The Spandrels of San Marco

The Spandrels of San Marco
Maria Schnitzmeier · CC BY-SA 3.0 · source
Name	The Spandrels of San Marco
Field	Evolutionary biology, Philosophy of science
Introduced	1979
Authors	Stephen Jay Gould, Richard Lewontin
Work	"The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme"

The Spandrels of San Marco. This term originates from a seminal 1979 paper in evolutionary biology, co-authored by Stephen Jay Gould and Richard Lewontin. The paper, presented at a meeting of the Royal Society of London and later published in the Proceedings of the Royal Society of London, used an architectural analogy to launch a powerful critique of adaptationism within neo-Darwinism. It argued that not all features of organisms are direct products of natural selection, inspiring decades of debate in evolutionary developmental biology and the philosophy of biology.

Historical context and architectural origin

The paper's title refers to the spandrels—the tapering triangular spaces—found beneath the domes of the Basilica di San Marco in Venice. These architectural elements are necessary by-products of mounting a dome on rounded arches. Gould and Lewontin noted that while these spandrels in St Mark's Basilica are often beautifully decorated with mosaics depicting theological scenes, their existence is not *for* the artwork. The spandrels originated as a structural constraint in the design by architects of the Byzantine Empire, and the mosaics by later craftsmen were secondary, adaptive uses of the pre-existing space. This observation was drawn from broader principles in the history of architecture, contrasting with the purely adaptive narratives often applied to biological traits by proponents of the Modern Synthesis.

The spandrel metaphor in evolutionary biology

Gould and Lewontin imported this architectural concept into evolutionary theory as a direct challenge to what they termed the "adaptationist programme." They argued that biologists, like the Pangloss character in Voltaire's Candide, often assume every trait is an optimal adaptation forged by natural selection. The spandrel metaphor illustrated that many biological features are analogous to architectural constraints: they arise as non-adaptive consequences of other, selected-for changes in development or structure. Examples might include the hollow nature of bones, which provides lightness for movement but also creates space for marrow, a later-utilized feature. This framework emphasized the role of developmental constraints, phylogenetic history, and allometry in shaping organismal form, beyond just adaptive utility.

Key arguments and examples

The paper presented several key lines of argument against strict adaptationism. First, it highlighted the problem of "just-so stories," where plausible but untested adaptive narratives are accepted as sufficient explanation. Second, it emphasized pleiotropy, where a gene influences multiple traits, so one selected trait may drag along other, non-adaptive ones. Third, it discussed allometry and correlated growth, where changes in size necessarily alter shape due to mechanical constraints. Gould and Lewontin cited specific biological examples, such as the large size of the human brain, which they suggested might be a side effect of overall body size increase rather than a direct target of selection for intelligence. They also referenced the work of D'Arcy Thompson on growth and form, and the neutral theory of molecular evolution proposed by Motoo Kimura, as alternative explanatory frameworks.

Reception and influence in evolutionary theory

"The Spandrels of San Marco" was immediately influential and controversial, becoming one of the most cited papers in modern evolutionary biology. It was central to debates at the 1980 conference "The Role of Behavior in Evolution" at the University of Chicago and fueled the growth of evolutionary developmental biology (evo-devo). The paper encouraged researchers to consider phylogenetic constraints, historical contingency, and the role of developmental pathways in evolution, as explored by scientists like Brian Hall and Rudolf Raff. Its arguments resonated with the perspectives of the Santa Fe Institute on complex systems and influenced the philosophy of science, particularly discussions on the units of selection and the structure of evolutionary theory.

Criticisms and subsequent debate

The paper faced significant criticism from prominent adaptationists. John Maynard Smith acknowledged its importance but argued it caricatured the careful work of many evolutionary biologists. Richard Dawkins, in his books The Extended Phenotype and later works, defended the adaptationist approach, arguing that the paper's central metaphor was flawed; he contended that what biologists consider adaptations are more like the arches, not the spandrels, and that the "Borgia" frescoes within them could still be seen as optimally designed. Daniel Dennett similarly criticized the paper in Darwin's Dangerous Idea, labeling it a "rhetorical masterpiece" but scientifically misleading. Subsequent empirical work, such as that on the Hox genes and cis-regulatory elements, has shown that both adaptation and structural constraint play crucial, intertwined roles, leading to a more nuanced modern synthesis that incorporates insights from both sides of the debate.

Category:Evolutionary biology Category:Philosophy of science Category:Scientific controversies