Three Forms of Subtle Bilateral Variation
Fluctuating asymmetry refers to a pattern of bilateral variation where variation on the right and left sides is both random and independent. It tends to be small (around 1% of trait size or less) and has become popular as a measure of 'quality', 'stress', 'health' or 'fitness' (e.g., increased stress experienced during development often yields increased fluctuating asymmetry= wider spread to the bilateral variation). However, its developmental origins remain obscure, though much debated. Both destabilizing (developmental noise) and stabilizing (developmental stability) forces may influence its magnitude.
Directional asymmetry arises when (i) one side is larger than the other on average, and (ii) the larger member of a bilateral pair tends to be on the same side.
Antisymmetry (sometimes referred to as 'random' asymmetry when applied to conspicuous asymmetries) arises when (i) one side is larger than the other on average, but (ii) the larger member of a bilateral pair occurs on either the right or left side at random.
Directional asymmetry and antisymmetry may either be quite subtle (a few percent of trait size) or quite conspicuous. Both imply (though do not demonstrate) repeatable effects of environment or genotype on asymmetry and thus may not qualify for use as measures of developmental precision.