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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. While our model suggests that RI produced by epigenetic loci will generally be weaker than that produced by genetic loci, epigenetic barriers to gene flow are not necessarily negligible. As per the definition of RI proposed by Westram et al. (2022), the measurement of RI by introgression of neutral loci among populations attributes all RI to genetic differences. We can, therefore, imagine an epigenetic locus whose state (in this example methylated or unmethylated) encodes a phenotype and a neutral genetic locus that is linked with such epigenetic locus (Figure 1a), similar to a genetic locus (Figure 1b). [Extracted from the article]