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Many studies have investigated the loss of adaptive potential in endangered or exploited species experiencing recent population declines. Less research has studied adaptive genomic variation in small populations known to have persisted for long periods, despite the unique contribution that such populations provide for determining mechanisms underlying population persistence in evolutionary and conservation modeling. Small populations of Brook trout (Salvelinus fontinalis) have persisted in Cape Race, Newfoundland for >12 000 years. We used genotyping-by-sequencing data to investigate the demographic history and adaptive genomic variation of 26 populations with effective population sizes (Ne) ranging from 11 to 442, and to explore mechanisms underlying long-term persistence. We show that all populations experienced demographic declines following postglacial colonization but have remained in their current, small Ne state for thousands of years. We also reveal greater homozygosity in adaptive alleles within the smallest populations and found signatures of adaptive divergence in small populations relating to several abiotic and biotic factors. Our study illustrates how the demographic history of a species can influence the adaptive dynamics of small populations persisting over long time periods. [ABSTRACT FROM AUTHOR]

ABSTRACT Island ecosystems, particularly vulnerable to environmental challenges, host many endangered native species. Diadromous fish, in particular, are threatened throughout their marine and freshwater habitats. The conservation of these species requires an in‐depth understanding of their genetic diversity and structure, to better understand their adaptive potential. We investigated fine‐scale population diversity and structure in native brook charr (Salvelinus fontinalis) by genotyping 10 microsatellite loci in 244 individuals at three spatial scales in Saint‐Pierre and Miquelon, France. We found limited genetic variability across the archipelago, with particularly low genetic diversity in one island, Langlade. A significant difference in allelic richness was also detected among the three islands, indicating a difference in genetic composition across the archipelago, probably induced by historical stocking actions on both Saint‐Pierre and Miquelon. Finally, a strong genetic structure was detected across the archipelago among hydrosystems (overall FST = 0.19) and even within several of them. The presence of predominant interisland gene flow combined with complete genetic isolation from certain hydrosystems suggests that this contemporary genetic structure is the result of both natural demographic processes during the species postglacial colonization and recent restocking actions. The complex genetic structure of such isolated brook charr subpopulations highlights the importance of considering fine‐scale genetic structure in conservation management.