Evolvability in vertebrate segmentation

The number of vertebrae in the axial skeleton of vertebrates is extremely diverse, and reflects adaptations to a diverse range of habitats and lifestyles. The capacity for heritable evolutionary change in the number of vertebrae - its evolvability - is underpinned by the process of somitogenesis, which determines the number of somites that form in the early embryo. However, despite the evolvability of somitogenesis having been crucial for the success of the vertebrates across evolutionary history, the developmental sources of evolvability in somitogenesis are still unknown. Here, we review the evolution of somitogenesis and vertebral number, and attempt to identify sources of evolvability within this important developmental process.