Homology and convergence in morphology
 
 
 

Pentadactyl pattern in vertebrate forelimbs
 
 
 
 

vertebrate forelimbs

 

This resemblence is often cited as evidence for common ancestry.
 
 

Is the evidence presented selectively?

Is the evidence truly distinctive to only one interpretation?

Counter evidence not often discussed:
 
 

a.  Lack of correspondence of developmental pathways in some cases
 

"The classic examples of this problem are salamanders.  In most vertebrate limbs, development of the digits proceeds from posterior to anterior - that is, in the tail-to-head direction.  This accurately describes frogs, but their fellow amphibians, salamanders, do it differently.  In salamanders, development of the digits proceeds in the opposite direction, from head to tail.  The difference is so striking that some biologists have argued that the evolutionary history of salamanders must have been different from all other vertebrates, including frogs."

J. Wells, Icons of Evolution, pg 72.
 

The pentadactyl patterrn develops very differently in humans and in frogs.  In humans, programmed cell death (apoptosis) divides the ridges into five regions that then develop into digits (fingers and toes).  But in frogs, the digits grow outwards from buds, as cells divide.

see  Langman's Medical Embyology, 7th ed. 1995 pg 154-157; and Tyler, M. J., Australian Frogs:  a natural history, 1999, pg 80.
 
 

b.  pentadactyl pattern in vertebrate hindlimbs
 

"But the hindlimbs of all vertebrates also conform to the pentadactyl pattern and are stikingly similar to the forelimbs in bone structure and in their detailed embryological development.  Yet no evolutionist claims that the hindlimb evolved from the forelimb, or that hindlimbs and forelimbs evolved from a common source."

M. Denton, Evolution:  A Theory in Crisis, 1986, pg 151.

"There is no doubt that in terms of evolution the fore- and hindlimbs must have arisen independently, the former supposedly evolving from the pectoral fins of a fish, the latter from the pelvic fins.  Here is a case of profound resemblence which cannot be explained in terms of a theory of descent.

M. Denton, Evolution:  A Theory in Crisis, 1986, pg 151.

"Whatever the ultimate explanation for this remarkable pattern turns out to be, there seems little intellectual satisfaction in attributing one case of correpondence to evolution while refusing it in the other."

M. Denton, Evolution:  A Theory in Crisis, 1986, pg 153.

 
 

c.  There are many examples of striking morphological similarities that are clearly not due to common ancestry
 

Certain marsupial and placental mammals

  placentals and marsupials

 

 
The placentals on the left and the marsupials on the right are said to have evolved independently.  In this view, all of the placentals are more closely related to each other than they are to any of the marsupials, and vice versa.  So the house mouse is believed to be more closely related to the gray wolf than to the marsupial mouse.
 

Common ancestry is viewed by many as the only logical explanation for hierarchical patterns of morphological similarity.  However, examples of morphological similarities which are not due to common ancestry abound.  The term "convergence" is used for the phenomenon of natural selection apparently arriving at a similar complex design more than once.  Convergence is ubiquitous.
 

"My view is that the ubiquity of convergence makes it crucial for understanding the history of life."

Simon Conway Morris, New Scientist Nov 16, 2002 pg 26-29.
 
 

How does the ubiquity of convergence impact the use of morphological similarities as evidence for common ancestry?