Spandrels of San Marco
Some traits may be byproducts of other structures rather than direct adaptations.
The Mechanism: Stephen Jay Gould and Richard Lewontin used architectural spandrels as an analogy for traits that arise as byproducts of structural constraints. The critique challenged adaptationist explanations that assign a direct evolutionary function to every feature.
The Philosophical Pivot: Explaining a trait by its present usefulness can confuse what it does now with why it arose. Historical constraints and developmental processes may shape biology in ways that do not look like optimal design.
The Horizon: The argument encouraged evolutionary biology to examine multiple causal routes, not only adaptation. It does not show that most traits are non-adaptive; each claim requires evidence.
A plausible story about function can be seductive because it makes complexity feel intentional. Scientists need tests that could distinguish an adaptation from a byproduct.
Not everything useful was made for its usefulness.
A feature can be useful without having been built for the job it now performs.
What else could explain a trait I am tempted to call an adaptation simply because it seems useful?