Phylotypic stage theory

Abstract

Brian Hall[1xHall, B.K. Trends Ecol. Evol. 1997; 12: 461–463Abstract | Full Text PDF | PubMedSee all References[1]provides a thought-provoking and scholarly review of the phylotypic stage theory, and its relation to patterns of development in different phyla. He discusses a recent paper on this subject by MKR, JH and colleagues[2xRichardson, M.K. et al. Anat. Embryol. 1997; 196: 91–106Crossref | PubMed | Scopus (150)See all References[2]. His assessment contributes to current debate concerning the validity of the phylotypic stage as applied to the vertebrates.We regard the phylotypic stage as an archetype and not a real entity. Like Owen's archetype, the vertebrate phylotype applies to all vertebrates in general, but to no one species in detail. Other authors have equated the phylotypic stage with the ancestral condition. We do not support this view because, although primitive patterns of morphogenesis and gene expression are inherited from common ancestors[3xSlack, J.M., Holland, P.W., and Graham, C.F. Nature. 1993; 361: 490–492Crossref | PubMedSee all References[3], these patterns can be modified during evolution. In some vertebrate clades (e.g. eutherian mammals) relatively little evidence of embryonic variation is apparent[2xRichardson, M.K. et al. Anat. Embryol. 1997; 196: 91–106Crossref | PubMed | Scopus (150)See all References[2]. In these cases a phylotypic stage can be recognized insofar as it represents a set of synapomorphic character states (i.e. a cluster of taxic homologies). But in other groups (e.g. anurans and caecilians), embryonic variation—some of it related to divergence in adult body plan rather than to larval adaptations—makes a common stage problematic[2xRichardson, M.K. et al. Anat. Embryol. 1997; 196: 91–106Crossref | PubMed | Scopus (150)See all References, 4xRichardson, M.K. et al. Development. 1998; 125: 151–160PubMedSee all References].The identity of the vertebrate phylotypic stage as an archetype is confirmed when one tries to match it to primary data on morphogenesis[5xSee all References, 6xRichardson, M.K. Dev. Biol. 1995; 172: 412–421Crossref | PubMed | Scopus (106)See all References]. For example, Ballard's pharyngula and the phylotype (tailbud stage) of Slack et al.[3xSlack, J.M., Holland, P.W., and Graham, C.F. Nature. 1993; 361: 490–492Crossref | PubMedSee all References[3]represent different stages; a tailbud zebrafish lacks pharyngeal arches and is not therefore a pharyngula[7xKimmel, C.B. et al. Dev. Dynam. 1995; 203: 253–310Crossref | PubMedSee all References[7]. There is a real problem here: as argued by many biologists (reviewed in Gould[5xSee all References[5]), the pervasive effects of heterochrony make the precise definition of common vertebrate developmental stages impossible. And recent analyses of primary data support this view, suggesting instead the recognition of a phylotypic period within which variations in the timing, sequence and diversity of character appearance may be included[5xSee all References[5].Thinking in terms of types, either as developmental stages or as putative ancestors, can be helpful in searching for order in the diversity of animal life. However we need to be aware of the limitations of typologism.