  We study fixation probabilities and times as a consequence of neutral genetic
drift in subdivided populations, motivated by a model of the cultural
evolutionary process of language change that is described by the same
mathematics as the biological process. We focus on the growth of fixation times
with the number of subpopulations, and variation of fixation probabilities and
times with initial distributions of mutants. A general formula for the fixation
probability for arbitrary initial condition is derived by extending a duality
relation between forwards- and backwards-time properties of the model from a
panmictic to a subdivided population. From this we obtain new formulae,
formally exact in the limit of extremely weak migration, for the mean fixation
time from an arbitrary initial condition for Wright's island model, presenting
two cases as examples. For more general models of population subdivision,
formulae are introduced for an arbitrary number of mutants that are randomly
located, and a single mutant whose position is known. These formulae contain
parameters that typically have to be obtained numerically, a procedure we
follow for two contrasting clustered models. These data suggest that variation
of fixation time with the initial condition is slight, but depends strongly on
the nature of subdivision. In particular, we demonstrate conditions under which
the fixation time remains finite even in the limit of an infinite number of
demes. In many cases - except this last where fixation in a finite time is seen
- the time to fixation is shown to be in precise agreement with predictions
from formulae for the asymptotic effective population size.
