posted 18. September 2003 23:56                   

quote:

---

The concept of a phylotypic stage, when all vertebrate embryos show low phenotypic diversity, is an important cornerstone underlying modern developmental biology. Many theories involving patterns of development, developmental modules, mechanisms of development including developmental integration, and the action of natural selection on embryological stages have been proposed with reference to the phylotypic stage. However, the phylotypic stage has never been precisely defined, or conclusively supported or disproved by comparative quantitative data. We tested the predictions of the ‘developmental hourglass’ definition of the phylotypic stage quantitatively by looking at the pattern of developmentaltiming
variation across vertebrates as a whole and within mammals. For both datasets, the results using two different metrics were counter to the predictions of the definition: phenotypic variation between species was highest in the middle of the developmental sequence. This surprising degree of developmental character independence argues against the existence of a phylotypic stage in vertebrates. Instead, we hypothesize that numerous tightly delimited developmental modules exist during the mid embryonic period. Further, the high level of timing changes (heterochrony) between these modules may be an important evolutionary mechanism giving rise to the diversity of vertebrates. The onus is now clearly on proponents
of the phylotypic stage to present both a clear definition of it and quantitative data supporting its existence.

---

posted 19. September 2003 04:02                    

• "Von Baer's law makes good sense" -- Gould is not referring to von Baer's laws (plural). He is a careful writer, and is referring to only one "law". Gould's article further makes that clear when it defines "von Baer's law" as a "principle of differentiation" "christened" by people other than von Baer, presumably evolutionists, (whom von Baer opposed). Therefore it cannot be von Baer's laws: (1) which contain no such "principle of differentiation," and (2) were clearly defined by von Baer. Gould's discussion is somewhat ambiguous for our purposes here. Gould's article does not give an explicit explanation of von Baer's laws, because it does not unambiguously address "von Baer's laws," and because what little 'explanation' it does provide is implicit and incomplete (that is, it relies on us to fill-in key pieces), and because his explanation is falsified by earliest ontogeny -- which shows considerable diversity, contrary to his explanation. I do not fault Gould for that, since his article was about personalities, Haeckle's faked embryological drawings, and the history of science -- not von Baer's laws. (in the March 2000 Natural History)
• "nothing in Darwinian theory implies or requires [the von Baer's law] validity" -- Gould is absolutely correct there! As I previously said, the information flow is in the opposite direction -- life's patterns give shape to an otherwise amorphous evolutionary theory. Here it is again from Gould: "the validity and relative frequency of von Baer's law remains an open, empirical question within evolutionary theory, an issue that can be resolved only by observational evidence from a wide variety of organisms."