Spandrels are features that emerge not as primary designs but as consequences of other structures. While initially an architectural term, this concept provides a powerful lens to interpret unexpected regularities in complex systems.
Spandrels in Architectural Design
In architecture, a spandrel refers to the triangular space formed between the curve of an arch and a rectangular frame, or between the tops of two adjacent arches. This space is not designed for a specific purpose; it arises as a byproduct of arch construction. Side-by-side arches create these triangular sections. Spandrels also describe the horizontal panel between a window’s top and the sill of the window above it in multi-story buildings. These spaces often conceal structural elements like floor slabs or support beams, and are frequently adorned with decorative elements.
The Concept’s Evolutionary Adaptation
The architectural concept of spandrels was adapted into evolutionary biology by Stephen Jay Gould and Richard Lewontin in their 1979 paper, “The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme.” They used the term to challenge pan-adaptationism, a view asserting every biological trait is a direct product of natural selection. Gould and Lewontin argued this perspective, which they likened to Voltaire’s Dr. Pangloss, often led to speculative “just-so stories” about a trait’s adaptive origin.
Some traits arise as byproducts or architectural constraints of other features under selection. These biological spandrels are not directly selected for utility but appear as incidental outcomes of developmental pathways. Like architectural spandrels, biological spandrels are incidental outcomes of evolutionary processes. This perspective broadened understanding of how complex organisms develop, integrating factors beyond direct selection.
Gould and Lewontin emphasized that organisms are integrated wholes, where changes in one part have correlated consequences elsewhere. They argued biological forms are constrained by evolutionary history and developmental processes, limiting pathways of change more than selective forces alone. Their critique called for a more pluralistic approach to evolutionary explanations, acknowledging genetic drift, developmental biases, and other non-adaptive factors as drivers of biological diversity.
Recognizing Biological Spandrels
Identifying a biological spandrel involves looking for features that emerge as non-adaptive byproducts of other selected traits or developmental constraints. These traits are not initially shaped by natural selection for a specific function, though they may later be co-opted for a different purpose through exaptation.
One commonly cited example is the human chin, a bony protrusion unique to humans. While adaptive explanations exist, current theories suggest it is a byproduct of the evolutionary reduction in overall jaw size and facial shortening. Studies indicate the chin’s evolutionary rate is higher than other primate features, suggesting it arose from selection on other facial structures, rather than direct selection for the chin itself.
Male nipples represent another clear example. All human embryos develop nipples before sex differentiation occurs. Since nipples form prior to the genetic switch triggered by the SRY gene on the Y chromosome, their presence in males is a byproduct of the shared early developmental pathway for both sexes, not an adaptation for male function.
The human belly button, or navel, also serves as a spandrel. It is the scar left after the umbilical cord detaches at birth. The umbilical cord is essential for fetal development, providing nutrients and removing waste. The navel itself has no direct adaptive function after birth; it is a necessary consequence of being born a placental mammal.
Complex structures like the human brain also exhibit spandrel-like features. The intricate folding of the cerebral cortex, which maximizes surface area within the cranial space, is a byproduct of rapid brain growth. While increased brain size is adaptive, the specific patterns of these folds are largely a mechanical consequence of cortical expansion, rather than each fold being individually selected for.
Another example is the pseudo-penis of female spotted hyenas. Female hyenas possess masculinized external genitalia, including an elongated clitoris resembling a penis and fused labia. While this structure functions in social displays and allows females to control mating, it presents reproductive challenges, as females must urinate, copulate, and give birth through this narrow canal. This feature is a byproduct of high levels of prenatal androgens in female hyenas, linked to the species’ female dominance and aggression, an adaptive trait.
Reshaping Evolutionary Perspectives
The spandrels concept influenced evolutionary thought, offering an alternative to strict adaptationist explanations. It highlighted that evolution is not solely driven by direct selection for optimal function. Gould and Lewontin’s critique pushed the field towards a pluralistic view, acknowledging multiple factors contribute to life’s diversity.
This perspective emphasized developmental constraints, which are biases or limitations on evolutionary pathways imposed by an organism’s existing developmental program. For instance, an organism’s fundamental body plan, or “Bauplan,” established early in embryonic development, can restrict possible changes later in evolution. These constraints mean certain evolutionary outcomes are more likely or unavoidable, regardless of selective pressures.
The spandrels concept also underlined historical contingency, suggesting evolution’s path depends on past events and accidental trait accumulation. This means if life’s tape were replayed, outcomes might differ, as evolutionary innovations can arise from initially non-adaptive byproducts. These byproducts can then be “exapted,” co-opted for new, beneficial functions unrelated to their origin. For example, feathers likely first evolved for thermoregulation but were later exapted for flight.
Recognizing spandrels and exaptations provided a nuanced understanding of how diverse biological forms arise, incorporating non-adaptive pathways and repurposing existing structures into evolution’s broader narrative. This framework expanded evolutionary biology beyond a purely functionalist interpretation, fostering appreciation for the interplay of selection, development, and chance. The debate spurred by the spandrels paper encouraged a more rigorous approach to identifying true adaptations and considering alternative explanations.
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