But does the theory identify such a mechanism?
From Whence New Traits and Form?
Neither allopatric speciation nor species selection can generate the new genetic and anatomical traits necessary to produce animal forms, let alone in the relatively brief time of the Cambrian explosion. As conceived by Gould and the other advocates of punctuated equilibrium, allopatric speciation just allows for the possibility of the rapid fixation of preexisting traits, not the generation of new traits. When a parent population splits into two or more daughter populations, each of the daughter populations retains a part, but usually not the whole, of the gene pool of the original population. No new genetic traits are generated by the geographical isolation of one part of a population from another.
It could be argued, of course, that mutations might occur during the process of speciation, thus generating new genetic traits. But as Gould and Eldredge conceived of it, allopatric speciation occurs much too rapidly to have a reasonable chance of mutations generating anything fundamentally new. Darwin recognized in On the Origin of Species that evolution is a numbers game: larger population sizes and more generations offer more opportunities for favorable new variations to arise. As he explained: “Forms existing in larger numbers will always have a better chance … of presenting further favourable variations for natural selection to seize on, than will the rarer forms which exist in lesser numbers.”28 Yet for the mechanism of allopatric speciation to generate new traits, it would need to generate significant changes in form in small “peripherally isolated” populations over relatively few generations.29 Because of these constraints, many biologists have concluded that allopatric speciation requires too much change too quickly to provide the theory of punctuated equilibrium with a biologically plausible mechanism for producing new traits or forms of animal life.
And that is why Gould and Eldredge, especially in their later formulations of the theory, envisioned new traits arising during long periods of stasis in larger populations rather than during short bursts of speciation. But a process in which traits arise “during long periods of stasis” does not constitute a “mechanism of unusual speed and flexibility,” though that is precisely what, according to Gould and Foote, punctuated equilibrium requires in order to explain the abrupt appearance of new animal forms.
If allopatric speciation does not produce a fast-acting trait-generating mechanism, does species selection? Again, the answer is no. Species selection does not account for the origin of the different anatomical traits that distinguish one species from another. Species selection, as conceived by the proponents of punctuated equilibrium, acts on species and traits that already exist. Indeed, when Stanley, Gould, and Eldredge envisioned natural selection acting to favor the most fit species over another in a competition for survival, they presupposed the existence of a pool of different species and, therefore, also the existence of some mechanism for producing the traits that characterize those different species. That mechanism, however, would necessarily need to generate those differentiating traits before species could enter into competition with each other. Species selection eliminates less fit species in a competition for survival; it does not generate the traits that distinguish species and establish the basis for interspecies competition.
So where do these traits come from? When pressed, Gould eventually acknowledged that the origin of anatomical traits themselves result from good, old-fashioned natural selection acting on random mutations and variations—that is, from the neo-Darwinian mechanism acting over long periods of time on large relatively stable populations. But that meant that punctuated equilibrium, to the extent it relies on mutation and natural selection, is subject to the same evidential and theoretical problems as neo-Darwinism. And one of those problems is that the neo-Darwinian mechanism does not act quickly enough to account for the explosive appearance of new fossil forms in the Cambrian period. Like allopatric speciation, species selection does not qualify as the kind of rapid and flexible mechanism that Gould elsewhere insisted his theory must have in order to explain the abrupt appearance of animal forms in the fossil record.
Novel Form and Mechanism
An even more profound difficulty with punctuated equilibrium as an explanation for the Cambrian explosion remains. Neither species selection nor allopatric speciation explains the origin of the representatives of the higher taxonomic categories—that is, the new animals representing new phyla and classes. Nor does it explain the structural and morphological features that distinguish animals from one another and earlier forms of life. Allopatric speciation explains how populations get separated from each other to form different species. Species selection describes how more fit species predominate over other species in a competition for survival. Neither mechanism gives any account of how the animals representing the specifically higher taxa or their distinctive anatomical novelties arose. Neither mechanism accounts, for example, for the origin of the compound eye of a trilobite, nor the gills of a Cambrian fish,30 nor the echinoderm body plan.
Many critics of punctuated equilibrium have noted this problem. As Richard Dawkins wrote in 1986: “What I mainly want a theory of evolution to do is explain complex, well-designed mechanisms like hearts, hands, eyes and echolocation. Nobody, not even the most ardent species selectionist, thinks that species selection can do this.”31 Or as paleontologist Jeffrey Levinton argued in 1988, “It is inconceivable how selection among species can produce the evolution of detailed morphological structures… . Species selection did not form an eye.”32
So where do these intricate structures come from? Again, when pressed, Gould resorted to the alleged power of the neo-Darwinian mechanism. As he wrote in his magisterial tome The Structure of Evolutionary Theory, published in 2002, the year of his death: “I do not deny either the wonder, or the powerful importance, of organized adaptive complexity.” He went on to concede, “I recognize that we know no mechanism for the origin of such organismal features other than conventional natural selection at the organismic level.”33
For this reason, few if any evolutionary biologists now regard punctuated equilibrium as a solution to the problem of the origin of biological form and novelty. As the evolutionary biologists Brian Charlesworth, Russell Lande, and Montgomery Slatkin have concluded, “genetic mechanisms that have been proposed [by proponents of punctuated equilibrium] to explain the abrupt appearance and prolonged stasis of many species are conspicuously lacking in empirical support.”34
Burst of Interest and Gradual Decline
Still, it may not be entirely fair to criticize punctuated equilibrium for failing to account for the Cambrian explosion. Gould, in particular, equivocated about whether he meant punctuated equilibrium to serve as a comprehensive theory of macroevolutionary change or just an account of how new species emerged from a pool of preexisting species. Strictly speaking, the mechanisms of allopatric speciation and species selection sought to explain the pattern of stasis and discontinuity among different species and not among the higher taxa. Thus, near the end of his career, Gould complained about his critics “misunderstanding” his theory by asserting that he “proclaimed the total overthrow of Darwinism” and “intended punctuated equilibrium as both the agent of destruction and the replacement.”35
Yet Gould and Eldredge, at least initially, advanced punctuated equilibrium as a bold new theory of evolutionary biology, giving the impression that it provided an ambitious solution to the problem of macroevolution and, by implication, events such as the Cambrian explosion. From 1972 to 1980, Eldredge and Gould presented a series of provocative scientific papers that portrayed punctuated equilibrium as a bold, and even revolutionary, alternative theory of macroevolution. Indeed, Gould himself referred to it explicitly as “a speciational theory of macroevolution.”36
In their second main paper, published in 1977, Gould and Eldredge made explicit their intention to position their theory as a “radical”37 challenge to neo-Darwinian gradualism and to replace it with a completely different under
standing of the mode and mechanism of evolutionary change. Sepkoski notes that in this 1977 article “the authors were more explicit about the exact nature of the conceptual reconfiguration their theory brought to macroevolution.”38 In particular, he argues that Gould and Eldredge “extended their model to propose a new and ‘general philosophy of change’ in the natural world.”39 Gould was no less radical in a widely cited 1980 paper in the journal Paleobiology in which he offered punctuated equilibrium as “a new and general theory” of evolution. There he also famously declared the synthetic theory of neo-Darwinism “effectively dead, despite its persistence as textbook orthodoxy.”40
Only after critics exposed punctuated equilibrium for lacking an adequate mechanism did Gould retreat to a more conservative formulation of the theory, making its reliance upon the neo-Darwinian mechanism explicit. From the early 1980s until his death in 2002, Gould made a series of concessions in particular about the inadequacy of speciation and species selection as mechanisms for generating complex adaptations. Thus, as Sepkoski notes, “Despite the brashness of many of his claims on behalf of punctuated equilibrium over the years, one is brought time and again back to the reconciliatory, even conservative justifications Gould made for his theory,” particularly, he notes, in The Structure of Evolutionary Theory, written in the years just before Gould’s death.41
In the end, Gould’s concessions to neo-Darwinism brought his thinking back into conflict with the pattern of sudden appearance in the fossil record that the theory of punctuated equilibrium was designed to explain. If Gould and Eldredge were right about the abrupt appearance of new forms of life in the fossil record, and if the neo-Darwinian mechanism needs as much time as evolutionary biologists and population geneticists (see Chapters 8 and 12) calculate, then the mutation and selection mechanism does not have enough time to produce the new traits needed to build the forms of life that first appear in the Cambrian period. But punctuated equilibrium, as initially formulated to rely mainly on allopatric speciation and species selection, fared no better, since neither mechanism gives any explanation for the origin of new traits. And so, in the end, punctuated equilibrium highlighted rather than resolved a profound dilemma for evolutionary theory: neo-Darwinism allegedly has a mechanism capable of producing new genetic traits, but it appears to produce them too slowly to account for the abrupt appearance of new form in the fossil record; punctuated equilibrium attempts to address the pattern in the fossil record, but fails to provide a mechanism that can produce new traits whether abruptly or otherwise. No wonder, then, that leading Cambrian paleontologists such as James Valentine and Douglas Erwin concluded in 1987, that “neither of the contending theories of evolutionary change at the species level, phyletic gradualism or punctuated equilibrium, seem applicable to [explaining] the origin of new body plans.”42
Spinning in Circles
In a sudden flash of insight in the humdrum of a Laundromat, Niles Eldredge realized that stasis in the fossil record represented evidence rather than mere investigative failure. But like laundry spinning around in a washing machine, the theory of punctuated equilibrium itself became caught in a dreary cycle of contradiction. On the one hand, “punk eek” made a bold attempt to describe more accurately, and even explain, the decidedly discontinuous pattern of the fossil record. On the other, its advocates were forced to concede both the inadequacy of their proposed mechanisms and their need to rely upon the neo-Darwinian process of mutation and selection in order to account for the origin of new genetic traits and anatomical innovations. After Gould appeared to jettison both gradualism and a reliance on the neo-Darwinian mechanism in order to bring evolutionary theory into conformity with the fossil record, he eventually acknowledged that he could not explain the origin of the forms of life documented in the fossil record apart from that same slow and gradually acting mechanism. Thus, though the theory of punctuated equilibrium was initially presented as a solution to the mysterious and sudden origin of animal forms, upon closer inspection, it failed to offer such a solution.
Nevertheless, the failure of punctuated equilibrium to provide a sufficient mechanism has raised questions about the adequacy of the mechanism that Gould ultimately did reaffirm as the explanation for the origin of novel biological form. Can the neo-Darwinian mechanism of natural selection acting on random mutations build new forms of animal life with all their complex adaptations? If so, is it possible that it could do so in the brief time allowed by the fossil record? If not, is it reasonable to think that it could build new forms of animal life if only more time were available? If so, how much time would the Darwinian mechanism need to build complex adaptations and new forms of animal life? In the next several chapters, I will address these fundamental questions at the heart of the Cambrian mystery—questions, in brief, about how to build an animal.
Part Two
How to Build an Animal
8
The Cambrian Information Explosion
When I was a college professor, I used to ask my students a question: “If you want your computer to acquire a new function or capability, what do you have to give it?” Typically, I would hear a smattering of similar answers from the class: “code,” “instructions,” “software,” “information.” Of course, all these are correct. And thanks to discoveries in modern biology, we now know that something similar is true of life: to build a new form of life from a simpler preexisting form requires new information.
To this point I’ve examined one main aspect of the mystery surrounding the Cambrian explosion: the mystery of the missing Precambrian ancestral forms expected on the basis of Darwin’s theory. The next group of chapters will examine a second, and perhaps more profound, aspect of the Cambrian mystery: that of the cause of the Cambrian explosion. By what means or process or mechanism could something as complex as a trilobite have arisen? Could natural selection have accomplished such a feat? To answer this question we will have to look more closely at what it takes to build a new form of animal life. And we’ll see that an important part of the answer to that question will have something to do with the concept of information.
The Darwinian Account of the Origin of Animal Form
As Darwin envisioned the process, natural selection can accomplish nothing without a steady supply of variation as a source of new biological traits, forms, and structures. Only after useful new variations arise can natural selection sift them from the chaff of unhelpful variations. If, however, the amount of variation available to natural selection is limited, then natural selection will encounter limits on how much new biological form and structure it can build.
Even in the late nineteenth century, many leading scientists recognized this. For this reason, there has been a long history of scientific controversy about just how much novelty natural selection can produce and about whether natural selection is a truly creative process. In fact, between 1870 and 1920 classical Darwinism entered a period of eclipse, because many scientists thought that it could not explain the origin and transmission of new heritable variation.1
Darwin favored a theory of blended inheritance that seemed to imply limitations on the amount of genetic variability.2 He thought that when parents with different traits combined germ cells during sexual reproduction, the resulting offspring would receive not one or the other set of their differing traits, but instead a compromise version. For example, if a male bird with red feathers on its wings mated with a female bird of the same species with white wing feathers, the theory implied that the two would likely produce offspring with pink wing feathers. As many of Darwin’s contemporaries pointed out, such instances of blending inheritance involved strict limitations on the range of traits that could possibly arise, depriving natural selection of the wide-ranging supply of variation it would need to produce truly fundamental changes in the form of animals. The pink-feathered offspring might later reproduce with a white-or red-feathered bird of the same species, producing a slightly lighter or darker shade of pink feathers. Nevertheless, descendants of the or
iginal white-and red-feathered pair would never produce green, blue, or yellow feathers in subsequent generations. If correct, blending inheritance would eventually lead to a bland, homogenous, variationless state in a population.
In the 1860s, the Austrian monk Gregor Mendel, widely regarded as the founder of modern genetics, showed in his work on garden peas that Darwin’s assumptions about blending inheritance were incorrect. The results of his studies created, at least initially, more problems for Darwinism. Mendel showed that the genetic traits of organisms typically have an integrity that resists blending. He showed this by cross-pollinating plants with yellow peas and plants with green peas. The plants in the subsequent generations produced either yellow or green peas, but nothing in between and nothing with an altogether different color.3
He also showed that the plants carried some kind of signal or instructions for building different traits even when the trait was not on display in a particular plant. He noticed, for example, that when he crossed pea plants with green and yellow seeds, the next generation had only yellow seeds, almost as if the capacity for generating green peas had been lost. But when he cross-pollinated the second-generation plants, the ones with only yellow peas, he found that both yellow and green peas emerged in the third generation, in a ratio of 3 to 1. From this Mendel hypothesized that the second-generation plants continued to carry signals, which he called “factors,” and later scientists called genes, for generating green peas even when those plants themselves did not display that trait.
Darwin's Doubt Page 17