| Description |
The vast majority of mutations are deleterious, and are eliminated by
purifying selection. Yet in finite asexual populations, purifying
selection cannot completely prevent the accumulation of deleterious
mutations due to Muller's ratchet: once lost by stochastic drift, the
most-fit class of genotypes is lost forever. If deleterious mutations
are weakly selected, Muller's ratchet turns into a mutational
``meltdown'' leading to a rapid degradation of population
fitness. Evidently, the long term stability of an asexual population
requires an influx of beneficial mutations that continuously
compensate for the accumulation of the weakly deleterious ones. Here
we propose that the stable evolutionary state of a population in a
static environment is a dynamic mutation-selection balance, where
accumulation of deleterious mutations is on average offset by the
influx of beneficial mutations. We argue that this state exists for
any population size /N/ and mutation rate /U/. Assuming that
beneficial and deleterious mutations have the same fitness effect s,
we calculate the fraction of beneficial mutations, $\epsilon$, that
maintains the balanced state. We find that a surprisingly low
$\epsilon$ suffices to maintain stability, even in small populations
in the face of high mutation rates and weak selection. This may
explain the maintenance of mitochondria and other asexual genomes, and
has implications for the expected statistics of genetic diversity in
these populations.
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