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Miller’s Biology Textbook on Gradualism vs. Punctuated Equilibrium and the Scope of the Cambrian Explosion

biologycover09_155.jpgAdopted for use in Texas, Ken Miller’s Pearson Biology textbook states that punctuated equilibrium (“punc eq”) only occurs as a model of evolutionary change “now and then.” (p. 549) In this regard, the textbook significantly understates the frequency of “stasis followed by sudden appearance,” the punc eq mode of change. Here is another major problem with the book, which I’ve been reviewing critically. See here, here, here, and here.

As Stephen Jay Gould and Niles Eldredge observed in Nature:

[T]here are a growing number of reports documenting an overwhelming relative frequency (often an exclusivity) for punctuated equilibrium in entire groups or faunas. Consider the lifetime testimonies of taxonomic experts on microfossils, on brachiopods and on beetles. Fortey has concluded for trilobites and graptolites “that the gradualistic mode does occur especially in pelagic or planktic forms, but accounts for 10% or less of observations of phyletic change, and is relatively slow”.

Other studies access all available lineages in entire faunas and assert the dominance of punctuated equilibrium. Stanley and Yang found no gradualism at all in the classic Tertiary molluscan sequences of the Gulf and Atlantic Coasts. With the exception of Gryphaea, Hallam detected no phyletic change in shape (but only for body size) in any Jurassic bivalve in Europe. Kelley documented the prevalence of punctuation for molluscs in the famous Maryland Miocene sequence, and Vrba has done the same for African bovids. Even compilations from the literature, so strongly biased by previous traditions for ignoring stasis as non-data and only documenting putative gradualism, grant a majority to punctuated equilibrium, as in Barnovsky’s compendium for Quaternary mammals, with punctuated equilibrium “supported twice as often as phyletic gradualism … the majority of species considered exhibit most of their morphological change near a speciation event, and most species seem to be discrete entities”. When controlled studies are done by one team in the field, punctuated equilibrium almost always seems to predominate. Prothero “examined all the mammals with a reasonably complete record from the Eocene-Oligocene beds of the Big Badlands of South Dakota and related areas in Wyoming and Nebraska … With one exception (gradual dwarfing in the oreodont Miniochoerus), we found that all of the Badlands mammals were static through millions of years, or speciated abruptly (if they changed at all).”1

Significantly, many sources note that “major groups” of organisms tend to appear abruptly, in a punctuated mode:

We are still in the dark about the origin of most major groups of organisms. They appear in the fossil record as Athena did from the head of Zeus — full-blown and raring to go, in contradiction to Darwin’s depiction of evolution as resulting from the gradual accumulation of countless infinitesimally minute variations.2

Many species remain virtually unchanged for millions of years, then suddenly disappear to be replaced by a quite different, but related, form. Moreover, most major groups of animals appear abruptly in the fossil record, fully formed, and with no fossils yet discovered that form a transition from their parent group.3

Even Stephen Jay Gould acknowledged that “transitions between major groups are characteristically abrupt.”4 The high frequency of this pattern of abrupt appearance — especially for “major groups” — is a major omission by Pearson that should have been corrected.

In response to the reviewer, Pearson cited the classic example of supposed gradual evolution: the horse series. Pearson admits this series shows both punctuated and gradual change, but Gould and Eldredge note that horse evolution really fits under a predominantly punc eq model:

Prothero and Shubin have shown that the most ‘firmly’ gradual is tic part of the horse lineage (the general, and false, exemplar of gradualism in its totality), the Oligocene transition from Mesohippus to Miohippus, conforms to punctuated equilibrium, with stasis in all species of both lines, transition by rapid branching rather than phyletic transformation, and stratigraphic overlap of both genera (one set of beds in Wyoming has yielded three species of Mesohippus and two of Miohippus, all contemporaries). Prothero and Shubin conclude: “This is contrary to the widely-held myth about horse species as gradualistically-varying parts of a continuum, with no real distinctions between species. Throughout the history of horses, the species are well-marked and static over millions of years. At high resolution, the gradualistic picture of horse evolution becomes a complex bush of overlapping, closely related species.”5

Thus, a very strong case can be made that the Pearson textbook seriously understates the frequency of punctuated equilibrium (i.e., stasis followed by abrupt appearance of new forms, especially major groups). Though it does mention the “punc eq” model, it should not state that abrupt appearance only happens “now and then.”

Another area where Pearson misrepresents the nature of the fossil record is on the scope of the Cambrian explosion. Specifically, Pearson fails to acknowledge that virtually all the major animal phyla appeared in the early Cambrian. In response to the reviewer, Pearson cites Budd and Jenson’s 2000 paper which argues that “very few bilaterian phyla sensu stricto have demonstrable representatives in the earliest Cambrian.” But Douglas Erwin and James Valentine’s more recent 2013 book, The Cambrian Explosion, explains that “all or nearly all the major phylum-level groups of living animals … had appeared by the end of the early Cambrian”:

Because many of those fossils represent complex groups such as vertebrates (the subgroup of the phylum Chordata to which humans belong) and arthropods, it seems likely that all or nearly all the major phylum-level groups of living animals, including many small softbodied groups that we do not actually find as fossils, had appeared by the end of the early Cambrian. This geologically abrupt and spectacular record of early animal life is called the Cambrian explosion.6

Likewise, a college-level biology textbook, The Invertebrates: A New Synthesis, states:

Most of the animal phyla that are represented in the fossil record first appear, “fully formed” and identifiable as to their phylum, in the Cambrian some 550 million years ago. . . . The fossil record is therefore of no help with respect to understanding the origin and early diversification of the various animal phyla.7

As Erwin and Valentine’s book shows, the Pearson textbook omits the most important fact about the scope of the explosion of diversity in the Cambrian. For the sake of accuracy, the statement should be changed to reflect the text I have indicated in bold above. Pearson’s treatment of this issue is misleading and should have been corrected.

References Cited:
[1.] Stephen Jay Gould and Niles Eldredge, “Punctuated equilibrium comes of age,” Nature, Vol. 366: 223-227 (November 18, 1993).
[2.] Jeffrey Schwartz, Sudden Origins: Fossils, Genes, and the Emergence of Species, p. 3 (Wiley, 1999).
[3.] C.P. Hickman, L.S. Roberts, and F.M. Hickman, Integrated Principles of Zoology, p. 866 (Times Mirror/Moseby College Publishing, 1988, 8th ed).
[4.] Stephen Jay Gould “The Return of Hopeful Monsters,” Natural History, Vol. 86: 22-24 (June-July, 1977).
[5.] Stephen Jay Gould and Niles Eldredge, “Punctuated equilibrium comes of age,” Nature, Vol. 366: 223-227 (November 18, 1993).
[6.] Douglas Erwin and James Valentine, The Cambrian Explosion, p. 5 (Roberts & Co., 2013) (emphasis added).
[7.] R.S.K. Barnes, P. Calow and P.J.W. Olive, The Invertebrates: A New Synthesis, pp. 9-10 (3rd ed., Blackwell Sci. Publications, 2001).

Casey Luskin

Associate Director, Center for Science and Culture
Casey Luskin is a geologist and an attorney with graduate degrees in science and law, giving him expertise in both the scientific and legal dimensions of the debate over evolution. He earned his PhD in Geology from the University of Johannesburg, and BS and MS degrees in Earth Sciences from the University of California, San Diego, where he studied evolution extensively at both the graduate and undergraduate levels. His law degree is from the University of San Diego, where he focused his studies on First Amendment law, education law, and environmental law.



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