Developmental constraints versus flexibility in morphological evolution

Abstract

Evolutionary developmental biology has encouraged a change of research emphasis from the sorting of phenotypic variation by natural selection to the production of that variation through development¹. Some morphologies are more readily generated than others, and developmental mechanisms can limit or channel evolutionary change². Such biases determine how readily populations are able to respond to selection³, and have been postulated to explain stasis in morphological evolution⁴ and unexplored morphologies⁵. There has been much discussion about evolutionary constraints^6,7,8 but empirical data testing them directly are sparse^9,10. The spectacular diversity in butterfly wing patterns¹¹ is suggestive of how little constrained morphological evolution can be. However, for wing patterns involving serial repeats of the same element, developmental properties suggest that some directions of evolutionary change might be restricted^12,13. Here we show that despite the developmental coupling between different eyespots in the butterfly Bicyclus anynana, there is great potential for independent changes. This flexibility is consistent with the diversity of wing patterns across species and argues for a dominant role of natural selection, rather than internal constraints, in shaping existing variation.