Commentary: The presence of bifurcations as a ‘third component of individual differences’: implications for quantitative (behaviour) genetics

Why is Gärtner's paper so interesting? It is for a number of reasons, but the most interesting is not the finding that phenotypic differences among animals still exist after standardization of genotype and environment (described in the first part of Gärtner's paper). These differences are expected to remain to a certain extent, because complete experimental control is impossible. More interesting is how large these differences are. Particularly interesting are the results of Gärtner's experiments in which he attempts to alter the amount of phenotypic variance by varying the amount of variance in environmental conditions and genetic influences (described in the second part). These results suggested the presence of an additional source of phenotypic variance besides the genotype and environment. Long before Gärtner's paper had been published, various researchers had speculated about the existence of such non-genetic, non-environmental (‘third’) source of variance. These included pioneering geneticists such as Sewall Wright (see the first path diagram ever), Sir Kenneth Mather and Jinks (p. 6) and Douglas Falconer and Mackay (p. 135). As we argue below, whereas this third source of variance was demonstrated in experimental organisms, it is also relevant to the interpretation of human quantitative (behaviour) genetic results (see also previous research).