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12-26-2005 5:14 PM
Michael Richardson has argued in this article  that there is no highly conserved embryonic stage in the vertebrates. This conserved stage is called phlylotypic because it represents the point of “maximum resemblance among members of a phylum or comparable higher Taxon “. It is thought to be resistant to evolutionary change, assuming that “differences among species arise through divergence at later stages of development”. (The phylotypic stage is part of the so called hourglass model, which claims that initial divergence leads to a near identical stage followed by subsequent divergence).
Richardson criticizes members of his own field - developmental biology - for assuming similarities based on sparse or even faulty evidence, whereas he seems to exempt at least some evolutionary biologists form this reproach, as can be seen by the following quote from :
Raff (1992) has pointed out that developmental biologists tend to emphasise the similarities among species, whereas evolutionary biologists emphasise the differences. The result is a long history of disagreement over the extent to which embryonic development is conserved during evolution (reviewed by McKinney and McNamara 1991; Hall 1992; Raff 1992, 1996; McNamara 1995; Richardson 1995).
This is partly because, with a few exceptions (e.g. Slack et al. 1993; Burke et al. 1995), many of our current ideas about the molecular basis of vertebrate development and evolution come from studies of a small number of laboratory species. The assumption that these findings are generally applicable to all vertebrates is not necessarily a valid one (Raff 1996).
In his article Richardson compares the tailbud stage of a wide range of vertebrates covering Agnathans, Cartilaginous fishes, Bony fishes, Amphibians, Reptiles, Birds and Mammals. He concludes:
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)
So, if Richardson is correct - that’s still under debate - what would be the consequence for the theory of evolution? Mainly that there is no need to explain why a certain stage of development is highly resistant to evolutionary change (one would have to demonstrate developmental constraints which prevent adaptation ).
And what would be the consequence for developmental biology? Mainly the replacement of the concept of a phylotypic stage with that of a phylotypic period (see  ). That means to accept that timing is an important factor of development , that therefore pharyngeal pouches, neural tube, somites and a chambered heart can develop at different times in different species. Therefore the concept of a common stage, where all developmental characters typical for the phylum appear, has to dropped.
An additional point would be to accept evolutionary adaptations during this period which foreshadow variations in the adult body plan.