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