Transcript Reversal Homoplasy

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)
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.
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/stand
ard/017614_caecilian.jpg
http://www.wildherps.com/images/herps/stand
ard/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
http://www.unexplainedmysteries.com/gallery/albums/userpics/22445/normal_ajolote.jpg
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.edwardtbabinski.us/images/spurs1.jpg
There are also partial losses: the Boa has vestigial pelvic limbs
Of course the alternative interpretation is a partial reversal of loss
http://dakotabirding.com/Snake_wlegs.jpg
There are several clear examples of reversals of reversals:
This snake has reversed the reversal blocking pectoral limbs
http://www.telegraph.co.uk/earth/wildlife/6187320/Snake-with-foot-found-in-China.html
This is another verified reversal of a reversal:
This snake has reversed the reversal blocking pelvic limbs
Of course humans will make false claims!
http://robandjan.files.wordpress.com/2008/08/snake1.jpg
This claim of pectoral limbs is clearly false:
This snake is in the act of swallowing a frog, NOT sprouting legs!
This is likely your cladogram from Page 4 of Cladistics
OG
E
B
A
-- long wing
-- large eye
D
C
-- black eye
-- wide neck
-- long leg
-- dark body
-- heavy leg
-- wide body
-- wide wing
-- long tail
What do we do with the newly-discovered Clade Critter?
OG
E
B
A
-- long wing
-- large eye
D
C
-- black eye
-- wide neck
-- long leg
-- dark body
-- heavy leg
-- wide body
-- wide wing
-- long tail
F
There are two possible explanations--which is most parsimonious?
OG
Two
Forward
Evolutions
F
dark -body
A homoplasy of
parallelism or…
convergence
A single evolution
But two reversals
B
A
D
C
-- dark R
-- black eye
body
-- long wing
-- wide neck
R
-- dark body
-- long leg
-- dark body
-- large eye
-- heavy leg
-- wide body
-- dark body
-- wide wing
-- long tail
F
Is it easier to evolve or to lose the characteristic?