Abstract
Phenotypic variation arises from genetic and environmental variation, as well as random aspects of development. The genetic (nature) and environmental (nurture) components of this variation have been appreciated since at least 1900. The random developmental component (noise) has taken longer for quantitative geneticists to appreciate. Here, I sketch the historical development of the concepts of random developmental noise and developmental instability, and its quantification via fluctuating asymmetry. The unsung pioneers in this story are Hugo DeVries (fluctuating variation, 1909), C. H. Danforth (random variation between monozygotic twins, 1919), and Sewall Wright (random developmental variation in piebald guinea pigs, 1920). The first pioneering study of fluctuating asymmetry, by Sumner and Huestis in 1921, is seldom mentioned, possibly because it failed to connect the observed random asymmetry with random developmental variation. This early work was then synthesized by Boris Astaurov in 1930 and Wilhelm Ludwig in 1932, and then popularized by Drosophila geneticists beginning with Kenneth Mather in 1953. Population phenogeneticists are still trying to understand the origins and behavior of random developmental variation. Some of the developmental noise represents true stochastic behavior of molecules and cells, while some represents deterministic chaos, nonlinear feedback, and symmetry breaking.
Highlights
Charles Darwin [1] (p. 127) recognized that individuals of the same species vary among one another, producing an “infinite diversity in structure, constitution, and habits.” He recognized that heritable variation is the raw material upon which natural selection acts
Population phenogenetics involves the marriage of developmental biology and population biology
By studying fluctuating asymmetry and phenotypic variation of bilaterally symmetrical traits, population phenogeneticists can estimate the upper limit on genotypic variation
Summary
Charles Darwin [1] (p. 127) recognized that individuals of the same species vary among one another, producing an “infinite diversity in structure, constitution, and habits.” He recognized that heritable variation is the raw material upon which natural selection acts. Devoted three chapters of Origin of Species to the problem of variation—variation under domestication, variation under nature, and the laws of variation. Despite an opening section of Origin of Species entitled “The Causes of Variation,” Darwin was, acutely aware that those causes were entirely unknown He openly admitted that our “ignorance of the laws of variation is profound. The non-genetic component, com called the environmental component, is attributable to macro- and 2moifc1r4o-environ variation, maternal effects, ontogenetic variation, and random aspects of developm fact, the environmental component is something of a misnomer, since random Developmental noise can be quantified by holding the genotype and environment constant and measuring the phenotypic variation This can be achieved most by examining the variation among genetically identical individuals (clones) raised in a common environment, or by examining the minor asymmetries of symmetrical structures— fluctuating asymmetries
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