Among several factors proposed as drivers of species diversification in the American tropics, the riverine barriers hypothesis plays a central role in Amazonian biogeography, being observed in many species. Although the hypothesis has great evolutionary importance, some species deviate from the expected biogeographic pattern. Their evolutionary histories are better explained when considering climatic oscillations or joint action of climatic fluctuations and river dynamics. Here, we used reduced representation genome sequencing and species distribution models to better understand the implications of landscape dynamics and climate shifts on species diversification in the Neotropics. The blue-crowned Manakin (Lepidothrix coronata) has one of the widest distributions amongst Manakin, with remarkable phenotypic variation across its distribution, thus an excellent model for this sort of study. We found evidence for the existence of four, geographically structured, evolutionarily independent lineages that diverged close to the Pliocene-Pleistocene boundary. Despite distribution models indicating habitat suitability crossing the rivers, we found no evidence of gene flow between the lineages, suggesting that the diversification process was mainly triggered by vicariant events, such as the establishment of the Amazonian rivers and the rise of the northern Andes.

Body size mediates life history, physiology, and inter- and intra-specific interactions. Within species, sexes frequently differ in size, reflecting divergent selective pressures and/or constraints. Both sexual selection and differences in environmentally-mediated reproductive constraints can drive sexual size dimorphism, but empirically testing causes of dimorphism is challenging. Manakins (Pipridae), a family of Neotropical birds comprising ~50 species, exhibit both male- and female-biased size dimorphism and are distributed across gradients of precipitation and elevation. Males perform courtship displays ranging from simple hops to complex aerobatic manoeuvres. We tested associations between sexual size dimorphism and (a) agility and (b) environment, analysing morphological, behavioural, and environmental data for 22 manakin species in a phylogenetic framework. Sexual dimorphism in mass was most strongly related to agility, with males being lighter than females in species performing more aerial display behaviours. However, wing and tarsus length dimorphism were more strongly associated with environmental variables, suggesting that different sources of selection act on different aspects of body size. These results highlight both the strength of sexual selection in shaping morphology—even atypical patterns of dimorphism—while demonstrating the importance of constraints and ecological consequences of body size evolution.

<b>Abstract</b><br/><p><span lang="EN-GB">Body size mediates life history, physiology, and inter- and intra-specific interactions. Within species, sexes frequently differ in size, reflecting divergent selective pressures and/or constraints. Both sexual selection and differences in environmentally-mediated reproductive constraints can drive sexual size dimorphism, but empirically testing causes of dimorphism is challenging. Manakins (Pipridae), a family of Neotropical birds comprising ~50 species, exhibit both male- and female-biased size dimorphism and are distributed across gradients of precipitation and elevation. Males perform courtship displays ranging from simple hops to complex aerobatic manoeuvres. </span><span lang="EN-GB">We tested associations between sexual size dimorphism and (a) agility and (b) environment, analysing morphological, behavioural, and environmental data for 22 manakin species in a phylogenetic framework. </span><span lang="EN-GB">Sexual dimorphism in mass was most strongly related to agility, with males being lighter than females in species performing more aerial display behaviours. However, wing and tarsus length dimorphism were more strongly associated with environmental variables, suggesting that different sources of selection act on different aspects of body size. These results highlight both the strength of sexual selection in shaping morphology—even atypical patterns of dimorphism—while demonstrating the importance of constraints and ecological consequences of body size evolution.</span></p>