Alejandro Cano, Alejandro Couce, Joanna Masel, Joshua L Payne, Arlin Stoltzfus, Jay F Storz

The idea that adaptive change is subject to biases in variation by a "first come, first served" dynamic is not part of classic evolutionary reasoning. Yet, predictable effects of biases in the introduction of variation have been reported in models of population genetics, in laboratory evolution, and in retrospective analyses of natural adaptation. This effect of "arrival bias" has potentially broad significance, given widespread contemporary interest in the role of mutational and developmental tendencies of variation, in the context of a traditional view that such ideas are incompatible with population genetics. Indeed, the idea is addressed at length in a recent commentary by Svensson (2022). Unfortunately this commentary misrepresents the theory, history, evidence, and even the basic concepts involved in research on the role of mutation biases in adaptation. Here we correct some of these misrepresentations and offer a clearer and more accurate account of contemporary empirical and theoretical results.