Biol. Rev. (2013), pp. 000 – 000. 1 doi: 10.1111/brv.12070 Genetic perspectives on northern population cycles: bridging the gap between theory and empirical studies Karin Nor´ en ∗ and Anders Angerbj ¨ orn Department of Zoology, Stockholm University, 106 91 Stockholm, Sweden ABSTRACT Many key species in northern ecosystems are characterised by high-amplitude cyclic population demography. In 1924, Charles Elton described the ecology and evolution of cyclic populations in a classic paper and, since then, a major focus has been the underlying causes of population cycles. Elton hypothesised that fluctuations reduced population genetic variation and influenced the direction of selection pressures. In concordance with Elton, present theories concern the direct consequences of population cycles for genetic structure due to the processes of genetic drift and selection, but also include feedback models of genetic composition on population dynamics. Most of these theories gained mathematical support during the 1970s and onwards, but due to methodological drawbacks, difficulties in long-term sampling and a complex interplay between microevolutionary processes, clear empirical data allowing the testing of these predictions are still scarce. Current genetic tools allow for estimates of genetic variation and identification of adaptive genomic regions, making this an ideal time to revisit this subject. Herein, we attempt to contribute towards a consensus regarding the enigma described by Elton almost 90 years ago. We present nine predictions covering the direct and genetic feedback consequences of population cycles on genetic variation and population structure, and review the empirical evidence. Generally, empirical support for the predictions was low and scattered, with obvious gaps in the understanding of basic population processes. We conclude that genetic variation in northern cyclic populations generally is high and that the geographic distribution and amount of diversity are usually suggested to be determined by various forms of context- and density-dependent dispersal exceeding the impact of genetic drift. Furthermore, we found few clear signatures of selection determining genetic composition in cyclic populations. Dispersal is assumed to have a strong impact on genetic structuring and we suggest that the signatures of other microevolutionary processes such as genetic drift and selection are weaker and have been over-shadowed by density-dependent dispersal. We emphasise that basic biological and demographical questions still need to be answered and stress the importance of extensive sampling, appropriate choice of tools and the value of standardised protocols. Key words: climate, Elton, cyclic, Arctic, Chitty hypothesis, density-dependent dispersal. CONTENTS I. Introduction ................................................................................................ 2 II. A brief description of population cycles .................................................................... 3 III. Microevolutionary processes ............................................................................... 3 (1) Direct consequences through genetic drift ............................................................. 5 (a) Prediction 1: population cycles reduce genetic variation ............................................ 5 (b) Prediction 2: population cycles cause temporal fluctuations in the amount of genetic variation ..... 5 (c) Prediction 3: population cycles increase genetic differentiation ..................................... 8 (d ) Prediction 4: population cycles cause temporal fluctuations in genetic differentiation .............. 8 (2) Direct consequences through selection ................................................................. 9 (a) Prediction 5: population cycles lead to temporal fluctuations in the frequency of adaptive genotypes 9 (b) Prediction 6: balancing selection maintains high genetic variation .................................. 10 * Author for correspondence (Tel: +46 8 16 40 37; Fax: +46 8 167715; E-mail: karin.noren@zoologi.su.se). Biological Reviews (2013) 000 – 000  2013 The Authors. Biological Reviews  2013 Cambridge Philosophical Society