Polyandry and non-random fertilisation maintain long-term genetic diversity in an isolated island population of adders (Vipera berus)

Conservation genetic theory suggests that small and isolated populations
should be subjected to reduced genetic diversity i.e., heterozygosity and
allelic diversity. Our 34 years study of an isolated island population of
adders (Vipera berus) in southern Sweden challenges this notion. Despite a
lack of gene flow and a yearly mean estimated reproductive adult
population size of only 65 adult adders (range 12 to 171), the population
has been able to maintain high levels of heterozygosity and allelic
diversity similar to that observed in two mainland populations. Even a
14-year major “bottleneck” i.e., a reduction in adult adder numbers,
encompassing at least four adder generations, did not result in any
reduction in the island adders’ heterozygosity and allelic diversity.
Female adders are polyandrous, and fertilisation is non-random, which our
empirical data and modelling suggest underpinning the island adders’
ability to maintain a high level of heterozygosity. Our empirical results
and subsequent modelling suggest that the positive genetic effects of
polyandry in combination with non-random fertilisation, often overlooked
in conservation genetic analyses, deserve greater consideration when
predicting long-term survival of small and isolated populations.