Unlinking the Speciation Steps: Geographical Factors Drive Changes in Sexual Signals of an Amazonian Nurse-Frog Through Body Size Variation

Geographical and environmental distances influence the divergence of characters among biological populations, especially on a macro spatial scale, making it difficult to interpret the individual contribution of these predictor variables in the process of population differentiation. Anurans are excellent models for multi-character evolutionary studies, due to their low vagility and frequent territoriality, causing certain environmental changes to result in barriers that isolate populations. Accordingly, we propose to test the correlation of environmental and geographical distances in the absence of obvious vicariant barriers with phenotypic and genotypic population characters using, as a study model, an Amazonian litter-frog (Allobates sumtuosus). Combining univariate and multivariate analyses, and Structural Equation Modeling, we tested the general hypotheses that geographical and environmental distances affect the variation in the morphometric, acoustic and genetic characters of this frog along a latitudinal gradient at a fine spatial scale. We found that the latitudinal variation was the most correlated with the variation of the studied characters, with an explanatory force always greater than 78%. Therefore, we suggest that there is a combined effect of latitude and environment on phenotypic characteristics in A. sumtuosus. These factors shape the acoustic characters of this species through pressures on body size, as confirmed by a regression analysis showing that larger body sizes resulted in lower-spectral frequency acoustic signals. This is because the climatic environmental gradient occupied by the species promotes changes in the area-volume relationship of individuals, resulting in larger body sizes towards the Equator. Although we observed a pronounced intrapopulation genetic structure, it was not associated with phenotypic variation. In summary, our study breaks down the stages of speciation for this Amazonian litter-frog, demonstrating that environmental factors lead to changes in the sexual signal due to the variation in body size.