Dollo's law of irreversibility

[6] Stephen Jay Gould suggested that irreversibility forecloses certain evolutionary pathways once broad forms have emerged: "[For example], once you adopt the ordinary body plan of a reptile, hundreds of options are forever closed, and future possibilities must unfold within the limits of inherited design.

They concluded that in order for this protein to evolve in reverse and regain its ability to bind two hormones, several independent neutral mutations would have to occur purely by chance with no selection pressure.

[12] Although the exact threshold for violations of Dollo's law is unclear, there are several case studies whose results dispute the validity of some interpretations.

[13][14] Frietson Galis observed that many of these studies are based on either molecular phylogenies or morphological cladistic analyses that are tenuous and subject to change.

[15] Other proposed 'exceptions' include the ocelli and wings of stick insects,[16][17] the larval stages of salamanders,[18][19] lost toes and re-evolution of oviparity in lizards,[20][21] lost lower teeth in frogs,[22] clavicles in non-avian theropod dinosaurs,[23] and neck, pectoral region, and upper limb musculature in primates, including the lineage leading to humans.