Dollo's law and the irreversibility of digit loss in Bachia.

Several recent studies conclude that exceptions to Dollo's law are more common than used to be thought. If the claims are true this would change our view on the role of developmental constraints in the evolution of body plans. One study claims the reevolution of lost digits in the lizard genus Bachia (Kohlsdorf and Wagner 2006). We evaluate this claim. We conclude that the proposed molecular phylogenetic tree is in conflict with evolutionary mechanisms concerning the biogeography of lizards and with morphology-based phylogenies. A reanalysis of the molecular data does not support the topology of the published tree. Furthermore, two implicit assumptions, that digit numbers are fixed and that polydactyly evolves independently from other characters, are incorrect. We conclude that there is no convincing support for reevolution of digits in Bachia. We discuss our findings in the light of the current evidence for the reversal of losses of complex traits. We conclude that in metazoans, exceptions to Dollo's law are mainly found among meristic traits that originate relatively late during embryogenesis, when developmental systems are more compartmentalized. Finally, our study shows that phylogenetic analyses should incorporate evolutionary mechanisms including constraints, variation, and selection, not only for correct phylogenetic reconstruction, but also for correct evolutionary inference.