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Common Patterns of Development in Animals

and their Evolutionary Significance

Presentation by Mark Steinhilber

Rapporteur, Keith L. Jackson

Based on the paper: Hall, B. K. 1996. Baupläne, phylotypic stages, and constraint. Evolutionary Biology. 29:215-261.

Presentation summary.&emdash;Body plan is an old concept. Aristotle noted the unity of plan in animals. Buffon proposed the ideal morphotype, a body plan that typifies a group of animals. Later workers noted that embryology is fundamental to body plan. Cuvier argued form determines function&emdash; a typological view. Geoffrey argued function determines form&emdash; an evolutionary view. Von Baer took this evolutionary view further by proposing the biogenic law, in which higher organisms evolve from lower organisms by addition to embryonic development. In stating this, he noted the embryological criterion of homology. Haeckel proposed the Gastrea hypothesis, in which the gastrulation stage is the point in ontogeny that all animals are most similar and proposed Gastrea as the hypothetical common ancestor. Darwin used these developments in embryology to support his theory of natural selection. Woodger (1945) proposed bauplän (German) to describe the common body plan of a taxon (usually referring to phyla).

An explosive radiation of baupläne occurred in the lower Cambrian. Several theories to explain this early diversity have been put forth. The hard body part theory proposes that diversity predated the Cambrian and hard body parts simultaneously arose in the early Cambrian. This theory has problems in not being parsimonious (i.e., hard body parts arising simultaneously in many taxa). Valentine proposed that a major extinction event at the end of the Precambrian opened new set of niches into which novel baupläne could fill. The end of the Permian also saw a massive extinction event, but no new baupläne, however. Other ideas on the cause of this early diversity include pervasive environmental change in the early Cambrian and Hox genes preadapting organisms for morphological radiation. Much of this diversity disappeared by the end of the Cambrian leaving all extant phyla, and baupläne. Theories explaining why new baupläne have not arisen include: reduction of evolutionary rates since the Cambrian&emdash; not likely as modern species are capable of rapid evolution; Gould proposed that all niches became full and there is no "space" for new baupläne to fit in; and developmental systems became too complex to easily be restructured.

How does a bauplän arise and why is it stable? There is three ways a bauplän can arise: saltation in which the entire bauplän evolves at once ("hopeful monster"); mosaic evolution in which the characters defining a plan evolve independently and eventually achieve the currently observed bauplän; or the correlated progressive hypothesis&emdash; slow evolution of all characters defining a bauplän (somewhat intermediate between the first two hypotheses?). Stability of bauplän may be the result of stabilising selection against nonconformists to the complex developmental system ("evolutionary ratchet") or evolutionary constraint. Examples of constraint of bauplän include: developmental mechanisms already being maximally efficient; canalisation, in which a standard end product is selected for regardless of development process (e.g., phylotypic stage, where all in a phylum show the greatest degree of similarity); or a limited genetic system (e.g., stable hox genes).

Have new baupläne have arisen since the Cambrian explosion? Perhaps. The sponge genus Asbestopluma lacks choannocytes and canal system typical of Porifera and has adapted unique spicules for a predatory way of life. Some argue that this is a new bauplän developed by adaptation.

Presentation and discussion April 6, 1998. Report distributed April 14, 1998.


Discussion

Discussants: Stephanie Zaklan and Gregory Dueck

Clarify bauplän. Is it a case of semantics or does this term have a morphological or molecular basis? If so is it a useful term? Discussion opens: One vote for no, followed by general nodding amongst the crowd. To be useful, are baupläne comparable? Does it constitute anything more than characters of a phylum? Can there be baupläne outside the phylum? General consensus that baupläne is poorly defined and not a very useful term. It seems to be a holdover of outdated ideas. Perhaps last common ancestor (hypothetical) is a more relevant term.

Clarify the distinction between bauplän and phylotypic stage. Are they the same? Discussion opens: No the phylotypic stage refers to a single stage of ontogeny. Baupläne refers to a developmental progression. Phylotype is a stage of a baupläne.

Lets make some working definitions. Discussion opens: Bauplän, totality of developmental stages. Phylotype, point in development when all members of a phylum show the greatest degree of similarity. Archetype, an ideal organism that embodies its taxon (an out dated concept, perhaps last common ancestor is more appropriate, reiterated).

Arrested development implies that lower organisms incomplete higher organisms. This seems to be a hold over from evolutionary progression or ladder ideology. In spite of this, it is still pervasive, even in cladistics. Discussion opens: Caldistics is specifically presented to be objective. Any reading into outgroups as being "primitive" and ingroups being "advanced" is missing the point.

Cuvier and Geoffrey's arguments on the importance of form and function are manifested in Halls stages of development in which form determines function until a turning point when function determines form. Is this turning point at the phylotypic stage or when embryonic gene products take over maternal? Discussion opens: Probably not based on gene products, as there is enormous variation there. There is probably not a distinct turning point, as development of different organ systems occurs at different rates..

Ontogeny recapitulating phylogeny was used as the ontogenetic criterion of cladistics to break tautology inherent with outgroup determination of character polarity. Haekel became the cladists' hero. No further discussion.

Constraints&emdash; why are new niches not filled by new body plans? Discussion opens: Perhaps existing body plans are preadapted to fill in new niches quickly before new body plans arise. Can any living organism give rise to a new phylum or does a phylum have any special significance? No, just our general ignorance of their interrelationships. In traditional classification, phylum has meaning (albeit subjective). Perhaps current phyla should be classified cladistically.

When are constraints most prominent? Would Hall's proposed experiment work? Discussion opens: No, his rough estimates of mortality are grossly overestimated and it is questionable of whether one stage of development in particular would be the bottleneck. How about two bottlenecks, or more? Try to prove the "evolutionary ratchet". Good luck.

Time constraint forces a reluctant end of discussion (2:05 P.M.).

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