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  Topic: Discussing "Explore Evolution", Have at it.< Next Oldest | Next Newest >  
Paul Nelson

Posts: 43
Joined: July 2007

(Permalink) Posted: July 20 2007,09:54   

Afarensis wrote:

The morphology that was transitioning was not based on size so "smoothing" the scaling to make them look similar is irrelevant.

Right.  But if size is irrelevant to the characters involved in the transitional series, and in any case is easily modified genetically, why not just depict the fossils using the same scale (so that the relative sizes of the actual specimens is clear to the reader)?  If size doesn't matter, showing the fossils as one might see them lined up in a museum drawer shouldn't be a problem.

The accurate representation of data is important, especially when most students will never see the actual fossils in question.

JAM, can you say which quotes from the box "Coming Out of Their Shell?" you find objectionable, and why?  Also, Burke's data were interpreted by Rieppel (2001) as disproving the "correlated progression" model for turtle evolution, advanced by Kemp and others.  Rieppel writes:

The turtle body plan is evidently highly derived, indeed unique among tetrapods.  The problem for an evolutionary biologist is to explain these transformations in the context of a gradualistic process.  Given the recently obtained developmental evidence [Rieppel cites Burke 1989 here], the theory of "correlated progression" presents an incomplete explanation of the turtle body plan....Ribs can only be located either deep to, or superficial to, the scapula.  There are no intermediates, and there is only one way to get from one condition to the other, which is the redirection of the migration, through the embryonic body, of the precursor cells that will form the ribs.

O. Rieppel, "Turtles as hopeful monsters," BioEssays 23 (2001):987-991; pp. 990-991.

For his part, Kemp responds:

[Correlated progression] stands in contrast to an alternative view of the origin of turtles, expressed most recently by Rieppel (2001 [citing Burke]), that the rib-vertebrae-carapace-limb complex is too radically different from the ancestral amniote condition to have evolved gradually, but must have resulted from a macromutational event caused by a radical change in early development.  The difficulty with Rieppel's hypothesis is that it must account for how this sudden developmental change also caused what must have been simultaneous, but functionally integrated shifts in many other traits, notably the musculature, limb function, central neural control of locomotion, ventilation mechanism, dietary shift away from faunivory and so on: it is unrealistic in the extreme that any single macromutation could have such a comprehensive effect.

T.S. Kemp, "The concept of correlated progression as the basis for a model for the evolutionary origin of major new taxa," Proc. R. Soc. B. 274 (2007):16671673; pp. 1669-1670.

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