Allele frequency fluctuations are associated with demographic cycling in Icelandic rock ptarmigan

Understanding how natural populations adapt to recurring environmental changes, such as demographic cycles, is a central question in evolutionary biology. Multi-annual population cycles impose fluctuating selective pressures, yet disentangling the resulting genomic signals from those of long-term directional selection and genetic drift has been challenging due to a lack of high-resolution temporal data. Here, we investigate the genomic consequences of demographic cycling in a population of rock ptarmigan ( Lagopus muta ) from northeastern Iceland. We analyzed whole-genome sequences from 91 individuals sampled over an 11-year period, encompassing two population peaks (in 2010 and 2018) and two troughs (in 2007 and 2013). Using a simulation-based framework, we identified 22,399 single-nucleotide polymorphisms (SNPs) exhibiting patterns of strong fluctuating selection. These patterns were highly correlated with fluctuating demography, consistent with density-dependent selection. The strongest fluctuating sites overlapped with numerous candidate genes that may be important in the context of density-depending selection. The most significant candidate, CTNNA2 (Catenin Alpha-2), is associated with startle response and anxiety-like behaviors that may be associated with predator avoidance. We also found candidate genes such as GUCY2C and TRAF2 , whose functions may intersect with key stages of host-parasite conflicts. Our findings suggest that multi-annual demographic cycles are a powerful evolutionary force driving rapid and widespread genomic changes, with density-dependent selection on traits potentially playing a key role in adaptation. This study highlights the significance of temporal genomics in clarifying the interplay between demography and natural selection in wild populations.