Molecular footprints of the H olocene retreat of dwarf birch in B ritain

Past reproductive interactions among incompletely isolated species may leave behind a trail of introgressed alleles, shedding light on historical range movements. B etula pubescens is a widespread native tetraploid tree species in B ritain, 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 hybridization 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 minimization of hybridization with B . pubescens , and avoidance of planting B . pendula near B . nana populations.