  We investigate the infinitely many demes limit of the genealogy of a sample
of individuals from a subdivided population subject to sporadic mass extinction
events. By exploiting a separation of timescales property of Wright's island
model, we show that as the number of demes tends to infinity the limiting form
of the genealogy can be described in terms of the alternation of instantaneous
'scattering' phases dominated by local demographic processes, and extended
'collecting' phases dominated by global processes. When extinction and
recolonization events are local, this genealogy is given by Kingman's
coalescent and the scattering phase influences only the overall rate of the
process. In contrast, if the vacant demes left by a mass extinction event can
be recolonized by individuals emerging from a small number of demes, then the
limiting genealogy is a colaescent with simultaneous multiple mergers. In this
case, the details of the within-deme population dynamics influence not only the
overall rate of the coalescent process, but also the statistics of the complex
mergers that can occur within sample genealogies. This study gives some insight
into the genealogical consequences of mass extinction in structured
populations.
