Research Article

Distinguishing Biological Trends from Adaptation

  • Lucas John Mix orcid logo (Harvard University)


Models of evolution commonly decompose change into deterministic and stochastic components. Different models, however, produce different concepts of signal and noise. Excursion tests correct for two distinct types of noise, sampling error and phenotypic drift, resulting in two distinct types of signal or trend. A minimal trend is a signal of historical change after correcting for sampling error; a directed trend is a model dependent measure of confidence that selection has occurred (correcting for both sampling error and drift). The history of random walks and excursion tests in paleobiology highlights the conceptual middle ground between historical pattern and underlying processes. In paleobiology, both minimal and directed trends reflect a causally agnostic, trait-level product of evolutionary and environmental processes. Directed trends provide evidence for directional causes, but the identity and relative strength of those causes requires a deeper understanding. Thus, the magnitude of minimal and directed trends should not be interpreted as the strength of selection for a specified trait. They can be a useful stepping stone in that direction when paired with a map of the middle ground and agreement about how genetic, macroevolutionary, and environmental processes contribute to evolutionary change.

Keywords: excursion test, drift, MBL Model, natural selection, random, trend

How to Cite:

Mix, L., (2022) “Distinguishing Biological Trends from Adaptation”, Philosophy, Theory, and Practice in Biology 14: 10. doi:



Published on
28 Jan 2022
Peer Reviewed