Gould and Lewontin described adaptationism as, “An attempt to explain the existence and the particular forms of any phenotypic trait as the result of natural selection.” In other words adaptationism is the belief that natural selection is the only important method of evolution while spandrels are phenotypic characteristics that did not originate by the direct action of natural selection and that were later co-opted for a current use. Gould and Lewontin provide an analogy of spandrelism with the concept of spandrels in Renaissance architecture. These tapered spaces between arches supported a dome and were an architectural by-product. The spandrels were not designed for the artistic purposes for which they were often employed. Overall, Gould and Lewontin believed that the practice of assuming an organism’s current use of a trait is the reason for its evolution is a ‘sloppy’ form of evolutionary thinking.
According to Pigliucci and Kaplan, the concept of spandrelism has changed over the past 20 years in that much discussion and general acknowledgment of the role of constraints, tradeoffs and costs in evolution has taken place. In general, selection (panglossianism) and constraints (spandrelism) are seen as the two deterministic participants in phenotypic evolution either working together or opposing one another. In addition, the effects of the far from constant environments has been investigated with a “wider range of theoretical tools”, showing that the external environment results in as much limitation to adaptive evolution as genetic or epigenetic constraints. Furthermore, empirical advances are allowing biologists to assess the actual balance between non-adaptive forces and selection in natural populations.
I did not fully understand how the experiments for the empirical advances occurred.
For more information on the advances since the Spandrels, see Pigliucci, M. and Kaplan, J. 2010. “The fall and rise of Dr. Pangloss: adaptationism and the Spandrels paper 20 years later.”