by Kenneth W. Krause.
Kenneth W. Krause is a contributing editor and “Science Watch” columnist for the Skeptical Inquirer. Formerly a contributing editor and books columnist for the Humanist, Kenneth contributes regularly to Skeptic as well. He may be contacted at email@example.com.
All too often lost among the mind-numbing, carnivalesque creationist-vs.-evolutionist sideshows currently haunting the pop media’s pseudo-intellectual backwaters are the genuinely scientific debates over evolution’s varied mechanisms and alleged limitations. What is the fundamental unit of evolutionary change? Is the process dominated by natural selection, developmental constraint, or chance? To what extent can selection explain and predict cultural and psychological phenomena? No questions could be more relevant to, among other things, a competent understanding of humanity—who we are and what we can reasonably expect from one another. And these are precisely the questions presented and debated among science’s most prolific authorities in philosopher Kim Sterelny’s updated edition of Dawkins vs. Gould.
Oxford zoologist, Richard Dawkins, and recently deceased Harvard paleontologist, Stephen Jay Gould, of course, agreed more than disagreed on almost every aspect of evolution. Dawkins, for example, never proposed that the effects of natural selection were limited to gene frequencies. Nor has he denied the essential role of organisms. Instead, Dawkins pointed out that because adaptations result only from extended series of small changes, the basic agents of evolution must persist over extended periods of time. At the organismal level, however, sexual reproduction is not the same as duplication. Only the gene (defined in The Selfish Gene as “any portion of chromosomal material that potentially lasts for enough generations to serve as a unit of natural selection”) is significant enough to cause change, on the one hand, and small enough to escape DNA recombination, on the other. Only gene lineages, in other words, possess histories deep enough to serve as the fundamental units of natural selection. To the organism, Dawkins relegated the substantial but secondary role of “survival machine” for genes or, more accurately, gene alliances.
Gould’s view was more traditional in Darwinian terms and perhaps more intuitive. Although he acknowledged the fact of selection at the genetic level, for him such was the exception to the general rule that evolution mostly acts upon organisms in local populations, or even on groups of organisms including species and clades (founding species and all their descendents). Genes more record the process than cause it, Gould argued, and their survival relies not simply on alliances with other genes, but more profoundly on various features of and modifications to the organism’s larger environment.
Dawkins, however, has alluded to at least two general phenomena demonstrating the considerable weakness of any organism-centered view of evolution. “Outlaw genes,” for example, rather than allying with other genes, appear to compete against them—potentially at the expense of the host organism’s fitness. Consider mitochondria, for example, which are inherited only maternally. Certain plants that are normally capable of producing both pollen and seed will occasionally develop “sex ratio distorter genes” causing organisms to produce only mitochondria carrying seed. Any mutation to such genes rendering daughter-making more likely would be favored, everything else being equal. Similarly, so-called “segregation distorter genes” will chemically sabotage the allele with which they are paired on the chromosome, even if it results in a less fit—in this case sterile—organism.
Other genes promote themselves by reaching beyond the organism’s physical margins. Recently, in The God Delusion, Dawkins reiterated how this works from the perspective of a living extended phenotype. “An animal’s behavior” he writes, “tends to maximize the survival of genes for that behavior whether or not those genes happen to be in the body of the particular animal performing it.” The classic example involves a parasitic brainworm that burrows into the head of an ant, causing it to climb toward the most exposed portions of leaves and blades of grass until a cow, the worm’s ultimate host, consumes it. Although the relevant genes reside in the parasite, their adaptive effect takes place in its host. Instead of adapting the organism to the environment, in broader terms, extended phenotype genes adapt the environment to the organism. As such, neither outlaws nor extended phenotypes can be comfortably fused into an organism-centered view of evolution, and for Sterelny, Dawkins’ reductive selfish gene theory of natural selection is therefore the more sustainable one.
But Gould, of course, de-emphasized the consequence of natural selection in the first place. For example, although both he and Dawkins recognized the supply of variation as a constraint on the evolutionary trajectory of any given population, Gould conceived of developmental biology as a brake on creativity or as an entrenchment of evolutionary patterns and features. Consider, for instance, Wallace Arthur’s study of two centipede families. Each of the 1000 or so species of Geophilomorphians display an odd number of body segments (every one of which has a pair of legs), though that number ranges outrageously from 29 to 191. A similar diversity of Lithobiomorphian species all have fifteen segments. Why no even numbers? Selective pressure is an unlikely answer, given the extreme range of leg pairs among Geophilomorphians.
In his Biased Embryos and Evolution, Arthur concluded that nature is incapable of selecting a Lithobiomorphian with, say, thirty segments because these creatures’ developmental mechanisms impose an absolute constraint against it. He pointed out as well that more relative biases in the variation reserve are sufficient to the same task. Sterelny offers a hypothetical, initially isolated population of ringtail possums the average weight of which is two kilograms. Comparatively sized predatory cats are introduced. One-kilo possums can hide; three-kilo possums can successfully defend themselves. All other factors being equal, which will be selected? The answer, Sterelny reasons, will depend not on external dynamics, but rather on the current possum population’s developmental bias.
Consider too one of Dawkins’ own examples in Climbing Mount Improbable. In general, shells vary in only one of three ways: their “flare” (the rates at which they uncoil in one plane), their “spire” (the rate at which they rise above that plane), and their “verm” (the rate at which their tube expands). Such dimensions allowed David M. Raup to represent the range of all possible shell variations as a cube (dubbed “Raup’s cube”), much of which remained unoccupied by a shaded area indicating existing shell shapes. Why so many missing variants? Have existing shells inherited an insufficient capacity to vary, or has selection determined the outcome? We simply don’t know yet, according to Sterelny. But Dawkins has emphasized the revolutionary rather than the conservative character of developmental biology. In Climbing Mount Improbable, he argued that, for the relevant lineages, the momentous invention of segmentation, for example, opened up an expansive new world of adaptive possibilities. Indeed, segmented arthropods, including spiders, crabs, and insects, are by far the most diverse group of animals on the planet.
While Sterelny admits that “[t]he integration of evolution and development is the hottest of hot topics in contemporary evolutionary theory,” his “best current guess” is that developmental biology does indeed produce significant biases in the variation available for natural selection. In support of his position, he points out that gene mutations involve movements, duplications, inversions, and deletions in addition to limited point mutations. These “middle-size” mutations cause changes to gene regulation, shifts in reading frames, and thus substantial alterations to traits that might well introduce variation supply biases. Touting his Ontogeny and Phylogeny as a “foundation of modern evolutionary developmental biology,” Sterelny predicts that Gould’s position in this regard “will probably be vindicated.”
So too does the author prefer Gould’s accent on the caprice of mass extinctions along with its implied, attendant claim that such calamities are qualitatively distinct from run-of-the-mill background extinctions. Recently, some have suggested that certain so-called mass events had actually resulted from routine climate change, competition, or merely localized bad luck. Much debate in particular has centered on the Cretaceous/Tertiary boundary 65 million years ago. Skeptics of the meteor impact’s significance wonder why crocodiles, turtles, and frogs survived while dinosaurs (excepting birds), pterosaurs, pliosaurs, and other marine reptiles did not, arguing that some such creatures had dwindled or even gone extinct prior to the impact.
Ever the committed fossil hunter, however, Gould highlighted the 570 million year old remains of the frond- and disc-shaped Ediacaran fauna (perhaps more akin to lichens than animals, according to others), their abrupt disappearance following the start of the Cambrian Period 543 million years ago, and the ensuing “explosion” of fossil evidence documenting the emergence of today’s major lineages. Similarly, Sterelny underscores the catastrophe that devastated 90 percent of animal species at the close of the Permian Period 251 million years ago. Again, although he certainly acknowledges the fact of mass extinction, Dawkins has demoted these events to simple interruptions in the omnipresent process of selection. For Gould, by contrast, such chance calamities were paramount, dealing the death card to some groups and a royal flush to others.
But the most incisive fissure between these two men opened over the relentlessly messy subject of human behavior. Gould savagely opposed sociobiology and, perhaps to a lesser extent, its descendant, evolutionary psychology, insofar as they presumed selective underpinnings. On the one hand, Randy Thornhill’s hypothesis that, under certain circumstances, sexually isolated men can improve their reproductive fitness through rape is easily criticized for its failure to consider, among other things, the obvious social costs of violence. On the other hand, Gould found adaptive theories of differential parental investment more promising. Some emotional propensities common to men and women, he suggested, might well be adaptive, given that males find advantage in spreading their sperm as widely as possible and that females are most successful when able to extract additional time, investment, and attention from males. Even so, Gould warned, the range of behaviors that can be explained in such Darwinian terms was severely limited.
Although Dawkins employed certain tools distinct from those standard to most sociobiologists and evolutionary psychologists—the meme in particular—he has remained steadfast in his commitment to the evolutionary paradigm as the optimal strategy for understanding human behavior—an important observation that inevitably leads us to a more heady and comprehensive evaluation of this classic intellectual clash. In some ways, Gould seemed to fear what he imagined to be science’s potentially injurious stabs into the soft, sensitive underbelly of human existence. Dawkins, by contrast, seems to fear almost nothing. Indeed, he manages to maintain a contagious optimism over the tender topics of sociality and morality precisely because he recognizes, first, that science is far and away the best if not the only honest and practical method of discovering truth, and, second, that the more fortunate realities of our evolved behavior—specifically the fact of cooperation—are both liberating and hopeful. In the end, Sterelny agrees more with Dawkins, and, despite a deep and humbling appreciating of Gould’s extraordinary talents and accomplishments, so do I. Had I not felt much as Dawkins apparently does, after all, I probably wouldn’t have bothered to write this article.