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Author Topic:   Is Talkorigin a propaganda site? The “phylotypic stage” argument
bernd
Member (Idle past 3981 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 1 of 4 (282907)
01-31-2006 1:58 PM


Randman claimed in this post Message 3 that talkorigin
quote:
(..) still insist the phylotypic stage is true, imo, resurrecting an unsubstantiated claim thoroughly refuted by Richardson's 1997 study titled:
There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development
Richardson criticizes in his article [1] the idea that all vertebrate pass through a virtual identical stage, which would hint at highly conserved developmental constraints. Instead he suggests that evolutionary mechanism can modify all embryonic stages, a concept which may help to explain macro evolutionary change:
In summary, evolution has produced a number of changes in the embryonic stages of vertebrates including:
1. Differences in body size
2. Differences in body plan (for example, the presence or
absence of paired limb buds)
3. Changes in the number of units in repeating series
such as the somites and pharyngeal arches
4. Changes in the pattern of growth of different fields
(allometry)
5. Changes in the timing of development of different
fields (heterochrony)
These modifications of embryonic development are difficult to reconcile with the idea that most or all vertebrate clades pass through an embryonic stage that is highly resistant to evolutionary change. This idea is implicit in Haeckel’s drawings, which have been used to substantiate two quite distinct claims. First, that differences between species typically become more apparent at late stages. Second, that vertebrate embryos are virtually identical at earlier stages. This first claim is clearly true. Our survey, however, does not support the second claim, and instead reveals considerable variability - and evolutionary lability - of the tailbud stage, the purported phylotypic stage of vertebrates. We suggest that not all developmental mechanisms are highly constrained by conserved developmental mechanisms such as the zootype. Embryonic stages may be key targets for macro evolutionary change


Myers does not claim that all developmental mechanism are highly constrained by developmental mechanism nor that the “phylotypic stage” is virtual identical in all vertebrates. He states in [2]:
Modern theories of development and evolution propose something that fits the observations, and that Wells cannot easily dismiss. Genes can be modified to act at virtually any point in development, so the theoretical constraint imposed by Haeckel is nonexistent. Variations between species at the earliest stages were a problem for Haeckel, but are not incompatible at all with modern developmental biology. There isn't even a requirement for absolute morphological identity at the phylotypic stage. As Wells points out, Michael Richardson has been identifying variation within that stage between species.

The main difference seems to be one of terminology, that is whether the expression “phylotypic stage” should be replaced for example by “phylotypic period” as Richardson suggests in [3]. Richardson’s proposal is - as far as I know - still under debate, therefore I wouldn’t criticize Myers for using the former term.


-Bernd
P.S.
Admin Nosy asked me to put this into a seperate thread, see Message 23



References

[1] MK Rich Ardson - MK Blog Rich
[2] Wells and Haeckel's Embryos
[3] MK Rich Ardson - MK Blog Rich

Replies to this message:
 Message 2 by AdminWounded, posted 01-31-2006 2:54 PM bernd has replied

  
AdminWounded
Inactive Member


Message 2 of 4 (282916)
01-31-2006 2:54 PM
Reply to: Message 1 by bernd
01-31-2006 1:58 PM


Do you feel this is distinct enough from the, 'Richardson on Haeckel, a revision' topic to warrant its own thread? Could they be combined into one?
If not I am happy to promote this topic on its own.
TTFN,
AW

This message is a reply to:
 Message 1 by bernd, posted 01-31-2006 1:58 PM bernd has replied

Replies to this message:
 Message 3 by bernd, posted 01-31-2006 6:00 PM AdminWounded has not replied

  
bernd
Member (Idle past 3981 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 3 of 4 (282957)
01-31-2006 6:00 PM
Reply to: Message 2 by AdminWounded
01-31-2006 2:54 PM


Request for promotion
Yes, I think they are distinct enough to warrant an own thread. In this thread the focus should be on the question whether talk.origins is a propaganda site, specifically whether their usage of the term phylotypic stage has been thoroughly refuted by Richardson’s 1997 study “There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development”. In other words the discussion should focus on the question whether there is a substantial difference between Myers and Richardson’s concept of the developmental period in question.
On the other hand I hope the thread “Richardson on Haeckel, a revision” will be dealing with the reasons for Richardson’s re-evaluation of Haeckel's theory and drawings in his study “Haeckel's ABC of evolution and development” (2002).
-Bernd

This message is a reply to:
 Message 2 by AdminWounded, posted 01-31-2006 2:54 PM AdminWounded has not replied

  
AdminWounded
Inactive Member


Message 4 of 4 (283050)
02-01-2006 4:38 AM


Thread moved here from the Proposed New Topics forum.

  
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