Pharyngula
From Conservapedia
Pharyngula can also refer to a blog written and posted by P.Z. Myers. See Pharyngula_(blog).
Pharyngula is the term used to describe the phylotypic stage of development in embryology.[1] Taxonomically diverse vertebrate embryos all seem to converge to a very similar morphology.[2] The term was coined in 1981 and stems from the characteristic pharyngeal arches that appear.[2] It is defined as:
| “ | The point during development when the basic body plan for a particular higher-level taxa is visible and when all the members of this taxon look most similar.[3] | ” |
It is thought by most researchers that this stage represents the basic vertebrate body plan in the common ancestor of all vertebrates.[2] Though there is some dispute as to how similar embryos are and to the reality of this stage. [4]
Contents |
Conserved morphology
Their 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. [5] 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. [4]
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. [1]
History and criticism
The concept of a highly conserved ontogeny dates back to 1828 and the work by Karl von Baer.[2] 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.[6] 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.[7] [8]
There has been some argument in biology about the reality of the pharyngula stage. Some biologist have argued that there is more variability then previously accepted and that perhaps there is really no phylotypic stage. However, most research to date has agreed that the vast majority of vertebrates do go through a highly similar morphological and conserved stage. [2]
References
- ↑ 1.0 1.1 Pharyngual Period
- ↑ 2.0 2.1 2.2 2.3 2.4 Irie, N., Sehara-Fujisawa, S. (2007). The vertebrate phylotypic stage and an early bilaterian-related stage in mouse embryogenesis defined by genomic information. BMC Biology 5:1. [1]
- ↑ SLACK, J. M. W., P. W. H. HOLLAND, AND C. F. GRAHAM. 1993. The zootype and the phylotypic stage. Nature 361:490–492.
- ↑ 4.0 4.1 Callazo, A. (2000). Developmental variation, homology and the pharyngula stage. Systematic Biology 49:1, 3-18. [2]
- ↑ Embryo Stages
- ↑ http://www.iconsofevolution.com/
- ↑ http://www.talkorigins.org/faqs/wells/haeckel.html
- ↑ http://chem.tufts.edu/AnswersInScience/Coyne-IconsReview.htm
