Weak integration allows novel fin shapes and spurs locomotor diversity in reef fishes.

In complex functional systems composed of many traits, selection for specialized function can induce trait evolution by acting directly on individual components within the system, or indirectly through networks of trait integration. However, strong integration can also hinder diversification into regions of trait space that are not aligned with axes of covariation among traits. As a result, non-independence among traits may limit capacity for functional expansion. We explore this dynamic in the evolution of fin shapes in 106 species from 38 families of coral reef fishes, a polyphyletic assemblage that shows exceptional diversity in locomotor function. Despite strong shared developmental pathways and expectations of a strong match between form and function, we find that species that share swimming mode show substantial disparity in fin shape, and preferred swimming mode is a poor predictor of fin shape. The evolution of fin shape is weakly integrated across the four functionally dominant fins in swimming (the pectoral, caudal, dorsal, and anal fins) and integration is weakened as derived swimming modes evolve. The weak integration among fins in the ancestral locomotor condition provides a primary axis of diversification while allowing for substantial off-axis diversification via independent trait responses to selection. However, the evolution of novel locomotor modes coincides with a loss of integrated axes of covariation among fins. Our study highlights the need for additional work on the functional consequences of fin shape in fishes and impact of evolutionary integration on functions other than locomotion.