Physics (Internal)

Dynamic mutation selection balance as an evolutionary attractor

This event is part of the Condensed Matter Theory Seminar Series.

Abstract: 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
$\ss$, we calculate the fraction of beneficial mutations, $\e$, that
maintains the balanced state. We find that a surprisingly low $\e$
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.