Signals of ecological decline and increasing genomic inbreeding in Western Hudson Bay polar bears

Ruth Rivkin, Evan S Richardson, Samuel Deakin, David McGeachy, Joshua Miller, Colin Garroway

Climate change is a leading cause of population declines. Predicting the evolutionary consequences of such declines can be challenging for long-lived species because slow generation times and the overlap of individuals from multiple generations within a population can obscure signals of genetic diversity losses. We integrated four decades of environmental, demographic, and genomic data from the Western Hudson Bay polar bear subpopulation to assess whether genomic signatures of inbreeding have accumulated during a well-characterized period of climate-mediated habitat loss. This subpopulation occurs at the species' southern range edge, where the consequences of climate change are particularly acute. Against the background of declining sea ice availability and population abundance, genomic inbreeding increased in 533 polar bears sampled from progressively later birth cohorts. The increase was concentrated in runs of homozygosity, whose lengths indicate shared ancestry approximately 3–16 generations ago. This interval overlaps with unregulated polar bear harvest in the 1900s, suggesting a legacy of historical overharvesting. While the signal of increasing inbreeding raises questions about then potential of evolutionary decline in the subpopulation, very recent inbreeding was rare, suggesting that the expression of inbreeding depression may be offset by extensive gene flow in the species.