The possible role of environmental calcum levels during the evolution of phenotypic diversity in Outer Hebridean populations of the Three‐spined stickleback,Gasterosteus aculeatus

RarelyGasterosteus aculeatuspopulations occur where many individuals have lost their protective dorsal spines, ventral spines, pelvic girdle and lateral bony plates. Reduction in predation pressure is currently viewed as the causative factor. On the lsle of North Uist (Outer Hebrides) 10 spine‐deficientG. aculeatuspopulations, sympatric with various fish and bird predators are described. All of the spine‐deficient morph sticklebacks are found in acidic oligotrophic peat moorland lakes and pools where Ca2+is ≤ 2.5 mg/1. It is proposed that calcium deficiency may have been an important selective agency in the evolution of skeletal reduction in these unusual populations.