spandrels

In architecture, a spandrel is the roughly curvilinear space between the curve of an arch and the bounding rectangular frame that encloses the arch. In classical masonry, arches are set within a rectangular opening; the gap between the arch and the rectangle creates a space that can be triangular or more complex in shape. Spandrels are often filled with decorative panels, inscriptions, or relief sculpture, and they may also help to stiffen the wall by transferring loads from the arch to the surrounding structure. The term is commonly used to describe both the interior voids above the arch and the exterior spaces to the sides of the arch in an arcade or bridge.

In evolutionary biology, spandrels refer to features that arise as byproducts of the geometry, development, or construction of other structures rather than as direct targets of selection. The term was popularized by Stephen Jay Gould and Richard Lewontin in 1979 to argue that some traits may be nonadaptive consequences of other design constraints. Spandrels can later acquire function through exaptation, but the concept is used to caution against assuming that every trait has a specific adaptive purpose. The idea has since spurred ongoing debate about how to distinguish adaptive from nonadaptive features in organisms.