ORIGINAL PAPER Contrasting Levels of Variation in Neutral and Quantitative Genetic Loci on Island Populations of Moor Frogs (Rana arvalis) Theresa Knopp Æ Jose ´ M. Cano Æ Pierre-Andre ´ Crochet Æ Juha Merila ¨ Received: 1 November 2005 / Accepted: 11 March 2006 / Published online: 3 June 2006 Ó Springer Science+Business Media B.V. 2006 Abstract Reduced levels of genetic variability and a prominent differentiation in both neutral marker genes and phenotypic traits are typical for many island populations as compared to their mainland conspecifics. However, whe- ther genetic diversity in neutral marker genes reflects ge- netic variability in quantitative traits, and thus, their evolutionary potential, remains typically unclear. More- over, the phenotypic differentiation on islands could be attributable to phenotypic plasticity, selection or drift; something which seldom has been tested. Using eight polymorphic microsatellite loci and quantitative genetic breeding experiments we conducted a detailed comparison on genetic variability and differentiation between Nordic islands (viz. Gotland, O ¨ land and Læsø) and neighbouring mainland populations of moor frogs (Rana arvalis). As expected, the neutral variation was generally lower in island than in mainland populations. But as opposed to this, higher levels of additive genetic variation (V A ) in body size and tibia length were found on the island of Gotland as compared to the mainland population. When comparing the differentiation seen in neutral marker genes (F ST ) with the differentiation in genes coding quantitative traits (Q ST ) two different evolutionary scenarios were found: while selec- tion might explain a smaller size of moor frogs on Gotland, the differentiation seen in tibia length could be explained by genetic drift. These results highlight the limited utility of microsatellite loci alone in inferring the causes behind an observed phenotypic differentiation, or in predicting the amount of genetic variation in ecologically important quantitative traits. Keywords Amphibia Æ Genetic diversity Æ Island Æ F ST Æ Q ST Introduction Presence of sufficient genetic variation is a prerequisite for populations to evolve in response to changes in the envi- ronment (Hedrick and Miller 1992; Frankham et al. 2002). The levels of genetic variability are generally high in large populations, but reduced in smaller ones due to the effects of genetic drift and inbreeding (Hartl and Clark 1997; Frankham et al. 2002; Keller and Waller 2002). Island populations are typically small, isolated, and have often gone through population bottlenecks at the time of their founding. Hence, it is not surprising that island populations of many species suffer from inbreeding depression (re- viewed in Frankham 1997), exhibit reduced genetic vari- ability (Browne 1977; Frankham 1997; Hinten et al. 2003), and as a consequence, might stand at a higher risk of extinction than their mainland conspecifics (Frankham 1998; Frankham et al. 2002; Gaggiotti 2003). Another typical feature for island populations is that they often are phenotypically differentiated from their T. Knopp (&) Æ J. M. Cano Æ J. Merila ¨ Ecological Genetics Research Unit, Department of Biological and Environmental Sciences, University of Helsinki, PO Box 65, Helsinki 00014, Finland e-mail: theresa.knopp@helsinki.fi P.-A. Crochet Department of Animal Ecology, Evolutionary Biology Centre, Uppsala University, Norbyva ¨gen 18D, 75236 Uppsala, Sweden P.-A. Crochet CNRS-UMR 5175, Centre d’Ecologie Fonctionnelle et Evolutive, 1919 route de Mende, 34293 Montpellier cedex 5, France Conserv Genet (2007) 8:45–56 DOI 10.1007/s10592-006-9147-4 123