Anyone who has been to the Northeast is likely familiar with the miniature Dunkin Donuts confection the donut hole, aka the Munchkin. After an unsuccessful online search, I called headquarters to learn more about the history of this sweet, only to find them quite silent on the issue. What follows is based on hopeful speculation for the sake of an example of spandrel and exaptation in evolutionary theory.
Gould and Lewontin (1979) came up with the terms spandrel and exaptation to provide an explanation for the origins of heritable traits that weren’t initially adaptations. An adaptation begins as a trait that is selected for a particular function that it serves the carrier. A spandrel is a leftover of an adaptation. It has no function and is not subject to natural selection. However, if that spandrel is co-opted for a particular function, it is considered an exaptation – and then can actually become subject to selection. Though it didn’t begin as a functional product, in the end it comes to be. For example, bird feathers were initially an adaptation for thermoregulation, and later were co-opted, or exapted, for the function of flight. Flight in this case didn’t originate as an adaptation, but was co-opted from the spandrel.
The Munchkin illustrates the concept of exaptation well, though it requires a baker as “selector”, whereas natural selection operates with no selector. A doughnut in this example is a circular piece of dough with an empty circular middle. Imagine that the shape is created by making a round of dough, and then cutting out the middle piece, leaving you with the doughnut and some extra dough. Imagine further that the baker typically throws the middle piece aside as it serves no purpose. The middle piece here is a spandrel – it serves no function to the baker, but is rather a leftover portion of the functional dough – the doughnut.
However, the baker decides that those leftover pieces are too much of a waste. She decides to roll them into a ball and sell them separately from the doughnuts. She gives them a name, Munchkin, and markets them to dieters, children, and dog-owners for treats, and finds that these formerly useless pieces are now bringing in money. They have been co-opted for the function of money making, and therefore the Munchkin is an exaptation. Now she finds them so popular, the baker is creating different flavors, and fun boxes in which to sell the doughnut holes.
What originated as a mere by-product of the doughnut has now come to serve the function of a profitable treat. Though the Munchkin, as an exaptation, will be subject to selection pressures. Perhaps customers will prefer chocolate rather than plain Munchkins, resulting in more chocolate Munchkins being made. Or perhaps the mere size of the Munchkin will allow it to find its way to more diverse environments than the doughnuts, resulting in more Munchkins being made than doughnuts. Whatever the end result, the Munchkin shows that while some features are not initially subject to selection pressures, if they come to serve a function, someday they just may be.
[Postscript: I hope Stephen J. Gould had a sense of humor, otherwise he is figuratively rolling in his grave at my Just-So Munchkin story.]
Buss, D. M., Haselton, M. G., Shackelford, T. K., Bleske, A. L., & Wakefield, J. C. (1998). Adaptations, exaptations, and spandrels. American Psychologist, 53(5), 533-548.
Gould, S. J., & Lewontin, R. C. (1979). The spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme. Proceedings of the Royal Society of London, 205(1161), 591-598.