The intensification of human activities has led to a drastic loss of suitable habitats for various taxa, followed by patterns of population decline and isolation. During the past decades, in the UK, the adder (Vipera berus) has experienced a significant population decline. Once highly abundant, this reptile is now confined to small and fragmented populations, making it a national conservation concern. Despite this, data regarding the genetic status of the species in the UK are extremely scarce and limited to a single microsatellite-based study.
Therefore, for the first time, a whole-genome sequencing approach was here employed to assess patterns of population structure and genetic health in four adder populations from Wales and England. Despite strong population structure and a lack of recent gene flow between the investigated populations, the latter were characterised by extremely high levels of average genome-wide heterozygosity. The recovered heterozygosity levels turned out to be higher/comparable to those from higly genetically diverse vertebrates. However, further investigations revealed hidden extensive patterns of genomic erosion within the assessed adder’s genomes, thus revealing overlooked inbreeding patterns. This discrepancy is likely to be the product of large and genetically diverse ancestral populations that have recently undergone rapid and extreme bottlenecks. This study highlights the efficiency of whole-genome sequencing in assessing patterns of population structure and genetic health. Moreover, the results call attention to the future persistence of the assessed adder populations, arguing for the potential necessity of human-mediated genetic conservation actions.