Quasi epigenetic equilibrium: the implications of plasticity and variability on evolution and extinction

The phenotypic effects of epigenetic modifications, and thus their evolutionary consequences, depend on how the modifications interact with the underlying genetics and the surrounding environment. These interactions lead to a complex model that has so far prevented general analytical progress, and thus limited our understanding. Here, we show that the timescale difference between epigenetic and genetic changes create a quasi-epigenetic equilibrium (QEE). The QEE allows us to tackle the complexity of population epigenetic models by reducing them to their underlying population genetic models with effective parameters. Using this technique, we show how epigenetics modifies key evolutionary parameters, such as the strength of selection and dominance, which can have drastic evolutionary consequences on mutation-selection balance. Further, we show how the QEE allows us to analytically investigate the effect of epigenetics on the probability of a population escaping extinction in a harsh environment via adaptation - evolutionary rescue - by altering the number of potential rescue lineages and their probability of establishing. These calculations show that whether epigenetics helps or hurts population persistence depends non-trivially on the frequency and stability of epigenetic modifications.