for example.

The most obvious explanation of phenotypic gaps among living species is extinc-

tion of intermediate forms that once existed—as the cetaceans themselves illustrate

(FIGURE 20.3). DNA sequences imply that, among living animals, whales are most

closely related to hippopotamuses. The earliest known fossilized members of the

cetacean lineage (see Figure 20.3A) were terrestrial, but not particularly similar to

living hippopotamuses—implying that the common ancestor of two quite different

forms need not have appeared precisely intermediate between them, because the two

phyletic lines may have undergone quite different modifications. Extinct cetaceans

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_20.03.ai Date 12-16-2016

(A) Indohyus

(B) Ambulocetus

(C) Rodhocetus

(D) Dorudon

(E) Phocoena

Pelvis and hindlimb

FIGURE 20.3 Reconstruction of stages in

the evolution of cetaceans from terrestrial

artiodactyl ancestors. (A) Eocene raoel-

lids, perhaps the sister group of Cetacea,

were terrestrial but show some evidence

of semiaquatic life. (B) The amphibious

Ambulocetus. (C) The middle Eocene Rod-

hocetus had the distinctive ankle bones of

artiodactyls, but had numerous cetacean

characters. (D) Dorudon, of the middle to

late Eocene, had most of the features of

modern cetaceans, although its nonfunc-

tional pelvis and hindlimb were larger.

(E) A modern toothed whale, the harbor

porpoise (Phocoena phocoena). The

nostrils, forming a blowhole, are far back

on the top of the head, accounting for the

peculiar shape of the skull.

(A skeleton after [107]; B–D skeletons

after [18, 32]; E, skeleton drawing by

Nancy Haver.)

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