Many paeans to adaptation proceeded beyond mere claims about omnipresence to assert optimalized excellence, or near organic perfection, as well. Convictions about the exclusive power (as well as the range) of natural selection emerge most clearly from such statements, as by Telford and Kennedy (1965, p. 3) (Kennedy later became the president of Stanford University and editor of Science magazine):
It is of profound importance for the nature of the organism that, due to natural selection, the evolutionary changes in organisms have either moved relentlessly in the direction of efficiency or have kept them attuned to a changing environment... Evolutionary adaptation thus suggests an extremely fine attunement between organism and environment. The organism doesn't merely get along; its whole life mode has been tempered and refined by the successful competition of generations of its ancestors with a multitude of differing genotypes. Thus even in the finest details of their organization, organisms are constructed and operate in a manner which makes sense in terms of the way they make their living.
From this assertion of omnipresence for adaptation in morphologies, physiologies and behaviors of the moment, these texts then proceed to ascribe the second great phenomenon of evolution — the production of diversity — to natural selection as well. Simpson et al. (1957, p. 405) extend selection's scope to all phenomena at all scales by writing: “The evolutionary process, viewed in broad perspective, is characterized by two major features: it produces diversity among living things, and it gives rise to their adaptation, their fitness to survive and reproduce efficiently in the environments they inhabit. These two features are interdependent: life's diversity is largely a diversity in adaptation.”
Speciation, although replete with nonadaptive elements in Mayr's canonical formulation, usually receives a textbook description as an even stronger affirmation of natural selection (because the process now operates in two separated lines, working its differential effects to produce just the right adaptations in both distinct and varying environments). Nelson et al. entitling their section “Speciation: The Results of Adaptation,” write in summary (1967, p. 235): “Natural selection operating on the variability present in the genotypes of populations can cause better adaptation of organisms to their environment. Coupled with reproductive isolation, these adaptations bring about speciation.”
Jones and Gaudin (1977, p. 548) introduce their discussion of speciation with a scenario of pure adaptation and extrapolation. (Their full text discusses other mechanisms, including polyploidy — but note the pride of place awarded to adaptation, and the argument that so separate and important a phenomenon as geographic isolation only provides an impetus by setting new selection pressures in a different environment):
The accumulation of adaptations can lead to the production of new species, a process called speciation.... Suppose a population of gophers living [Page 579] in a valley is divided in two by a river that cuts a channel through their valley. The two segments of the population are now effectively separated from one another, and any environmental differences that exist between the two regions of the valley will result in adaptations restricted to one side or the other of the river.... Different selective pressures now will be operating on opposite sides of the river. Given sufficient time, the two gopher populations may diverge quite extensively.
With speciation thus explained as an extended consequence of adaptation under certain environmental circumstances, the same argument can then be smoothly extended to life's full pattern in geological time. Alexander (1962, p. 826) tells students that all phylogeny flows from “the fact of adaptation.” I can hardly imagine a more gradualistic and meliorist account of evolution, with all death for improved existence, and all life in continual motion towards more and better:
We need only accept the fact of adaptation — the idea that organisms are fitted for the particular environments in which they live — to see the necessity for a process of organic evolution. The environment in which organisms live has not been constant . . . Organisms, of course, do not exist under conditions for which they are not adapted. They have, therefore, met these various conditions at different times and places; in order to persist under a changing environment they themselves have had to change. We may think of organic evolution, therefore, as the progressive change of plants and animals in harmony with the changes in their environments. The unadapted die out and disappear. Those organisms whose descendants can fit into the new conditions survive, expand in numbers and kinds, and take over the changing habitat.
Reduction and trivialization of macroevolution
The hypothesis of selection's Allmacht, and adaptation's ubiquity, rests upon the validity of extrapolation to the full range of geological time, for what power (or generality) can a well-formulated theory of local adaptation assert if the same process, by uniformitarian extension, cannot explain the origin of multicellularity, the rise of mammals, and the eventual emergence of human intelligence? Paradoxically perhaps, this extrapolationist assertion becomes, at the same time, the most vulnerable and the most essential of all synthetic propositions — vulnerable in necessary reliance upon a “consistency argument” in the absence of empirical proof, and essential because the theory becomes such a paltry and limited device if its explanatory range cannot extend beyond the compass of its directly observable effects.
No evolutionary assertion has been more commonly advanced in textbooks, or more superficially (and almost nonchalantly) proclaimed by fiat, than the claim that adaptation by natural selection must be fully sufficient to render life's entire history. In the last section, I documented the “promotion” of arguments about pervasiveness of adaptation in local circumstances, to speciation, to the entire tree of life. Capping this sequence, Howells (1959, [Page 580] p. 28) writes, “all this exploring, stopping, and rushing, in the pursuit of profitable adaptation, has resulted in the great family tree of the animals.”
Nelson et al. (1967, p. 239) briefly extol the full sequence — from the rule of selection in local populations, through speciation, to the origin and diversification of phyla: “Evolution in its simplest and broadest sense means changes in gene frequency over a period of time. Natural selection guides these changes . . . Over long periods the accumulation of changes may be sufficient to separate once similar populations into distinct groups. In the course of evolutionary history this divergence has apparently led to different classes (mammals, birds, fish, etc.), different phyla (insects and corals, for example), and even different kingdoms (plants and animals).”
The dominant high school text of the 1960's and 70's depicts the standard equine example of macroevolution as anagenetic gradualism guided by natural selection, thus making any definition of chronospecies arbitrary: “The fossil record shows that all these differences are the result of a series of many gradual changes. Each change that became established through natural selection must have been very slight; only when many such changes accumulated did they result in detectable differences. How can this long sequence of horses be divided into species?” (Biological Sciences Curriculum Study, Green Version, 1973, p. 621). The accompanying figure of the phylogeny of horses depicts the actual (and copiously branching) bush as a smooth ladder of progress (see Fig. 7-3).
Bonner (1962, pp. 52-53), another leading evolutionist who also wrote a popular text, argued that paleontologists can't study the mechanics of evolution directly, but professed complete confidence in the efficacy of microevolutionary selection:
Paleontologists as well as ecologists have been for some years studying the evolutionary factors we have discussed, and have continuously attempted to see how the fossil record, on the one hand, or the present-day distribution of animals and plants, on the other, fit in with this scheme. There seem to be no major discrepancies, and a general feeling that the mechanism of evolution is understood prevails, particularly in regard to the importance of selection and the method of formation of new species... Some groups such as the mol
lusks have been exceedingly slow in their progress while others, such as the mammals, have been very rapid. Again this can be totally understood in terms of selection in particular environments. No other hypothetical mechanisms seem to be necessary to account for the facts, as we know them.
Such confidence in microevolutionary sufficiency can only lead to a downgrading of paleontology — either to theoretical irrelevance, or to a status as a mere repository for results of processes that can only be elucidated by studying modern organisms (and may then be smoothly extrapolated across a million millennia). I do not think that this derogatory judgment originated by the conscious intent of most textbook authors. Rather, the marginalization of paleontology flows directly from the logic of pure extrapolation. The basic
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7-3. Standard textbook misdepiction of a copiously branching evolutionary lineage as a ladder of progress. This canonical view of the evolution of horses appeared in the 1973 Green Version of the most popular and most widely respected high school textbook produced by the Biological Sciences Cvrriculum Study.
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argument takes two forms. Some authors explicitly exclude paleontology from the theoretical game:
Evolution can be studied on the population level only with living organisms. The fossil record provides too few data to allow such treatment; it merely allows paleontologists to reconstruct the history of animal and plant groups. The population approach makes it possible to ask such questions as: what is the rate of evolution in a given species? What factors influence the course or rate of evolution? What conditions are necessary for evolution to begin or cease? (Baker and Allen, 1968, p. 524). [I do not see why paleontologists cannot address all three of these questions with data from the morphology of fossils and their temporal distribution.]
But I must confess that a stronger and more focused form of this argument has long evoked my deeper distress, and has served, in substantial measure, as the impetus for personal career choices in research, and for my eventual decision to write this book. I refer to the claim, repeated almost as a catechism, and obviously copied from textbook to textbook, that macroevolution poses no problem not resolvable by a further understanding of allelic substitutions directed by natural selection in contemporary populations. We may move smoothly from one gene to an entire Bauplan, and extrapolate upwards from a few generations to a geological era. No additional problems arise in temporal vastness. Macroevolution becomes little more than industrial melanism writ large. But can we even imagine, in a world dominated by effects of scale, that such a maximal extension of form and time will engage not a single force or principle beyond the factors fully in evidence at the lowest level? Can the smallest scales really provide an entirely sufficient model for the largest? Can a uniformitarianism this rigid truly be sustained? If so, then paleontology only represents a playground for the full display of microevolutionary muscle — and textbooks need not consider the fossil record as more than an archive of the pathways carved by this power.
Most standard textbooks make this confident assertion based on little beyond hope and tradition — thus making macroevolution a nonsubject. Bonner (1962, p. 48), for example, writes: “There is no reason to believe that these large changes are not the result of the very same mechanics of the small changes of industrial melanism. One involves a small step over a few years; the other involves many thousands of steps over millions of years.”
Curtis (1962, p. 712), in a best-selling text of the 60's, begins her short section on macroevolution by stating: “Can the same processes that slowly shape the seed of mustard weed or change the color of the peppered moth create the differences between elephants and daisies or between butterflies and redwood trees? Darwin believed so — all he felt that was needed was time, millions of years of slow change. Today, almost all evolutionists are, in principle, in general agreement with Darwin's conclusions.”
Several texts even present this canonical argument as their only statement about macroevolution. I end this chapter by quoting two striking examples of [Page 583] this trivialization and marginalization of macroevolution, each from the most important source in its respective genre. As mentioned above, BSCS textbooks (written by a semiofficial consortium of private and governmental sources, The Biological Sciences Curriculum Study) virtually cornered high-school markets during post-Sputnik years of the 1960's and 1970's. The 1968 version of Biological Science: An Inquiry Into Life includes a heading on “The Origin of Genera and Larger Groups.” But the text contains only two paragraphs, fully reproduced below:
The final question which we must ask about the forces of evolution is this: can mutation, recombination, selection, and barriers to cross-breeding explain the major trends of evolution, such as the divergence of catlike from doglike animals and the evolution of the horse from its small primitive ancestors?
The mechanisms that govern these major trends of evolution cannot be studied directly: they took place many thousands or millions of years ago. Nevertheless, a study of populations today, and of fossils, provides strong evidence that the same evolutionary forces in operation today have guided evolution in the past. One species evolves into two (or more). All the new species continue to evolve, becoming more different from one another until eventually we would classify them as different genera (1968, p. 203).
Life on Earth (1973) surely ranks as the most distinguished textbook of introductory college biology published during the 1970's. Written by a team of eight authors, and headed by two of the world's leading evolutionists (E. O. Wilson and T. Eisner), this book staked an explicit claim for groundbreaking novelty by linking appropriate expertise at the highest level with accessibility in style, and excellence in design and illustration. Chapter 28 on “The Process of Evolution” ends with the heading “Macroevolution.” The quotation below may seem limited in content, particularly for a college text, but I do not cite an excerpt. I have reproduced the book's entire section on macroevolution!
In this passage, the history of life becomes a simple extension of the story of the raspberry eye-color gene. (For the second edition, the authors switched to the standard case of industrial melanism, but did not alter the general argument at all.) Paleontologists may be burdened with an incomplete record, the authors assert, but as they look more carefully, the gap between the raspberry gene and the Cambrian explosion closes continually. I can only express my astonishment at such a limited, but definitive, assertion by applying Ethel Barrymore's famous closing line to this dismissal of macroevolution as a subject: “That's all there is, there isn't any more.”
Each of the examples of microevolution examined, involving shifts in the frequencies of small numbers of genes, could be multiplied a hundredfold from reports in the scientific literature. Biologists have been privileged to witness the beginnings of evolutionary change in many kinds [Page 584] of plants and animals and under a variety of situations, and they have used this opportunity to test the assumptions of population genetics that form the foundations of modern evolutionary theory. The question that should be asked before we proceed to new ideas is whether more extensive evolutionary change, macroevolution, can be explained as an outcome of these microevolutionary shifts. Did birds really arise from reptiles by an accumulation of gene substitutions of the kind illustrated by the raspberry eye-color gene?
The answer is that it is entirely plausible, and no one has come up with a better explanation consistent with the known biological facts. One must keep in mind the enormous difference in time scale between the observed cases of microevolution and macroevolution. Under natural conditions the nearly complete substitution of the melanic gene of the peppered moth took 50 years. Evolution of the magnitude of the origin of the birds usually, perhaps invariably, takes many millions of years. As paleontologists explore the fossil record with increasing care, transitions are being documented between increasing numbers of species, genera, and higher taxonomic groups. The reading f
rom these fossil archives suggests that macroevolution is indeed gradual, paced at a rate that leads to the conclusion that it is based upon hundreds or thousands of gene substitutions no different in kind from the ones examined in our case histories (1973, p. 792).
But, pace Ms. Barrymore, there is so much more — as research in the vibrant field of macroevolution, filling the pages of numerous journals (all founded after these dismissive comments), attests; as the development of a tight and powerful theory of hierarchical selection embodies (see Chapters 8 and 9); as the union of developmental with evolutionary biology displays (see Chapters 10 and 11); as our advancing understanding of genomic complexity asserts. Can we not feel the frustration of E. C. Olson as he queried the titans of the Modern Synthesis in Chicago? Can we not understand why a few iconoclasts never made their peace with such a comfortable and limiting orthodoxy? Can we not gain a visceral (and not only an intellectual) sense of C. H. Waddington's isolation and irritation when he made his famous comment on the limitations of population genetics (Waddington, 1967), and won admiration for his panache but no consideration for his content: “The whole real guts of evolution — which is, how do you come to have horses and tigers, and things — is outside the mathematical theory.”
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The Structure of Evolutionary Theory Page 93
