The tyranny of phylogeny-A plea for a less dogmatic stance on two-species comparisons: Funding bodies, journals and referees discourage two- or few-species comparisons, but such studies provide essential insights complementary to phylogenetic comparative studies.

Phylogenetically controlled studies across multiple species correct for taxonomic confounds in physiological performance traits. Therefore, they are preferred over comparisons of two or few closely-related species. Funding bodies, referees and journal editors nowadays often even reject to consider detailed comparisons of two or few closely related species. Here, we plea for a less dogmatic stance on such comparisons, because phylogenetic studies come with their own limitations similar in magnitude as those of two-species comparisons. Two-species comparisons are particularly relevant and instructive for understanding physiological pathways and de novo mutations in three contexts: in a purely mechanistic context, when differences in the regulation of a trait are the focus of investigation, when a physiological trait lacks a direct connection to fitness, and when physiological measures cannot easily be standardized among laboratories. In conclusion, phylogenetic comparative and two-species studies have different strengths and weaknesses and combining these complementary approaches will help integrating biology.