230 research outputs found
Deterministic and stochastic regimes of asexual evolution on rugged fitness landscapes
We study the adaptation dynamics of an initially maladapted asexual
population with genotypes represented by binary sequences of length . The
population evolves in a maximally rugged fitness landscape with a large number
of local optima. We find that whether the evolutionary trajectory is
deterministic or stochastic depends on the effective mutational distance
upto which the population can spread in genotype space. For
, the deterministic quasispecies theory operates while for
, the evolution is completely stochastic. Between these
two limiting cases, the dynamics are described by a local quasispecies theory
below a crossover time while above , the population
gets trapped at a local fitness peak and manages to find a better peak either
via stochastic tunneling or double mutations. In the stochastic regime
, we identify two subregimes associated with clonal
interference and uphill adaptive walks, respectively. We argue that our
findings are relevant to the interepretation of evolution experiments with
microbial populations.Comment: Revised version, to appear in Genetics. Note on the role of selection
in defining d_eff added; new figure 4 include
Rapid adaptation of a polygenic trait after a sudden environmental shift
Although a number of studies have shown that natural and laboratory
populations initially well-adapted to their environment can evolve rapidly when
conditions suddenly change, the dynamics of rapid adaptation are not well
understood. Here a population genetic model of polygenic selection is analyzed
to describe the short-term response of a quantitative trait after a sudden
shift of the phenotypic optimum. We provide explicit analytical expressions for
the time scales over which the trait mean approaches the new optimum. We find
that when the effect sizes are small relative to a scaled mutation rate, the
genomic signatures of polygenic selection are small to moderate allele
frequency changes that occur in the short-term phase in a synergistic fashion.
In contrast, selective sweeps, i.e., dramatic changes in the allele frequency
may occur provided the size of the effect is sufficiently large. Applications
of our theoretical results to the relationship between QTL and selective sweep
mapping and to tests of fast polygenic adaptation are discussed
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