Abstract
Poleward range expansions are observed for an increasing number of
species, which may be an effect of global warming during the past
decades. However, it is still not clear in how far these expansions
reflect simple geographical shifts of species ranges, or whether
new genetic adaptations play a role as well. Here, we analyse the
expansion of the wasp spider Argiope bruennichi into Northern Europe
during the last century. We have used a range-wide sampling of contemporary
populations and historical specimens from museums to trace the phylogeography
and genetic changes associated with the range shift. Based on the
analysis of mitochondrial, microsatellite and SNP markers, we observe
a higher level of genetic diversity in the expanding populations,
apparently due to admixture of formerly isolated lineages. Using
reciprocal transplant experiments for testing overwintering tolerance,
as well as temperature preference and tolerance tests in the laboratory,
we find that the invading spiders have possibly shifted their temperature
niche. This may be a key adaptation for survival in Northern latitudes.
The museum samples allow a reconstruction of the invasion's genetic
history. A first, small-scale range shift started around 1930, in
parallel with the onset of global warming. A more massive invasion
of Northern Europe associated with genetic admixture and morphological
changes occurred in later decades. We suggest that the latter range
expansion into far Northern latitudes may be a consequence of the
admixture that provided the genetic material for adaptations to new
environmental regimes. Hence, global warming could have facilitated
the initial admixture of populations and this resulted in genetic
lineages with new habitat preferences.
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