Abstract
AbstractAimClarifying 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.LocationUK, Ireland.MethodsWe analysed the genetic variation at 12 microsatellite loci of 381 individuals sampled from one historical and 14 modern populations of grey squirrels.ResultsOur 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 conclusionsOur 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.
Highlights
The release of a few individuals of an alien species in a new area leads sometimes to what some authors call a ‘genetic paradox’, that is the ability of bottlenecked populations with low genetic diversity, low evolutionary potential and low reproductive fitness to become invasive (Frankham, 2005; Roman & Darling, 2007)
Our results show a reduction in allelic richness and heterozygosity in all the examined populations that were partially derived from past translocations from the same source in Woburn
The genetic diversity of a population founded from multiple introductions from different sources is expected to be similar or even higher than that of a single population from the native range (Kolbe et al, 2004; Dlugosch & Parker, 2008), but our results clearly do not support this prediction for grey squirrels in the UK
Summary
The release of a few individuals of an alien species in a new area leads sometimes to what some authors call a ‘genetic paradox’, that is the ability of bottlenecked populations with low genetic diversity, low evolutionary potential and low reproductive fitness to become invasive (Frankham, 2005; Roman & Darling, 2007). Recent work has shown that the genetic paradox can be explained by the merging of propagules from genetically differentiated source populations that lead to increased diversity compared to the source population (Lockwood et al, 2005; Simberloff, 2009), as is for example the case for anole lizards introduced in Florida (Kolbe et al, 2004) and ladybirds introduced in the USA (Kajita et al, 2012). Anole lizards showed higher local genetic diversity in the invaded area than lizards in their native range, following the merging and interbreeding of at least eight introductions from different source locations, explaining higher genetic diversity and migration rates, and outstanding invasion success of the species
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