Abstract
We ask the question "when will natural selection on a gene in a spatially
structured population cause a detectable trace in the patterns of genetic
variation observed in the contemporary population?". We focus on the situation
in which 'neighbourhood size', that is the effective local population density,
is small. The genealogy relating individuals in a sample from the population is
embedded in a spatial version of the ancestral selection graph and through
applying a diffusive scaling to this object we show that whereas in dimensions
at least three, selection is barely impeded by the spatial structure, in the
most relevant dimension, $d=2$, selection must be stronger (by a factor of
$łog(1/\mu)$ where $\mu$ is the neutral mutation rate) if we are to have a
chance of detecting it. The case $d=1$ was handled in Etheridge et al. (2015).
The mathematical interest is that although the system of branching and
coalescing lineages that forms the ancestral selection graph converges to a
branching Brownian motion, this reflects a delicate balance of a branching rate
that grows to infinity and the instant annullation of almost all branches
through coalescence caused by the strong local competition in the population.
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