Presentation on theme: "Reversal Homoplasy The Case of Tetrapods. Tetrapod limbs are complex Ball joint at girdle (pectoral or pelvic) Proximal segment has one heavy bone Hinge."— Presentation transcript:
Reversal Homoplasy The Case of Tetrapods
Tetrapod limbs are complex Ball joint at girdle (pectoral or pelvic) Proximal segment has one heavy bone Hinge joint at elbow or knee Distal segment has two bones for rotation of hand/foot Small cuboidal bones at wrist and ankle for flexibility of hand/foot position Long metacarpal/metatarsals for palm/instep Phalanges for the digits (fingers/toes)
Tetrapod limbs are stereotypical The complex structures are shared among these tetrapods: Fishes (some are more plesiomorphic) Amphibians Reptiles Birds Mammals Conclusion: 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)
Reversal of a complex trait is comparatively parsimonious Forward evolution of a leg requires modification of: Bones Muscles Connectives Vascular Paths Neural Paths Reversal 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.
Tetrapod evolution has been reversed multiple times! Thus, it must be easily done… i.e. is parsimonious Legless amphibians: caecilians Legless lizards: glass lizards Ajolote: mole lizards Snakes So 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.
A photo of parent Caecilian with offspring R635RmqosDI/AAAAAAAANYc/jwcNDv1suD4/flesh+eating+amphibian+caecilians%5B2%5D
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 mysteries.com/gallery/albums/userpics/22445/normal_ajolote.jpg
There are also partial losses: the Boa has vestigial pelvic limbs Of course the alternative interpretation is a partial reversal of loss
There are several clear examples of reversals of reversals: This snake has reversed the reversal blocking pectoral limbs
This is another verified reversal of a reversal: This snake has reversed the reversal blocking pelvic limbs
Of 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!
-- wide wing -- long tail This is likely your cladogram from Page 4 of Cladistics -- heavy leg -- wide body -- large eye -- long leg -- dark body -- black eye -- wide neck-- long wing OG E B A D C
-- wide wing -- long tail What do we do with the newly-discovered Clade Critter? -- heavy leg -- wide body -- large eye -- long leg -- dark body -- black eye -- wide neck-- long wing OG E B A D C F
-- wide wing -- long tail There are two possible explanations--which is most parsimonious? -- heavy leg -- wide body -- large eye -- long leg -- dark body -- black eye -- wide neck -- long wing OG F B A D C F dark -- body A single evolution But two reversals -- dark body -- dark body R -- dark body R Two Forward Evolutions Is it easier to evolve or to lose the characteristic? A homoplasy of parallelism or… convergence