1. Reversal Homoplasy
The Case of Tetrapods

2. 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)

3. 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)

4. Reversal of a complex trait is comparatively parsimonious
Forward evolution of a leg requires modification of:
Bones
Muscles
Connectives
Vascular Paths
Neural Paths
Reversal of a complex trait is comparatively parsimonious
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.

5. 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.

6. terrestrial amphibians
Caecilians:
terrestrial amphibians

7. A photo of parent Caecilian with offspring
R635RmqosDI/AAAAAAAANYc/jwcNDv1suD4/flesh+eating+amphibian+caecilians%5B2%5D

8. Or 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

9. There are also partial losses: the Boa has vestigial pelvic limbs
Of course the alternative interpretation is a partial reversal of loss

10. There are several clear examples of reversals of reversals:
This snake has reversed the reversal blocking pectoral limbs

11. This is another verified reversal of a reversal:
This snake has reversed the reversal blocking pelvic limbs

12. 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!

13. This 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

14. What 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 body F
-- wide wing
-- long tail

15. Is it easier to evolve or to lose the characteristic?
There are two possible explanations--which is most parsimonious?
OG F B A D C
-- dark
bodyR
-- black eye
dark --
body
-- long wing
Two
Forward
Evolutions
-- wide neck
-- dark bodyR
-- long leg
-- dark body
-- large eye
A homoplasy of
parallelism or…
convergence
-- heavy leg
-- wide body
A single evolution
But two reversals
-- dark body F
-- wide wing
-- long tail
Is it easier to evolve or to lose the characteristic?