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Aim Clarifying whether multiple introductions of a species remain relatively isolated or merge and interbreed is essential for understanding the dynamics of invasion processes. Multiple introductions from different sources can result in a mixture of genetically distinct populations, increasing the total genetic diversity. This mixing can resolve the 'genetic paradox', whereby in spite of the relatively small numbers of introduced individuals, the augmented diversity due to this mixing increases adaptability and the ability of the species to spread in new environments. Here, we aim to assess whether the expansion of a successful invader, the Eastern grey squirrel, was partly driven by the merger of multiple introductions and the effects of such a merger on diversity. Location UK, Ireland. Methods We analysed the genetic variation at 12 microsatellite loci of 381 individuals sampled from one historical and 14 modern populations of grey squirrels. Results Our data revealed that current UK population structure resembles a mosaic, with minimal interpopulation mixing and each element reflecting the genetic make-up of historic introductions. The genetic diversity of each examined population was lower than a US population or a historical UK population. Numbers of releases in a county did not correlate with county-level genetic diversity. Inbreeding coefficients remain high, and effective population sizes remain small. Main conclusions Our results support the conclusion that rapid and large-scale expansion in this species in the UK was not driven by a genetic mixing of multiple introduced populations with a single expansion front, but was promoted by repeated translocations of small propagules. Our results have implications for the management of grey squirrels and other invasive species and also demonstrate how invaders can overcome the genetic paradox, if spread is facilitated by human-mediated dispersal. [ABSTRACT FROM AUTHOR]