2025

Lehtonen, Jussi; Parker, Geoff A.

The optimality approach is a method for investigating evolutionary adaptations. It involves constructing one or more mathematical models based on the constraints and selective forces that are hypothesized to have generated a given trait by natural selection. The aim is not to demonstrate that traits are optimal. Rather, an optimality model serves as a null hypothesis for what is expected should selection be able to operate freely to drive a given phenotypic trait to a local equilibrium or stable state. Should a model's predictions match empirical observations, this gives some evidence that the selective forces and constraints that have shaped the trait have been correctly identified, and its likely adaptive function understood. As such, the optimality approach at its simplest adds mathematical rigor to verbal arguments about adaptive function that have prevailed in many branches of biology since (and even before) Darwin, although modern methods often permit reasoning well beyond what is possible with verbal arguments alone. Should a model's predictions not approximate to empirical observations, our understanding of the selective forces operative may be incorrect or incomplete, or genetic and other constraints may prevent evolution from reaching the phenotypic optimum, suggesting that further research is required to understand the mismatch. Here, we give an outline of optimality theory's history in evolutionary biology, the techniques used with some examples, and outline some general critiques of the method.