>>>3) The homology of vertebrate limbs: the main problem with all similarities between functional features is that they may need to be similar in order to function: thus, they cannot be evidence for common descent.
This is incorrect; functional necessities only demonstrate a potential for convergence or a reason for common design, but they can reflect common descent as well. Furthermore, the similarities between vertebrate limbs are perhaps most striking in the disparities of function, with fins, wings, legs, and arms all using the same basic genes and structures yet performing widely differing functions. The existence of numerous transitional forms between different types of limbs also points to the common evolutionary heritage.
>Similar cladistic trees may be a consequence of the features concerned being interdependent. The function of limbs is certainly a consequence of the functions of many component parts. Now, if two components are functionally interdependent, their evolution is interdependent, and we would expect the two cladistic trees formed for the two features in a group of taxa to be similar, no matter how this came about. <
However, the level of interdependence varies. Flight is associated with lengthening of various forelimb bones, but exactly which bones varies between pterosaurs, birds, and bats.
>Just as a similarity between the same feature in two taxa may be a consequence of either common descent or common requirements, so a similarity between the cladistic trees constructed for two component features in the same group of taxa may be a consequence of either a descent acording to the cladistic tree found or of the two components being interdependent. This case has also been described in PSCF 44 (2/1992), 80-94. Of course, if the compound function under consideration has been formed in different ways in different taxa, there will be at least some different interdependencies between the component functions or features, and there might be a stronger argument for evolution.<
Among vertebrate limbs, wings have been formed three ways; flippers and strong limbs for fast running, over seven different ways apiece; and grasping hands, at least four ways. There are generally similar patterns in many of these (e.g., fast running generally involves fusion of bones, strengthening the ankle; loss of side toes and other extra bones; and lengthening of the bones), but the details vary greatly.
As a whole, and taking into consideration the likely sources of error, cladograms based on multiple independent genes and morphological features match well, as do those based on the same gene but different samples (e.g., two analyses that studied different taxa representing the same higher taxa).
Dr. David Campbell
"Old Seashells"
Biology Department
Saint Mary's College of Maryland
18952 E. Fisher Road
St. Mary's City, MD 20686-3001 USA
bivalve@mail.davidson.alumlink.com, 301 862-0372
Fax: 301 862-0996
"Mollusks murmured 'Morning!'. And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!
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