Epigenetics role in evolution

When we look at the epigenetic machinery as a whole, with its complexity and its tight links to the genetic background, epigenetic mechanisms could contribute to or impact local adaptation and evolution in a few different ways. Excluding plasticity induced genetic accommodation (Table 1), which is not a direct mechanism and is not exclusively driven by epigenetics, we can envision at least four main direct mechanisms in which epigenetic marks could contribute to local adaptation and evolution:

1. Epimutations can arise stochastically and over evolutionary time they could undergo natural selection and, when functionally relevant, produce stable phenotypic variation (see phenotypic plasticity chapter). This would work in a similar manner as genetic mutations, with the difference that epimutations arise at a higher rate than genetic mutations but are also much more likely to revert (Becker et al. 2011; Johannes and Schmitz 2018).

2. Environmentally induced epigenetic variation could arise in response to changing environmental conditions and, if stably inherited over generations, could provide a means of rapid evolution (Richards 2006; Whitelaw and Whitelaw 2006).

3. DNA methylation variation may generate further genetic variation when transposable elements TEs are released (Dubin et al. 2015; Secco et al. 2015). This could result in different environmental cues releasing different TE families or in epigenetically distant accessions being more likely to generate different genetic variation.

4. In addition to these mechanisms, it is relevant to mention that methylated DNA has a higher mutation rate due to 5-methylcytosine being more prone to C to T transitions by deamination (Cambareri et al. 1989; Pfeifer 2006). This could increase the likelihood of epigenetically distant individuals to accumulate genetic mutations in different regions of the genome.

Nevertheless, in order to be taken into account, the main candidate mechanisms need to meet some basic requirements that are discussed in the next chapters. Most importantly, evolutionary relevant epigenetic variation has to be at least partially “stable”, meaning heritable through successive generations (see section “Stability of epigenetic marks”), have an effect on the phenotype (see section “Phenotypic effects”) and should be “pure” i.e. not rely on DNA sequence variation for its heritability (see section “Genetics – epigenetics”).