The Spandrels of San Marco...

You'll have to excuse the lack of posts dear readers, this has been one of the most unproductive weeks I've had in a long, long time. This is mostly due to the fact that I've been sick since Tuesday and I think that I've been responsible for a significant portion of the Kleenex corporation's earnings this month...

I've decided to get myself back in to the scientific mindset (as opposed to the 'Oh dear Darwin, I'm dying! Someone put me out of my misery!' mindset) by blogging about a classic paper in the field of evolutionary biology, and one that I recently re-read during a rather extensive literature review I've been doing over the past few weeks. The paper is:

Gould SJ, Lewontin RC. 1978. The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proc R Soc Lond B Biol Sci 205:581-598.

It can be accessed in its entirety in HTML format here.

Spandrels are the tapering triangular spaces formed by the intersection of two rounded arches at right angles, and are the necessary architectural by-products of the attempt to mount a dome on such a structure. Serving no purpose in an of themselves, spandrels were co-opted by artists who used them as canvases upon which to draw/sculpt some of the most memorable and beautiful pieces of artwork found in historical European monuments. Such is the beauty of spandrel artwork that an uninformed observer may actually come to believe that spandrels themselves were designed for the purpose of showcasing such amazing artwork.

The spandrels on the ceiling of King's College Chapel.

However, as we know, spandrels were not designed 'for' anything: They exist only as a contingency of the architectural constraints involved in mounting such a structure. The spandrel art may represent an architectural 'adaptation' (i.e., using an existing structure for a new purpose), however we could not explain the overall architecture of the chapel in the context of what the spandrel is now being used for. Or as the authors of the paper say in bringing this back to a biological analogy, "If blushing turns out to be an adaptation affected by sexual selection in humans, it will not help us to understand why blood is red."

Stephen Jay Gould and Richard 'Dick' Lewontin criticize what they perceive as a growing 'adaptationist programme' in U.S. evolutionary science. They liken this to the caricatured Dr. Pangloss from Voltaire's Candide. Pangloss, of course, argued that we live in the 'best of all possible worlds', which we can be sure of because of the obvious fact that everything is so perfectly adapted to its place:

"It is demonstrable," said he, "that things cannot be otherwise than as they are; for as all things have been created for some end, they must necessarily be created for the best end. Observe, for instance, the nose is formed for spectacles, therefore we wear spectacles."

The 'Panglossian Paradigm' therefore represents a faith in the near limitless power of natural selection to shape variation of individual traits such that they are perfectly adapted to their function, whatever that function may be. If it is shown that traits cannot be 'atomized' and selected individually (for example selecting for jaw length may increase jaw width as well because the structures are correlated), then the paradigm assumes that organisms represent an optimally selected state in a situation of trade-offs among competing demands. Finally, while acknowledging that other agents of evolutionary change exist (such as genetic drift and founder effects), the adaptationist assumes that their action is very limited, and cannot explain that most 'important' biological structures.

As the authors argue, "One must not confuse the fact that a structure is used in some way... with the primary evolutionary reason for its existence and conformation." They place particular emphasis on notions of the evolution of behavior as an adaptive trait. While no doubt many behaviors are adaptive (e.g., sexual desire or the fight-or-flight response), when it comes to extremely specific behaviors, particularly in humans, the question of genetic adaptation becomes much less clear. Apparently anthropologist Michael Harner proposed that Aztec human sacrifice arose as a solution to chronic shortage of meat, which E.O. Wilson then quoted as evidence of adaptive genetic predisposition towards carnivory in humans. As Gould and Lewontin point out:

"We strongly suspect that Aztec cannibalism was an "adaptation" much like evangelists and rivers in spandrels, or ornamented bosses in ceiling spaces: a secondary epiphnomenon representing a fruitful use of available parts, not a cause of the entire system. To put it crudely: a system developed for other reasons generated an increasing number of fresh bodies; use might as well be made of them. Why invert the whole system in such a curious fashion and view an entire culture as the epi-phenomenon of an unusual way to beef up the meat supply."

Furthermore, the authors argue that such adaptationism doesn't even meet the criteria of good science in that it is not falsifiable.

Often, evolutionists use consistency with natural selection as the sole criterion and consider their work done when they concoct a plausible story. But plausible stories can always be told. The key to historical research lies in devising criteria to identify proper explanations among the substantial set of plausible pathways to any modern result.

If one 'plausible' explanation is somehow discounted (for example, it turns out that Aztecs had no shortage of meat), one need only propose a new 'plausible' explanation and continue assuming that the trait represents an adaptation 'for' something. There's never a need to demonstrate that the trait represents a genetic 'adaptation' in the first place; it's simply assumed from the beginning.

Gould and Lewontin then devote a lengthy part of the paper to the consideration of alternatives to adaptationism, including genetic drift in small populations (such as those generally assumed to occur under the allopatric speciation model), traits arising as a correlation with the adaptation of another altogether different trait, phenotypic plasticity (which may explain much of specific human behaviour), and finally different populations devising alternative solutions to the same problem. In the latter case, there doesn't have to be an adaptive reason why each population evolved a different solution - it could simply result from unequal distributions of alleles and the stochastic properties of genic selection.

Finally, the authors bring up the much unappreciated subject of developmental constraint. Selection is not completely free to act on all traits individually and evolution cannot produce adaptations out of nothing. Genetic variation must exist for selection to have anything to act upon. There's a reason why there are no six-legged vertebrates, or why birds don't have arms in addition to their wings. All vertebrates develop from the vertebrate 'body-plan', something that is specified very early in development and changes to which have massive pleiotropic effects (i.e., small changes effect many things in the body, and thus it's more likely that something will go wrong). All vertebrates exist as variants on a basic whole-body plan, not as decomposed units that were individually selected, optimized, and reassembled.

This paper, written fairly early on in Stephen Jay Gould's career as an essayist, laid out the foundation of the theory and intellectual philosophy that would dominate his writing from then on. Similarly, though Lewontin's work had already begun to embrace a non-selectionist, plurastic view of evolution, the tone of much of his later work (at least in what I've read) was summarized in this manuscript. Although written 30 years ago, The spandrels of San Marco and the Panglossian paradigm remains an important contribution to evolutionary theory, and is worth a read every now and again in order to remind us to keep an open mind with regards to the diversity of mechanisms underlying evolutionary change.

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