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.
Caecilians:
terrestrial amphibians
http://www.wildherps.com/images/herps/standard/017614_caecilian.jpg
http://www.wildherps.com/images/herps/standard/017612_caecilian.jpg
http://scienceblogs.com/zooillogix/caecilian.bmp
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
http://www.unexplained-m
ysteries.com/gallery/albums/userpics/22445/
norm
al_ajolote.jpg
There are also partial losses: the Boa has vestigial pelvic limbs
Of course the alternative interpretation is a partial reversal of loss
http://www.edwardtbabinski.us/images/spurs1.jpg
There are several clear examples of reversals of reversals:
This snake has reversed the reversal blocking pectoral limbs
http://dakotabirding.com/Snake_w
legs.jpg
This is another verified reversal of a reversal:
This snake has reversed the reversal blocking pelvic limbs
http://www.telegraph.co.uk/earth/wildlife/6187320/Snake-with-foot-found-in-China.htm
l
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!
http://robandjan.files.wordpress.com/2008/08/snake1.jpg
-- 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 evolutionBut two reversals
-- dark body
-- dark bodyR
-- dark bodyR
TwoForwardEvolutions
Is it easier to evolve or to lose the characteristic?
A homoplasy ofparallelism or…convergence
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