The Convergent Evolution of Snake-like Forms by Divergent Evolutionary Pathways in Squamate Reptiles

Convergent evolution of phenotypes is considered evidence that evolution is deterministic. Establishing if such convergent phenotypes arose through convergent evolutionary pathways is a stronger test of determinism. We studied the evolution of snake-like body shapes in six clades of lizards, each containing species ranging from short-bodied and pentadactyl to long-bodied and limbless. We tested whether body shapes that evolved in each clade were convergent, and whether clades evolved snake-like body shapes following convergent evolutionary pathways. Our analyses showed that indeed species with the same numbers of digits in each clade evolved convergent body shapes. We then compared evolutionary pathways among clades by considering patterns of evolutionary integration and shape of relationship among body parts, patterns of vertebral evolution, and models of digit evolution. We found that all clades elongated their bodies through the addition, not elongation, of vertebrae, and had similar patterns of integration. However, patterns of integration, the body parts that were related by a linear or a threshold model, and patterns of digit evolution differed among clades. These results showed that clades followed different evolutionary pathways. This suggests an important role of historical contingency as opposed to determinism in the convergent evolution of snake-like body shapes.