Stasis, change, and functional constraint in the evolution of animal body plans, whatever they may be
The phrase "body plan" or "bauplan" has been used to mean (1) the characteristic features of a phylum or other taxon of high rank, (2) architectural features of animals (such as symmetry; modular units; types of body walls, body cavities, body openings, and body subdivisions; types of supporting structures; position and structure of organ systems), (3) traits characteristic of an especially invariant stage in a life history (phylotypic stage), or (4) patterns of gene expression that first indicate the development of regions of the body. Multiple meanings of bodyplan within one argument can be misleading, but under all four meanings, body plans of animals have changed after stasis for long periods and after stasis during divergence of other traits. Change in body plans is often associated with an identifiable change in a functional constraint. Examples include decreases in body size and changes in requirements for feeding or locomotion. These observations support the hypothesis that functional constraints contribute to stasis in body plans. There is evidence that ancestral developmental processes constrain directions of evolutionary changes in body plans. There is little evidence that developmental processes prevent changes in body plans, but evidence for developmental constraint is more difficult to obtain than evidence for functional constraint.
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