LAUSR

Reproductive isolation (RI) is often considered an essential component of speciation; however, its definition varies, and it is challenging to measure, making it difficult to compare across studies. To help overcome these difficulties, Westram et al. (2022) suggest to define RI as ‘a quantitative measure of the effect of genetic differences on gene flow’, specifically at neutral loci. Here, we consider this definition of RI in the context of epigenetic variation. We define epigenetics as molecular interactions with DNA that influence gene expression without changes in the underlying nucleotide sequence (see Box 1 for background on epigenetic variation). Furthermore, we consider a process as epigenetic, regardless of whether it is transmitted across generations or not. As we will see, intergenerational transmission of epigenetic state is not a prerequisite to influence RI. In the following, we establish a framework through which to better quantify epigenetic influences on population divergence and speciation by building upon the model of RI at neutral loci established by Westram et al. (2022). Epigenetics can underlie phenotypic plasticity. A large body of work has examined phenotypic plasticity (WestEberhard, 2003), including its effects on gene flow and speciation (Fitzpatrick, 2012; Klemetsen, 2010; Otte et al., 2016; ThibertPlante & Hendry, 2011). However, focussed studies that have directly examined epigenetic variation as a mechanism producing barriers to reproduction (LafonPlacette & Köhler, 2015; Smith et al., 2016) and speciation (Greenspoon et al., 2022) are only now emerging.