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

In 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. Thus, non-independence among traits may limit 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 expectations of a strong match between form and function, we find subtantial fin shape disparity across species that share a swimming mode. The evolution of fin shape is weakly integrated across the four functionally dominant fins in swimming 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 off-axis diversification via independent trait responses to selection. However, the evolution of novel locomotor modes coincides with a loss of integration among fins. Our study highlights the need for additional work on the functional consequences of fin shape in fishes.