Spandrel

The term “spandrel” was introduced into evolutionary biology by Stephen Jay Gould and Richard Lewontin in their influential 1979 paper, “The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme.” Drawing an analogy from architecture, they described spandrels as spaces created by the necessary geometry of structural elements, such as the curved triangular areas formed between the tops of arches and a circular dome. These architectural spandrels are not designed for their own sake but are inevitable byproducts of constructing arches and domes. However, they are often subsequently decorated with intricate artwork, giving the appearance of being primary design elements.

The Architectural Metaphor in Biology

Gould and Lewontin argued that many biological traits might be analogous to architectural spandrels: non-adaptive byproducts of other evolved features that may or may not later be co-opted for a function. They contended that evolutionary biologists, particularly those adhering to a strong adaptationist program, often fall into the trap of assuming that every discernible trait must be an adaptation—a feature that arose through natural selection for its current function. This tendency, which they termed the “Panglossian paradigm” (referencing Voltaire’s Dr. Pangloss, who believed everything in the world was for the best possible purpose), leads to the invention of adaptive stories for traits that might be better explained as developmental constraints, genetic drift, or spandrels.

For example, the human chin, a prominent bony protrusion at the front of the lower jaw, has been proposed as a spandrel. It is not clear what selective advantage a chin might confer. Instead, it may be an unavoidable consequence of the differential growth rates of two parts of the jaw during human evolution, specifically the reduction of the dental arch relative to the alveolar bone (Lieberman, 2011). Similarly, the male nipple, while having no known function, is an inevitable byproduct of mammalian development, where a common developmental pathway for mammary glands exists in both sexes before sexual differentiation.

Implications for Evolutionary Psychology

The spandrel concept has been particularly relevant in evolutionary psychology, a field often characterized by its strong emphasis on adaptationist explanations for human cognitive and behavioral traits. Critics of evolutionary psychology, such as David Buller (2005), frequently invoke the spandrel concept to challenge claims that specific psychological mechanisms are direct adaptations to ancestral environments. They argue that many complex human traits, such as artistic ability, musicality, or even certain aspects of consciousness, might not be adaptations in themselves but rather emergent properties or byproducts of other cognitive adaptations, such as advanced language capabilities, theory of mind, or general intelligence.

For instance, the ability to play a musical instrument might not be a direct adaptation but a spandrel arising from the sophisticated motor control, auditory processing, and pattern recognition abilities that were selected for other purposes, such as tool use, language, or navigation. Similarly, a general capacity for symbolic thought, which is clearly adaptive for problem-solving and communication, might produce the capacity for abstract mathematics or philosophical reasoning as a spandrel.

Debates and Nuances

The extent to which spandrels actually occur in evolution, and their significance relative to adaptations, remains a subject of debate. Gould and Lewontin’s paper provoked a strong reaction, leading to a re-evaluation of adaptationist assumptions across biology. However, many evolutionary biologists and psychologists argue that while spandrels certainly exist, they are less common or less significant than Gould and Lewontin implied, especially for complex, costly traits.

Proponents of a robust adaptationist program, such as Tooby and Cosmides (1992), acknowledge the existence of spandrels but maintain that the design features of complex psychological mechanisms are overwhelmingly likely to be adaptations. They argue that the intricate, specialized, and efficient design of many cognitive systems points strongly to a history of natural selection for specific functions, rather than being accidental byproducts. They also point out that even if a trait originates as a spandrel, it can subsequently be co-opted by natural selection and refined for a new function, becoming an exaptation (Gould & Vrba, 1982). For example, feathers may have initially evolved for thermoregulation (a spandrel or adaptation for warmth) and later were exapted for flight.

Distinguishing between an adaptation, a spandrel, and an exaptation can be challenging. Researchers typically rely on evidence of complex design, efficiency, specificity, and universality within a species to infer adaptation. The spandrel concept serves as an important cautionary principle, reminding researchers to consider alternative, non-adaptive explanations and to avoid premature adaptive storytelling when analyzing biological traits, particularly in the complex domain of human cognition and behavior.