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| Paper IPM / P / 6481 |
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| Abstract: | |||||
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Wright proposed that there is a "shifting balance" between selection within
demes, random drift, and selection between demes at different "adaptive
peaks". We investigate the establishment and spread of new adaptive peaks,
considering a chromosome rearrangement, and a polygenic character under
disruptive selection. When the number of migrants (Nm) is small, demes
fluctuate independently, with a bias towards the filter peak. When Nm is
large, the whole population can move to one of two stable equilibria,
and so can be trapped near the lower peak. These two regimes are
separated by a sharp transition at a critical Nm of order 1. Just below this
critical point, adaptation is most efficient, since the shifting balance
greatly increases the proportion of demes that reach the global optimum. This
is so even if one peak is only slightly filter than the other
(∆W=1/N),
and for both strong and weak selection (Ns << 1 or Ns >> 1). Provided that
Nm varies sufficiently gradually from place to
place, the fiver peak can be established in regions where Nm=1, and can
then spread through the rest of the range. Our analysis confirms Wright's
argument that if selection, migration and drift are of the same order, the
"shifting balance" leads to efficient evolution towards the global optimum.
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