Parallelism

23 Flower evolution is characterized by widespread repetition, with adaptations to pollinator 24 environment evolving in parallel. Recent studies have expanded our understanding of the 25 developmental basis for adaptive floral novelties—petal fusion, bilateral symmetry, heterostyly, 26 and floral dimensions. Here we highlight patterns of trait evolution and review developmental 27 genetic mechanisms underlying floral novelties. We discuss the diversity of mechanisms for 28 parallel adaptation, the evidence for constraints on these mechanisms, and how constraints 29 help explain observed macroevolutionary patterns. We describe parallel evolution resulting from 30 similarities at multiple hierarchical levels—genetic, developmental, morphological, functional— 31 which indicate general principles in floral evolution, including the central role of hormone 32 signaling. An emerging pattern is mutational bias that may contribute to rapid patterns of parallel 33 evolution, especially if the derived trait can result from simple degenerative mutations. We argue 34 that such mutational bias may less likely govern the evolution of novelties patterned by complex 35 developmental pathways.