  Some species exhibit very high levels of DNA sequence variability; there is
also evidence for the existence of heritable epigenetic variants that
experience state changes at a much higher rate than sequence variants. In both
cases, the resulting high diversity levels within a population (hyperdiversity)
mean that standard population genetics methods are not trustworthy. We analyze
a population genetics model that incorporates purifying selection, reversible
mutations and genetic drift, assuming a stationary population size. We derive
analytical results for both population parameters and sample statistics, and
discuss their implications for studies of natural genetic and epigenetic
variation. In particular, we find that (1) many more intermediate frequency
variants are expected than under standard models, even with moderately strong
purifying selection (2) rates of evolution under purifying selection may be
close to, or even exceed, neutral rates. These findings are related to
empirical studies of sequence and epigenetic variation.

Populations and Evolution