This is the second of a series of posts reviewing Michael Denton’s new book Evolution: Still a Theory in Crisis.
“Gaps among known species are sporadic and often small. Gaps among known orders, classes, and phyla are systematic and almost always large.” George Gaylord Simpson, “The History of Life,” in ed. Sol Tax, Evolution After Darwin (Chicago: University of Chicago Press, 1960), 1:149.
“Unfortunately, the origins of most higher categories are shrouded in mystery; commonly new higher categories appear abruptly in the fossil record without evidence of transitional ancestral forms.” D.M. Raup and Steven M. Stanley, Principles of Paleontology (San Francisco: W.H. Freeman and Co., 1971), 306.
“The known fossil record fails to document a single example of phyletic evolution accomplishing a major morphologic transition and hence offers no evidence that the gradualistic model can be valid.” Steven M. Stanley, Macroevolution: Pattern and Process (San Francisco: W. H. Freeman, 1979), 39.
As I noted in part one of this series, Dr. Denton has no doubt that Darwinian evolution occurs. Nor should he. The term “bauplan” comes from the German “building plan” or “building scheme,” and is often translated in biology as “body plan.” In chapter 2 (entitled Galápagos), Denton explains that the biosphere is replete with examples of minor variations of bauplans – such as the beaks of the finches of the Galápagos islands made famous by Darwin – that are doubtless due to purely Darwinian processes. These minor variations of bauplans are often classified under the term “microevolution.”
Before we go any further, let us anticipate an objection. Darwinists who post in these pages often howl in indignation over the term “microevolution.” They say the term is never used by “real” biologists. Perhaps they do not consider George Gaylord Simpson to be a “real” biologist:
“Micro-evolution involves mainly changes within potentially continuous populations, and there is little doubt that its materials are those revealed by genetic experimentation. Macro-evolution involves the rise and divergence of discontinuous groups, and it is still debatable whether it differs in kind or only in degree from microevolution.” George Gaylord Simpson, Tempo and Mode in Evolution (New York: Columbia University Press, 1944), 97
Or Stephen Jay Gould:
“As a Darwinian, I wish to defend Goldschmidt’s postulate that macroevolution is not simply microevolution extrapolated . . .” Stephen Jay Gould, The Return of Hopeful Monsters, Natural History, 86 (June/July 1977), 24, 30
Or Douglas H. Erwin and James W. Valentine:
“Explanations of the Cambrian radiation of invertebrate marine phyla and classes have focused on species selection or traditional microevolutonary processes.” Douglas H. Erwin and James W. Valentine, “‘Hopeful Monsters,’ Transposons, and the Metazoan Radiation,” Proceedings of the National Academy of Sciences, USA 81 (September 1984): 5482-5483
Eminent biologists have recognized and referred to the distinction between microevolution and macroevolution for nearly one hundred years. Let us hope that we can dispense with the “real biologists don’t use the term ‘microevolution’” canard once and for all.
Denton uses the Galápagos finch beaks as an example of microevolution. The finches of the various islands are closely related in terms of nest architecture, egg coloration and DNA. Yet in other respects, including their beak morphology, they can be quite different. Denton quotes Darwin’s correct inference regarding the origin of these differences: “Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.” Darwin was also correct about the causal mechanism that resulted in these changes: “Darwin also inferred (again rightly, as the work of subsequent researchers on Galápagos has amply confirmed) that the major causal mechanism responsible for their adaptive divergence—the shaping of their beaks for example—is the simple mechanism of natural selection.”
Denton concludes that evolution on this scale is caused by purely Darwinian mechanisms:
As far as the evolution of finch beaks is concerned, there is no need either at the morphological or genetic level to call for any causal agency other than cumulative selection. Here I concur with classic Darwinism. The beaks are clearly adaptations and their evolution is entirely explicable within a classic functionalist framework. . . . The lesson of the Galápagos, and one of the repeated mantras of Evolution: A Theory in Crisis (see Chapters 2 and 4) is simply this: Cumulative selection will work its magic as long as the novelty of interest is adaptive and there is a functional continuum (at the morphological or genetic level) leading from a putative ancestor species or structure A to a descendant species or structure B.
(emphasis in the original)
In summary, Denton has absolutely no problem with attributing evolutionary change to Darwinian mechanisms so long as the two conditions Darwin himself asserted are necessary are in place: (1) the novelty is adaptive (otherwise there is nothing for natural selection to “select” for); and (2) there is a functional continuum – there can be no “gaps” in functional intermediates.
This brings us to another important term: “saltation.” In Origin of Species Darwin quoted six times the Latin sentence natura non facit saltus. It means “nature does not take jumps.” The word saltus means “leap” or “jump,” and in evolutionary theory a drastic or sudden change in a line of organisms (a “jump”) is called a “saltation.” Thus, Darwin himself asserted that his theory does not allow for saltations. Stephen Jay Gould added in his 2002 The Structure of Evolutionary Theory that Darwinian evolution requires variation for natural selection to act upon, AND that variation must, by definition, be very small. Otherwise, it would be the variation itself, and not natural selection, that would account for the evolutionary change.
Thus Darwinists, beginning with Darwin himself, have always insisted that the theory absolutely requires the “functional continuum” to which Denton alludes. Saltations are not possible under the theory. Gould goes so far as to say “For this reason . . . saltationist (or macromutational) theories have always been viewed as anti-Darwinian.” Stephen Jay Gould, The Structure of Evolutionary Theory (Cambridge, MA: Belknap Press [Harvard], 2002), 111.
Functionalist evolutionary theory demands a continuous chain of functional intermediates for natural selection to work at all:
Darwin’s interminable series of transitional forms is necessary for straightforward mechanistic reasons (how else can one get from A to B by cumulative selection?), but it is also essential if the sole agency of change is to be natural selection. Where a complex adaptation—no matter how complex—can be reached in a series of tiny adaptive steps, then natural selection can indeed function, in Dawkins’s description, as a blind watchmaker and change A into B no matter how complex the transition, without any other causal agency being involved.
But what if the continuous chain of functional intermediates is missing? Here we get to the nub of the matter, because Denton claims that
many of the taxa-defining homologs actualized during the course of evolution have never been shown to be adaptive and even in the case of those homologs which are apparently adaptive, functional continuums are either unknown or very hard to envisage.
And if this is the case, certain conclusions follow ineluctably:
To acknowledge their absence [i.e., the absence of a continuous chain of functional intermediates] is to acknowledge that the paths of evolution must have been ordered and directed by additional causal factors, i.e., that cumulative selection is not the sole or even the major directive agency.
Denton writes that this “need for adaptive continuums brings us to the nub of the problem, the core contention of Evolution: A Theory in Crisis, and the major point defended here: Practically all the novel, taxa-defining homologs of all the main taxa are not led up to via adaptive continuums. Moreover, as argued later in this book, many of these novel Bauplans do not convey any obvious impression of being adaptive . . .”
Denton concludes the chapter with this delightful observation of fine irony:
It is ironic that the very evidence for believing that microevolution has indeed occurred in cases like the finches—an empirically known or readily envisaged continuum of forms leading from an ancestral form A to descendant form B—is precisely the evidence that is lacking when attempting to account for macroevolution and the origin of the defining features (feathers, hands, mammary glands, hair, the placenta, flowers, body plan, etc.) of the major taxa.