Pharyngula

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Pharyngula can also refer to a blog written and posted by P.Z. Myers. See Pharyngula_(blog).

Pharyngula is a term used by evolutionists to describe a hypothetical phylotypic stage of development in embryology.[1] It is mistakenly thought by most evolutionists that this stage represents the basic vertebrate body plan in the common ancestor of all vertebrates.[2] There is currently a dispute among scientists as to how similar embryos are and to the reality of this stage. [3]

Contents

Conserved morphology

There are six stages to embryonic development, and the pharyngula stage is towards the middle. In the early stages of development there is significant diversity in the morphology of embryos, this diversity decreases over time till the pharyngula stage where they are most similar (often difficult for anyone but trained embryologist to differentiate), and finally in the last stages of development morphology diversifies again. [4] It is hypothesized that the reason the pharyngula stage is so morphologically constrained is that this is the point where sequential activation of hox genes is initiated so any strong deviations from the developmental plan would lead to drastic changes in the final phenotype of the organism. [3]

Observable structures

All the major structures of the vertebrate are developed by this period. This includes:

  • notochord
  • dorsal hollow nerve cord
  • post-anal tail, and
  • paired branchial arches (pharyngeal arches)

All major tissue types of differentiated and all major organs are present. By the time the embryo leaves the pharyngula stage it will appear very similar to its adult form.

History and criticism

The concept of a highly conserved ontogeny dates back to 1828 and the work by Karl von Baer. Baer's work was cited by Charles Darwin and used in support of his Theory of Evolution. The concept was made famous though by Ernst Haeckel in 1874 with the publication of his drawings of the conserved stage. Haeckel was mainly pushing the concept of recapitulation in which he hypothesized that ontological development repeated the evolutionary steps of the organism. Recapitulation has since been discredited and is not accepted by any modern biologist. Haeckel has been accused of falsifying his embryonic drawings, most notably by Jonathan Wells in his book Icons of Evolution.[5] Some biology text books used Haeckel's drawings for many years after it was known they were faked. However, most modern biology textbooks only use them now for historical reference and actual photos of embryos are used to discuss the pharyngula stage.[6][7][8]

See also

Pharyngula can also refer to a blog written and posted by P.Z. Myers. See Pharyngula_(blog).

References

  1. http://zfin.org/zf_info/zfbook/stages/phar.html Pharyngual Period]
  2. http://creation.com/embryonic-recapitulation-questions-and-answers
  3. 3.0 3.1 Callazo, A. (2000). Developmental variation, homology and the pharyngula stage. Systematic Biology 49:1, 3-18. [1]
  4. Embryo Stages
  5. http://www.iconsofevolution.com/
  6. http://creation.com/embryonic-recapitulation-questions-and-answers
  7. http://creation.com/appendix-1-common-arguments-for-evolution-that-have-been-rejected
  8. http://chem.tufts.edu/AnswersInScience/Coyne-IconsReview.htm
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