**This post is a part of the series on the 2020 AGA Presidential Symposium – Genes as Environment: Indirect Genetic Effects on Evolution, Agriculture, & Medicine**
About the Blog Author: Dr. Michael J. Wade is an Distinguished Professor and evolutionary geneticist at the Indiana University in Bloomington, Indiana. He studies the role of interactions, between genes, between individuals and between species, in evolution, with a special focus on genetically subdivided populations. His publications can be found at his website.
I have long been fascinated by the fact that an offspring trait, like body size, could evolve even if it had no direct heritability, because of change in the distribution of maternal genetic variation (Wolf et al. 1998; Wolf and Wade 2009) affecting offspring body size. This finding, surprising from the viewpoint of single trait evolution, is readily understandable as a correlated response to indirect selection on another trait, even if that trait is in mothers of the previous generation. Famously, in quantitative genetics, maternal effects have long been considered ‘special’ environments, where methods like paternal half-sibs or randomized cross-fostering are used to prevent such environmental influences from contaminating estimates of trait heritability. Maternal genetic effects are the most common of indirect genetic effects (IGEs) (Wade 2021) and, like all IGEs, they arise from the genes of some individuals (mothers) and affect the environment of others (offspring). IGEs change our paradigm from a binary view of nature (genes) versus nurture (environments) to a more wholistic view of nature, nurture and the nurturers, those who span both nature and nurture.
In much of evolutionary theory, genetic and environmental variation are partitioned into separate, distinct components. For example, the Price Equation (Frank and Slatkin 1992) partitions the change in mean fitness of a population into a genetic and an environmental component. The genetic component equals the additive genetic variance for fitness, capturing the essence of Fisher’s Fundamental Theorem (Queller 2017). The second component, the effect of environmental change on mean fitness, is often assumed to be zero so that it can ‘usefully be set aside as being of secondary interest’ (Queller 2017). I wondered where maternal genetic effects, which are both genetic and environmental, would appear in the components of the Price Equation. Because genotype-by-environment interaction (G x E) is the typical way to entangle the effects of genes and environments, I wondered whether maternal genetic effects would partition similarly to G x E effects in the Price Equation. Could maternal genetic effects (or IGEs in general) elevate the second term of the Price Equation from evolutionary obscurity to prominence? That is what my paper (Wade 2022) is about.