Exaptation

Spandrels and Emergentism

Some forms of flight appear to have arisen through co-option or readaptation of the lift afforded by feathers, which most scientists believe originally provided a selective advantage because of their insulating qualities that aided body temperature regulation. A metaphor that is sometimes used to explain exaptation is that of a spandrel, a term borrowed from architecture, which describes the v-shaped space between two archways. The shape of a spandrel is not the direct intention of architectural plans but rather the result of placing two archways adjacent to each other. Architecture led directly to the development of arches as a feature in the construction of buildings. Spandrels were simply a by-product of this process.

Spandrels have a correlate in nature, in the principle of emergentism. A feature or characteristic may exist in the world, or emerge during evolution (phylogenesis) or cognitive development (ontogenesis), that itself is not a direct consequence of evolutionary pressures but rather the outcome of the conditions surrounding its emergence. Think of the flow of water following the path of least resistance. Water exhibits no intentionality but is simply subject to the resistances that are presented by the landscape. The shape of honeycomb cells in beehives is hexagonal, not because bees have adapted a hexagon-making gene but because the hexagonal shape completely tiles in two-dimensional space. Thus, hexagon-shaped containers in three dimensions provide greater structural integrity than round cylinders do, as the surface tensions in hexagonal [Page 433]cells are evenly distributed along a maximum area. Whether the bee intends to produce this shape or the roundness of cylindrical cells simply deforms under pressure into hexagons, the emergent nature of stacked hexagonal containers is the consequence of the laws of physics rather than natural selection.

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