Genetic variation and structure of house sparrow populations: is there an island effect? HENRIK JENSEN,* RUNE MOE,* INGERID JULIE HAGEN,* ANNA MARIE HOLAND,* JAANA KEKKONEN, † JARLE TUFTO ‡ and BERNT-ERIK SÆTHER* *Department of Biology, Centre for Conservation Biology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway, †Department of Biological and Environmental Sciences, University of Helsinki, PO Box 65, Helsinki Yliopisto, Helsinki, 00014, Finland, ‡Department of Mathematical Sciences, Centre for Conservation Biology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway Abstract Population genetic structure and intrapopulation levels of genetic variation have important implications for population dynamics and evolutionary processes. Habitat fragmentation is one of the major threats to biodiversity. It leads to smaller population sizes and reduced gene flow between populations and will thus also affect genetic structure. We use a natural system of island and mainland populations of house spar- rows along the coast of Norway to characterize the different population genetic proper- ties of fragmented populations. We genotyped 636 individuals distributed across 14 populations at 15 microsatellite loci. The level of genetic differentiation was estimated using F-statistics and specially designed Mantel tests were conducted to study the influence of population type (i.e. mainland or island) and geographic distance on the genetic population structure. Furthermore, the effects of population type, population size and latitude on the level of genetic variation within populations were examined. Our results suggest that genetic processes on islands and mainland differed in two important ways. First, the intrapopulation level of genetic variation tended to be lower and the occurrence of population bottlenecks more frequent on islands than the main- land. Second, although the general level of genetic differentiation was low to moder- ate, it was higher between island populations than between mainland populations. However, differentiation increased in mainland populations somewhat faster with geo- graphical distance. These results suggest that population bottleneck events and genetic drift have been more important in shaping the genetic composition of island popula- tions compared with populations on the mainland. Such knowledge is relevant for a better understanding of evolutionary processes and conservation of threatened popula- tions. Keywords: founder effect, F-statistics, genetic structure, genetic variation, isolation by distance, Passer domesticus Received 20 May 2011; revision received 4 December 2012; accepted 11 December 2012 Introduction Genetic variation plays a central role for long-term pop- ulation viability (Lande & Barrowclough 1987), adapta- tion through natural selection (Willi et al. 2006) and speciation (Carson & Templeton 1984). Knowledge of genetic variation is therefore of paramount importance to understand processes that relate to both conservation and evolution (Falconer & Mackay 1996; Storfer 1996). However, because parameters such as population differ- entiation and genetic diversity may vary between differ- ent populations according to their demographic history, the latitude where they are situated, migration and genetic drift, the genetic properties of a given popula- tion may not be directly extrapolated to other Correspondence: Henrik Jensen, Fax: +47 7359 6100; E-mail: Henrik.Jensen@ntnu.no © 2013 Blackwell Publishing Ltd Molecular Ecology (2013) doi: 10.1111/mec.12226