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Symmetry Breaking and the Evolution of Development
A. Richard Palmer
Because of its simplicity, the binary-switch nature of left-rightasymmetry permits meaningful comparisons among many differentorganisms. Phylogenetic analyses of asymmetry variation, inheritance,and molecular mechanisms reveal unexpected insights into howdevelopment evolves. First, directional asymmetry, an evolutionarynovelty, arose from nonheritable origins almost as often asfrom mutations, implying that genetic assimilation ("phenotypeprecedes genotype") is a common mode of evolution. Second, themolecular pathway directing hearts leftwardthe nodalcascadevaries considerably among vertebrates (homologyof form does not require homology of development) and was possiblyco-opted from a preexisting asymmetrical chordate organ system.Finally, declining frequencies of spontaneous asymmetry reversalthroughout vertebrate evolution suggest that heart developmenthas become more canalized.
Systematics and Evolution Group, Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, Canada, and Bamfield Marine Sciences Centre, Bamfield, BC V0R 1B0, Canada.
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