Variational properties hold a fundamental role in shaping biological evolution, exerting control over the magnitude and direction of evolutionary change elicited by microevolutionary processes that sort variation, such as selection or drift. We studied the Tyrannus genus, as a model for examining the conditions and drivers that facilitate the repeated evolution of exaggerated, secondary sexual traits in the face of significant functional limitations. We study the role of allometry, sexual selection, and their interaction on the diversification of tail morphology in the genus, assessing whether and how they promoted or constrained phenotypic evolution. The exaggerated and functionally-constrained long feathers of deep-forked species, T. savana and T. forficatus, independently diverged from the rest of the genus following the same direction of main interspecific variation common to the entire cluster of species. However, at a macroevolutionary scale those axes summarising both sexual dimorphism and allometric variation of the deep-forked species were aligned with the between-species maximum variation axis of non deep-forked species. Thus, we are presenting evidence of amplified divergence via the co-option and reorientation of allometric shape variation involved in a sexual selection process that repeatedly drove morphology along a historically favoured direction of cladogenetic evolution.

Variational properties hold a fundamental role in shaping biological evolution, exerting control over the magnitude and direction of evolutionary change elicited by microevolutionary processes that sort variation, such as selection or drift. We studied the genus Tyrannus as a model for examining the conditions and drivers that facilitate the repeated evolution of exaggerated, secondary sexual traits in the face of significant functional limitations. In particular, we explore the role of allometry, sexual selection and their interaction, on the diversification of tail morphology in the genus, assessing whether and how they promoted or constrained phenotypic evolution. Non‐deep‐forked species tend to show reduced sexual dimorphism and moderate allometric variation in tail shape. The exaggerated and functionally constrained long feathers of deep‐forked species, T. savana and T. forficatus, which show both marked sexual dimorphism and allometric tail shape variation, independently diverged from the rest of the genus following the same direction of main interspecific variation accrued during the evolution of non‐deep‐forked species. Moreover, the latter direction is also aligned with axes summarising sexual dimorphism and allometric variation on deep‐forked species, a feature lacking in the rest of the species. Thus, exaggerated tail morphologies are interpreted as the result of amplified divergence through reorientation and co‐option of allometric variation by sexual selection, repeatedly driving morphology along a historically favoured direction of cladogenetic evolution.