Data from: Molecular footprints of the Holocene retreat of dwarf birch in Britain

Past reproductive interactions among incompletely isolated species may
leave behind a trail of introgressed alleles, shedding light on historical
range movements. Betula pubescens is a widespread native tetraploid tree
species in Britain, occupying habitats intermediate to those of its native
diploid relatives, B. pendula and B. nana. Genotyping 1134 trees from the
three species at 12 microsatellite loci we found evidence of introgression
from both diploid species into B. pubescens, despite the ploidy
difference. Surprisingly, introgression from B. nana, a dwarf species
whose present range is highly restricted in northern, high-altitude peat
bogs, was greater than introgression from B. pendula, which is
morphologically similar to B. pubescens and has a substantially
overlapping range. A cline of introgression from B. nana was found
extending into B. pubescens populations far to the south of the current B.
nana range. We suggest that this genetic pattern is a footprint of a
historical decline and/or northwards shift in the range of B. nana
populations due to climate warming in the Holocene. This is consistent
with pollen records that show a broader, more southerly distribution of B.
nana in the past. Ecological niche modelling predicts that B. nana is
adapted to a larger range than it currently occupies, suggesting
additional factors such as grazing and hybridisation may have exacerbated
its decline. We found very little introgression between B. nana and B.
pendula, despite both being diploid, perhaps because their distributions
in the past have rarely overlapped. Future conservation of B. nana may
partly depend on minimisation of hybridisation with B. pubescens, and
avoidance of planting B. pendula near B. nana populations.