Polyommatus icarus butterflies in the British Isles: evidence for a bottleneck RIEN DE KEYSER 1 , TIM G. SHREEVE 1 , CASPER J. BREUKER 1 , ROSEMARY S. HAILS 2 and THOMAS SCHMITT 3 * 1 Department of Biological and Medical Sciences, Faculty of Health and Life Sciences, Oxford Brookes University, Headington, Oxford OX3 0BP, UK 2 Centre for Ecology & Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford OX10 8BB, UK 3 Department of Biogeography, FB VI, Trier University, D–54286 Trier, Germany Received 23 January 2012; revised 22 March 2012; accepted for publication 22 March 2012 Phylogeographical research has revealed several paradigm patterns of postglacial range expansion from the Mediterranean peninsulas to more northern parts of Europe. These range expansions have consequences for the genetic constitution of populations. Although many studies have been performed in mainland Europe, the colonization history of the British Isles is relatively poorly studied; the genetic consequences of the last glacial readvances and the climate optimum conditions, as well as the implications of the recent climatic conditions on the population genetic structures, are little understood. Therefore, we selected the common blue butterfly Polyommatus icarus as a model species for understanding more generally the colonization patterns of the British Isles and the genetic dynamics on these islands. Allozyme analyses of this butterfly show a rather high genetic diversity over continental Europe without major genetic differentiation. The situation on the British Isles is completely different. Here, populations show a much lower genetic diversity compared to mainland Europe. The genetic constitution is well differentiated from that observed on the European mainland, and the genetic differentiation among popula- tions in Britain is stronger than at the European scale. These results support the hypothesis that a relatively cold-tolerant species such as the common blue could have colonized the British Isles early during the late glacial period and survived the last glacial readvances in small refugia in the South. The retraction of this species in small isolated populations could have caused the genetic impoverishment found. The subsequent forest climax during the climate optimum possibly restricted further expansion of this early succession species to small pockets all over the British Isles, resulting in the genetic patchwork that is still observed. Additionally, the relatively cool and rainy conditions one these islands might have caused bottlenecks, possibly enforcing these genetic patterns. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 107, 123–136. ADDITIONAL KEYWORDS: allozyme electrophoresis – biogeography – common blue – postglacial range expansion – phylogeography. INTRODUCTION The biogeography of Europe has been studied for some time (Reinig, 1937; de Lattin, 1967) and the introduction of genetic marker systems as analytical tools has helped to improve understanding of biogeo- graphical structures (Avise et al., 1987; Hewitt, 1996, 1999, 2004), particularly of those taxa without fossil or subfossil remains. For most species groups, includ- ing the Lepidoptera, there is a general consensus that the current ranges of many alpine and arctic species (i.e. cold-adapted species) are restricted by warm Holocene conditions, whereas the current distribu- tion of the majority of species (i.e. warm-adapted species) have arisen from range expansions during the Holocene from Mediterranean refugia (Dennis, Williams & Shreeve, 1991; Dennis & Schmitt, 2009). Holocene range expansion patterns differ between taxa but there are repetitive patterns within groups of species (Dennis, Shreeve & Williams, 1995a; *Corresponding author. E-mail: thsh@uni-trier.de Biological Journal of the Linnean Society, 2012, 107, 123–136. With 3 figures © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 107, 123–136 123 Downloaded from https://academic.oup.com/biolinnean/article/107/1/123/2701499 by guest on 28 June 2021