2Tetrapod limbs are complex Ball joint at girdle (pectoral or pelvic)Proximal segment has one heavy boneHinge joint at elbow or kneeDistal segment has two bones for rotation of hand/footSmall cuboidal bones at wrist and ankle for flexibility of hand/foot positionLong metacarpal/metatarsals for palm/instepPhalanges for the digits (fingers/toes)
3Tetrapod limbs are stereotypical The complex structures are shared among these tetrapods:Fishes (some are more plesiomorphic)AmphibiansReptilesBirdsMammalsConclusion: rather than evolving complex limb form and function separately and identically among all these groups of species (i.e., many homoplasies), the tetrapod leg design evolved just once in a common ancestor (i.e., more parsimonious)
4Reversal of a complex trait is comparatively parsimonious Forward evolution of a leg requires modification of:BonesMusclesConnectivesVascular PathsNeural PathsReversal of a complex trait is comparatively parsimoniousReversal could be just ONE point mutation in ONE gene that normally puts limb development into motion.Without the first step, the rest does not happen.
5Tetrapod evolution has been reversed multiple times! Thus, it must be easily done… i.e. is parsimoniousLegless amphibians: caeciliansLegless lizards: glass lizardsAjolote: mole lizardsSnakesSo some gene functioning early in tetrapod development can mutate (become defective), rendering some ancestral species (and its descendants) legless.All the rest of the tetrapod genes that had evolved to make the limbs, are made useless by this one mutation.
7A photo of parent Caecilian with offspring R635RmqosDI/AAAAAAAANYc/jwcNDv1suD4/flesh+eating+amphibian+caecilians%5B2%5D
8Or it is a lizard that has reversed only its pelvic limbs This ajolote is a snake that reversed the reversal blocking only pectoral limbs (note: belly scale pattern and limb location)Or it is a lizard that has reversed only its pelvic limbs
9There are also partial losses: the Boa has vestigial pelvic limbs Of course the alternative interpretation is a partial reversal of loss
10There are several clear examples of reversals of reversals: This snake has reversed the reversal blocking pectoral limbs
11This is another verified reversal of a reversal: This snake has reversed the reversal blocking pelvic limbs
12Of course humans will make false claims! This claim of pectoral limbs is clearly false:This snake is in the act of swallowing a frog, NOT sprouting legs!
13This is likely your cladogram from Page 4 of Cladistics OG E B A D C-- black eye-- long wing-- wide neck-- long leg-- dark body-- large eye-- heavy leg-- wide body-- wide wing-- long tail
14What do we do with the newly-discovered Clade Critter? OG E B A D C-- black eye-- long wing-- wide neck-- long leg-- dark body-- large eye-- heavy leg-- wide bodyF-- wide wing-- long tail
15Is it easier to evolve or to lose the characteristic? There are two possible explanations--which is most parsimonious?OG F B A D C-- darkbodyR-- black eyedark --body-- long wingTwoForwardEvolutions-- wide neck-- dark bodyR-- long leg-- dark body-- large eyeA homoplasy ofparallelism or…convergence-- heavy leg-- wide bodyA single evolutionBut two reversals-- dark bodyF-- wide wing-- long tailIs it easier to evolve or to lose the characteristic?