I (PR) wrote:
> >>>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.
David Campbell (DC) "bivalve" <bivalve@mail.davidson.alumlink.com> (Wed,
30 May 2001 16:34:01 -0400) wrote:
> 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.
PR: In complex systems like limbs there are many different signals
together, similarities, dissimilarities, similar or different positive
or negative selection, convergence, population dynymics aspects, etc.,
perhaps also some genuine indications of common descent. I should have
pointed out more clearly that my claim that a similarity may indicate a
common functional requirement would apply to a single signal, not
directly to a composite of many signals. Of course it may also be due to
genetic relatedness. Similarly, a dissimilarity may indicate either lack
of relatedness (on a given level) or a different functional requirement.
Your statements regarding the disparity of limb functions and
transitional forms are again different signals requiring interpretation.
Taking all evidence together may very well lead us to the conclusion of
common descent. But my point was that a specific single similarity is,
in itself, ambiguous if it goes together with a similar functional
requirement, a fact which is usually ignored. Similarly, a specific
single dissimilarity is ambiguous if it goes together with dissimilar
functional requirements. The only unambiguous phylogenetic indicators
are signals unconnected to any functional requirement. But it may be
impossible to demonstrate such independence.
PR:
> >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. <
DC:
> 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.
PR: Again, the signals are ambiguous. The similarity between the limb
bones may correlate with a similar developmental situation and a similar
requirement for certain types of limb movements, e.g. twisting, whereas
the dissimilarities between pterosaur, bird, and bat forelimb bones may
correlate with differences between their developmental programs and/or
other differences in their lifestyles. The decision of concluding to a
certain phylogenetic tree is a matter of judgment after considering all
signals in context. But again, the interpretational ambiguities are not
usually taken into consideration.
PR:
> >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.<
DC:
> 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.
PR: We would have to look into the details of each case, most of which I
don't know. I expect there is a somewhat different set of developmental
and functional requirements in each case.
DC:
> 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).
PR: This may be true in general, but have you ever seen anyone (let
alone a textbook) consider the problem of independence between genes and
morphological features, which is a requirement for the reliability of
similar phylogenetic trees confirming each other?
-- -------------------------------------------------------------- Dr Peter Ruest Biochemistry Wagerten Creation and evolution CH-3148 Lanzenhaeusern Tel.: ++41 31 731 1055 Switzerland E-mail: <pruest@dplanet.ch - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - In biology - there's no free lunch - and no information without an adequate source. In Christ - there is free and limitless grace - for those of a contrite heart. --------------------------------------------------------------
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