BILATERAL VARIATION AND THE EVOLUTIONARY ORIGIN OF MACROSCOPIC ASYMMETRIES

Given that characters exhibiting macroscopic asymmetry have evolved in a wide variety of taxa, heritable variation for bilateral asymmetry m ust have arisen at some point in their history. The recognition that h eritable variation may underlie some statistical asymmetries not only raises concerns about the incautious use of statistical estimates of F A in studies of developmental stability, but it suggests some intrigui ng questions about the possible evolutionary origins of macroscopic as ymmetries. First, we developed an additive model of bilateral variatio n based on some simple assumptions about the developmental control of bilateral variation. Second, using a new approach for studying statist ical asymmetries, we conducted an analysis of bilateral variation in e ight metrical traits of a harpacticoid copepod (Tigriopus californicus ) to search for novel forms of statistical asymmetries. The model we d eveloped revealed three independent statistical asymmetries of potenti al evolutionary significance: a) a previously unrecognized form of asy mmetry (referred to here as normal covariant asymmetry), b) antisymmet ry, and c) directional asymmetry. Because each pattern of variation wo uld seem to require different amounts and kinds of developmental-genet ic information [a- only negative feedback between sides (bilateral inh ibition), b- both bilateral inhibition and average departure from symm etry (bilateral offset), c- bilateral inhibition, bilateral offset, an d a consistent overdevelopment of one side or the other (side-bias con trol)], those requiring less information would seem more likely to rep resent earlier stages in the evolution of macroscopic asymmetries. Our analysis of bilateral variation in Tigriopus revealed no evidence for any form of statistical asymmetry other than fluctuating asymmetry. H owever, a significant positive covariation between sides, even after c orrection for body size variation, suggested that factors influencing relative limb length (whether genetic or environmental) affected both sides equally rather than one side at the expense of the other. Finall y, we note that certain statistical asymmetries (directional asymmetry , any form of covariant asymmetry) may render characters unreliable fo r estimating developmental stability because, unlike pure fluctuating asymmetry, they may signal a genetic component to asymmetry variation.