Phylogenetic Structure and Environment Explain Body Size in North American Killifishes

Aim Investigate maximum‐length patterns in North American killifishes within an evolutionary macroecological framework by examining how body size evolved, how it varies across the geographic range, and how large‐scale environmental variables shape body‐size geography. Location North America. Taxon North American killifishes (Fundulidae, Cyprinodontidae, Goodeidae and Profundulidae). Methods We documented maximum total length of killifish within one‐degree grid cells covering North America. We assessed phylogenetic signal using a Phylogenetic Signal Representation (PSR) curve and employed Principal Coordinates of Phylogenetic Structure (PCPS) to integrate the geographic distributions of lineages as a numerical variable alongside environmental variables and latitude. Relative contributions of latitude, environmental and phylogenetic variables were evaluated through hierarchical partitioning and variance partition analysis. Results The evolutionary pattern of Maximum TL was better described by a multiple‐optima Ornstein‐Uhlenbeck model, identifying seven distinct adaptive regimes across the phylogeny, indicating stabilizing selection toward multiple phenotypic optima. Forward selection identified maximum temperature of the warmest month and intra‐annual variability in net ecosystem production as key bioclimatic predictors. Variance partitioning revealed that phylogenetic structure and bioclimatic variables are strongly correlated, jointly explaining the majority of variation. While these variables share structure with the latitudinal gradient, the unique component of phylogenetic variation explained a substantial proportion of the pattern. Main Conclusions Body size in North American killifish exhibits phylogenetic conservatism manifested as evolutionary stasis. Although temperature and latitude strongly predict its geographic distribution, creating a pattern suggestive of Bergmann's rule, the distribution of lineages among families primarily drives this relationship. The observed latitudinal cline is a biogeographic artefact of lineages retaining ancestral sizes while expanding their distributions northward or southward, severely limiting the extent of in situ adaptation to new environments. These findings underscore the importance of integrating phylogenetic information to accurately interpret broad‐scale patterns.