The current issue of Cell is dedicated to the rapidly changing field of epigenetics, loosely defined at the ensemble of chromatin and DNA modifications that structure the genome and control gene expression. The only review I've worked my way through yet is this one: Timescales of Genetic and Epigenetic Inheritance.

According to classical evolutionary theory, phenotypic variation originates from random mutations that are independent of selective pressure. However, recent findings suggest that organisms have evolved mechanisms to influence the timing or genomic location of heritable variability. Hypervariable contingency loci and epigenetic switches increase the variability of specific phenotypes; error-prone DNA replicases produce bursts of variability in times of stress. Interestingly, these mechanisms seem to tune the variability of a given phenotype to match the variability of the acting selective pressure. Although these observations do not undermine Darwin's theory, they suggest that selection and variability are less independent than once thought.

The authors limit themselves largely to bacteria and yeast, but they document a great deal of evidence that parameters like the mutation rate change in response to external stimuli, perhaps in almost a "directed" fashion. Some work in primates has shown a correlation between the amount of mutation raining down on a population and the amount of beneficial mutations that arise. Could "directed" mutagenesis be an explanation for this?

The main points from the paper are summarized in the figure below; essentially, epigenetic inheritance is a way for a population to respond to environmental changes that occur on time scales shorter than those needed for genetic evolution.

Labels: Epigenetics, Evolution