Different macroevolutionary trajectories lead to contrasting ecogeographical patterns in two widespread frog radiations

Large-scale evolutionary radiations often take place through marked
climatic gradients that could promote morphological variation. However,
opposite trends to the well-known ecogeographical rules can be observed in
closely related clades across different geographic regions. Here, we
compared macroscale patterns of morphological diversification between two
geographically separated evolutionary radiations of widespread frogs to
assess how morphological diversity has arisen and tested whether distinct
ecogeographical rules operate at broad spatial scales. We combined
phylogenetic, morphological, climatic, and microhabitat data for 65
species of Rana and Lithobates and used phylogenetic comparative methods
to identify which factors have played a major role in the phenotypic
diversification of the two radiations. Morphological evolution in Eurasian
subclades closely followed phylogenetic relatedness, with slow and
constant rates of body size diversification through time. By contrast,
American species showed accelerated rates of body size evolution in one
subclade. Adaptive models (OU) best explained the morphological evolution
of this radiation despite the lack of significant correlations with
climatic variables, suggesting that selective pressures might have driven
adaptation. Discordant ecogeographical trends were also found between
radiations: Eurasian species followed Bergmann’s rule and the water
conservation hypothesis, while no clear trend was observed among American
species. Overall, different evolutionary trajectories between the two
radiations lead to contrasting ecogeographical patterns, highlighting that
distinct selective pressures and adaptive mechanisms can play very
different roles in the evolution of phylogenetically related clades. Our
research demonstrates that ecogeographical rules should not be considered
universally applicable but rather context-dependent, and that widespread
radiations are valuable systems to uncover mechanisms regulating these
general patterns and potential constraints hindering their emergence.