Fluctuating asymmetry

As a form of biological asymmetry, fluctuating asymmetry (FA), along with anti-symmetry and direction asymmetry, refers to small, random deviations away from perfect bilateral symmetry. This deviation from perfection is thought to reflect the genetic and environmental pressures experienced throughout development, with greater pressures resulting in higher levels of asymmetry. Examples of FA in the human body include unequal sizes (asymmetry) of bilateral features in the face and body, such as left and right eyes, ears, wrists, breasts, testicles, and thighs.

Research has exposed multiple factors that are associated with FA. As measuring FA can indicate developmental stability, it can also suggest the genetic fitness of an individual. This can further have an effect on mate attraction and sexual selection, as less asymmetry reflects greater developmental stability and subsequent fitness. Human physical health is also associated with FA. For example, young men with greater FA report more medical conditions than those with lower levels of FA. Multiple other factors can be linked to FA, such as intelligence and personality traits.

Fluctuating asymmetry (FA) can be measured by the equation: Mean FA = mean absolute value of left sides - mean absolute value of right sides.

The closer the mean value is to zero, the lower the levels of FA, indicating more symmetrical features. By taking many measurements of multiple traits per individual, this increases the accuracy in determining that individual's developmental stability. However, these traits must be chosen carefully, as different traits are affected by different selection pressures.

This equation can further be used to study the distribution of asymmetries at population levels, to distinguish between traits that show FA, directional asymmetry, and anti-symmetry. The distribution of FA around a mean point of zero suggests that FA is not an adaptive trait, where symmetry is ideal. Directional asymmetry of traits can be distinguished by showing significantly biased measurements towards traits being larger on either the left or right sides, for example, human testicles (where the right is more commonly larger), or handedness (85% are right handed, 15% are left handed). Anti-symmetry can be distinguished by the bimodal distributions, due to some adaptive functions.

Fluctuating asymmetry (FA) is often considered to be the product of developmental stress and instability, caused by both genetic and environmental stressors. The notion that FA is a result of genetic and environmental factors is supported by Waddington's notion of canalisation, which implies that FA is a measure of the genome's ability to successfully buffer development to achieve a normal phenotype under imperfect environmental conditions. Various factors causing developmental instability and FA include infections, mutations, and toxins.

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