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