27 May 2012
Unlike the subject of my previous column in this series, James A. Shapiro, she does not present her ideas as revolutionary or as a mortal threat to the Darwinian worldview.
In fact, she goes out of her way to associate her ideas with those of Darwin himself, which were in some respects more radical that those of his twentieth-century epigones.(1)
And yet, I shall argue that West-Eberhard—who is a researcher at the Smithsonian Tropical Research Institute in Panama, as well as a professor of biology at the University of Costa Rica—has made a foudational contribution to a new and revolutionary approach to evolutionary theorizing that bids fair (whatever her expressed intentions) to turn mainstream Darwinism on its head.
West-Eberhard’s ideas are crucial for one main reason: The Darwinian project is intended, more than anything else, to demonstrate that teleology, or purpose, can be eliminated from our theoretical understanding of the living world.
West-Eberhard’s work helps to upend that project by showing how purposiveness (or goal-directedness) lies at the heart of any realistic explanatory framework in evolutionary biology. In other words, her contribution consists in demonstrating that, far from eliminating purpose from nature, evolution in fact presupposes it.
In a nutshell, West-Eberhard’s thesis is contained in the title of her magnum opus, Developmental Plasticity and Evolution (Oxford UP, 2003).
What is developmental plasticity? It is the property that all living things possess of being able to compensate during ontogenetic development for variations in either internal or external conditions. Note that “compensation” is a teleological concept. It implies that there is a particular end- or goal-state that one is trying to attain by means of the compensatory maneuvers.
In the case of an organism, the developmental process is aiming at the viable adult form. If a perturbation occurs during this process—be it genetic, physiological, or environmental—then compensatory changes will occur elsewhere within the organism to ensure, insofar as possible, that the viable adult form is reached in spite of the perturbation. It is important to note that such compensatory changes need not be spatially or temporally isolated, but may often result in cascades of changes throughout the organism, affecting multiple organ systems in the present and the future.
West-Eberhard calls this compensatory power—which all organisms seem to possess—”developmental (or phenotypic) plasticity.” The entire suite of resulting compensatory changes she refers to as “phenotypic accommodation.” Here is how she defines these two closely interrelated ideas:
Phenotypic plasticity enables organisms to develop functional phenotypes despite variation and environmental change via phenotypic accommodation—adaptive mutual adjustment among variable parts during development without genetic change.(2)
The reason why developmental plasticity and phenotypic accommodation are important from an evolutionary point of view is that taking them into consideration forces us to radically rethink the standard Darwinian account of natural selection.(3)
Darwinism is dedicated to extirpating teleology from biology, root and branch. The way it purports to do so is by invoking chance against the backdrop of the metaphysical assumption of mechanism.
A point mutation is introduced in random fashion in the genome. The causal consequences of this event then propagate as a sequence of mechanical, hence deterministic, changes through the rest of the organism. If the resulting phenotype happens by chance to be more successful than competing forms, then the original mutation is retained (“selected”) and that genotype, together with its mechanistically related phenotype, are gradually propagated throughout the population.
This scenario does indeed appear devoid of teleology. It has long been accepted as a matter of faith by the majority of biologists. On the other hand, it has always seemed absurd to a small but tenacious minority of scientists, practically from 1859 right through until the present.
West-Eberhard’s emphasis on the role of developmental plasticity in evolution manages to break this long-unresolvable debate wide open by demonstrating the fundamental conceptual role that a clearly teleological process plays in the tacit explanatory framework of the theory of natural selection.
Darwinists have felt convinced by the standard picture because they have tacitly relied upon the fact that organisms possess phenotypic plasticity—in short, are not machines.
Critics of Darwinism have remained skeptical of the standard picture because they have kept its mechanistic premises more clearly in view: If you swap out the parts of a machine at random, you will never get an improvement in its functioning.
Well, it turns out the Darwinists were right, in this restricted sense: You can swap out the parts of an organism. But only because all the other parts will automatically compensate for the change. That is, only because organisms are not machines. But, of course, in that case, Darwinism can hardly be said to eliminate teleology from the natural world.
So, the critics end up with the preponderance of evidence and logic on their side: If the Darwinists had been right and organisms really were machines, then evolution would have been impossible.
West-Eberhard vindicates the possibility of evolution by rejecting the machine metaphor and placing the adaptive power of living beings (developmental plasticity and phenotypic accommodation) at the center of the evolutionary process.
Her textbook is replete with concrete examples of developmental plasticity. My favorite one is known is “Slijper’s goat,” which I hope may supersede Darwin’s finches someday as our standard model for thinking about evolution.
Everhard Johannes Slijper (1907–1968) was a Dutch zoologist who during World War II published a report on a goat that had been born without forelegs.(4) He had reared the animal to the age of one year, then sacrificed it in order to study its anatomy.
With human help, the goat had learned to hop about on its hind legs. Slijper demonstrated that this ability was supported by a whole suite of coordinated changes in the animal’s skeleton and musculature. In fact, upon dissection the animal’s body was found to resemble that of a kangaroo more closely than that of a normal goat.
In case all of this seems hard to credit, West-Eberhard documents numerous similar examples. Moreover, since seeing is believing, the reader is invited to watch the short video clip below of a fully bipedal dog that is alive today—Faith the Dog.
What’s the point of the Slijper’s goat example? Admittedly, this case is extreme, but pathological cases are often helpful in throwing light on normal phenomena in biology. While a deformity this extreme would certainly have led to the animal’s death absent human intervention, the point is that all natural variations must undergo a similar, if less extreme, compensatory developmental process.
As West-Eberhard explains:
. . . the point is to dramatize how a change in one aspect of the phenotype—in this case the front legs—can lead to correlated changes that show a degree of complexity and functional integration that we usually assume to require generations of natural selection and genetic change at many loci.(5)
The example of Slijper’s goat is useful because it helps us to represent to ourselves the abstract notions of “developmental plasticity” and “phenotypic accommodation” in a more vivid way. In cases like Slijper’s goat and Faith the Dog, we can see the power of these phenomena in action. But they are equally active in less extreme cases, as well. In fact, whenever any change whatsoever is introduced at the genetic level, some goal-directed compensation must take place in order for a viable adult form to be attained.
In a phrase:
Responsive phenotype structure is the primary source of novel phenotypes . . .(6)
This means that the process of natural selection presupposes the phenomena of developmental plasticity and phenotypic accommodation. Therefore, far from being cast out of biology, teleology turns up at its very heart. In this way, evolution is vindicated, but Darwinism is turned on its head.
To avoid teleology, Darwinism must posit random genetic changes that result in random phenotypic changes. But West-Eberhard’s work shows us there is no such thing as a random phenotypic change. Instead, we can now see that all phenotypic change is goal-directed.
Thus, the evolutionary process has depended upon the inherent, teleological capability of all living things to adapt themselves to circumstances, within and without. It is this capability that explains evolution, not the other way around.(7)
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(1) Mary Jane West-Eberhard, “Toward a Modern Revival of Darwin’s Theory of Evolutionary Novelty,” Philosophy of Science, 2007, 75: 899–908.
(2) Mary Jane West-Eberhard, Developmental Plasticity and Evolution (Oxford UP, 2003); p. 51.
(3) These ideas are not absolutely new, of course. For example, developmental (or phenotypic) plasticity is clearly related to such ideas as Conrad Waddington’s “chreode,” Walter B. Cannon’s “homeostasis,” and Claude Bernard’s “milieu intérieur.” However, West-Eberhard’s developmental plasticity is more dynamic and flexible than any of those concepts. In this respect, her ideas hark back to Hans Driesch’s “harmonious-equipotential system,” or even Etienne Geoffroy Saint-Hilaire’s “unity of composition.” In any case, West-Eberhard’s signal contribution has been to stress developmental plasticity as a significant contributing factor in evolution, rather than writing it off in typical Darwinian fashion as just another result of natural selection.
(4) E.J. Slijper, “Biologic-anatomical Investigations on the Bipedal Gait and Upright Posture in Mammals, with Special Reference to a Little Goat, born without Forelegs,” Proceedings of the Koninklijke Nederlandsche Akademie van Wetenschappen, 1942, 45: 288–295, 407–415.
(5) Mary Jane West-Eberhard, Developmental Plasticity and Evolution (Oxford UP, 2003); p. 52.
(6) Ibid.; p. 503.
(7) For a more succinct presentation of her ideas, including the Slijper’s goat example, see Mary Jane West-Eberhard, “Phenotypic Accommodation: Adaptive Innovation Due to Developmental Plasticity,” Journal of Experimental Zoology, Part B: Molecular and Developmental Evolution, 2005, 304B: 610–618.