FROM SYMMETRY TO ASYMMETRY - PHYLOGENETIC PATTERNS OF ASYMMETRY VARIATION IN ANIMALS AND THEIR EVOLUTIONARY SIGNIFICANCE

Phylogenetic analyses of asymmetry variation offer a powerful tool for exploring the interplay between ontogeny and evolution because (i) co nspicuous asymmetries exist in many higher metazoans with widely varyi ng modes of development, (ii) patterns of bilateral variation within s pecies may identify genetically and environmentally triggered asymmetr ies, and (iii) asymmetries arising at different times during developme nt may be more sensitive to internal cytoplasmic inhomogeneities compa red to external environmental stimuli, Using four broadly comparable a symmetry states (symmetry, antisymmetry, dextral, and sinistral), and two stages at which asymmetry appears developmentally (larval and post larval), I evaluated relations between ontogenetic and phylogenetic pa tterns of asymmetry variation, Among 140 inferred phylogenetic transit ions between asymmetry states, recorded from 11 classes in five phyla, directional asymmetry (dextral or sinistral) evolved directly from sy mmetrical ancestors proportionally more frequently among Larval asymme tries, In contrast, antisymmetry, either as an end state or as a trans itional stage preceding directional asymmetry, was confined primarily to postlarval asymmetries, The ontogenetic origin of asymmetry thus si gnificantly influences its subsequent evolution, Furthermore, because antisymmetry typically signals an environmentally triggered asymmetry, the phylogenetic transition from antisymmetry to directional. asymmet ry suggests that many cases of laterally fixed asymmetries evolved via genetic assimilation.