Social Epistasis Amplifies the Fitness Costs of Deleterious Mutations, Engendering Rapid Fitness Decline Among Modernized Populations

Of Mice and Men: Empirical Support for the Population-Based Social Epistasis Amplification Model (a Comment on )

“…With negative selection against the carriers of these spiteful mutations restored, population growth continues until a stable equilibrium is established. Thus, Kalbassi et al (2017) found, as Woodley of Menie et al (2017)’s model predicts, that restoration of adaptive behavior occurred in mouse communities when Nlgn3 was re-expressed in Nlgn3 y/- mice (this restoration simulates the effects of negative selection as reflected in Woodley of Menie et al, 2017’s model).…”

“…Woodley of Menie et al (2017) predicted that results similar to Kalbassi et al (2017)’s would be found in mice subjected to the proper experimental design. Kalbassi et al (2017) are to be commended for devising a better procedure than that contained in the provisional experiment offered by Woodley of Menie et al (2017)—which is a variant of the “mouse utopia” experiments of Calhoun (1973)—wherein two mice colonies were to be bred under cornucopian conditions.…”

“…Most saliently, a paper published earlier this year integrated a great deal of research on social epistasis, i.e., interorganismal gene-gene interactions, and mutation load in humans, mice, and other organisms to develop the novel thesis that the fitness costs of the accumulation of certain kinds of deleterious mutations under conditions of relaxed negative selection in humans are externalized onto noncarrier individuals and thereby amplified via the damage that these mutations do to populations’ “group-level extended phenotype[s]” (Woodley of Menie et al, 2017). This theory, which was termed the social epistasis amplification model, is based partly on the biological literature concerning eusocial insects, in which the term “social epistasis” was coined (e.g., Linksvayer, 2007).…”

“…Deletion of Nlgn3 is potentially a “spiteful mutation” (Woodley of Menie et al, 2017). A mutation is spiteful if it degrades the fitness of carriers and also undermines, or incurs opportunity costs on, the fitness of conspecifics with whom carriers enter into social epistatic transaction, e.g., by imposing sociocultural conditions that disincentivize procreation.…”

“…A mutation is spiteful if it degrades the fitness of carriers and also undermines, or incurs opportunity costs on, the fitness of conspecifics with whom carriers enter into social epistatic transaction, e.g., by imposing sociocultural conditions that disincentivize procreation. Woodley of Menie et al (2017) ran structured population models to explore the effects of spiteful mutations, which found that under relaxed negative selection, these mutations reach fixation because population growth offsets their individual-level fitness costs. But once a critical prevalence of mutations is present, their negative impact on the fitness of noncarriers causes rapid population decline.…”