Abstract
Fluctuating asymmetry, the random deviation from perfect symmetry, is a widely used population-level index of developmental instability, developmental noise, and robustness. It reflects a population’s state of adaptation and genomic coadaptation. Here, we review the literature on fluctuating asymmetry of human populations. The most widely used bilateral traits include skeletal, dental, and facial dimensions; dermatoglyphic patterns and ridge counts; and facial shape. Each trait has its advantages and disadvantages, but results are most robust when multiple traits are combined into a composite index of fluctuating asymmetry (CFA). Both environmental (diet, climate, toxins) and genetic (aneuploidy, heterozygosity, inbreeding) stressors have been linked to population-level variation in fluctuating asymmetry. In general, these stressors increase average fluctuating asymmetry. Nevertheless, there have been many conflicting results, in part because (1) fluctuating asymmetry is a weak signal in a sea of noise; and (2) studies of human fluctuating asymmetry have not always followed best practices. The most serious concerns are insensitive asymmetry indices (correlation coefficient and coefficient of indetermination), inappropriate size scaling, unrecognized mixture distributions, inappropriate corrections for directional asymmetry, failure to use composite indices, and inattention to measurement error. Consequently, it is often difficult (or impossible) to compare results across traits, and across studies.
Highlights
Research on the developmental instability of human populations, as estimated by fluctuating asymmetry, has blossomed only in the past 35 years
There are no consistent relationships between asymmetry and size in the data, whether we looked at males and females positive relationships between asymmetry and size in the data, whether we looked at males and separately or pooled
Given that others have found no relationship between tooth asymmetry and tooth size [65,66], the elevated asymmetry of the Down syndrome sample could have been magnified by the transformation; Down syndrome individuals have smaller teeth and scaling by tooth size would have increased the relative asymmetry of smaller teeth
Summary
Research on the developmental instability of human populations, as estimated by fluctuating asymmetry, has blossomed only in the past 35 years. The domain of population geneticists and evolutionary biologists studying Drosophila, Mus, and other species of plants and animals, nearly 60 percent of the more than 900 papers published in 2015 that mentioned fluctuating asymmetry in their titles, their text, or as a keyword, mentioned humans. Fluctuating asymmetry is the random developmental variation of a trait (or character) that is perfectly symmetrical, on average It is estimated as the variance of the difference between right and left sides, Var(di ), where di = Ri − Li and Ri is the value of a trait on the right side and Li is the value of the same trait on the left side of individual i.
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