Summary (ZL): This paper reports the results of a study of genetic diversity investigated at two spatial levels—for the island as a whole, and at the level of subdivisions along the length of the island. Considering the long and narrow shape of the island, and the social dynamics of the band system found in free-living horses, some genetic substructuring may exist in the population. To examine this possibility, 177 individuals sampled (using tissues collected from horses that died of natural causes) were assigned, based on their birth location, to one of four subdivisions (I to IV) of roughly equal distance west to east along the length of the island. Each subdivision was treated as a subpopulation for computation of genetic parameters. This was a very broad and preliminary examination. Birth location only roughly represents the home range of the family band, and although all four subdivisions encompassed vegetated terrain, the amount of vegetated terrain in each was not equal.
Although significant heterozygote deficiency was indicated at island-wide scale, the results from the four subdivisions suggest that the island-wide measure of heterozygosity does not represent the true genetic diversity in the Sable population. Fine-scale spatial substructuring in the population was detected, with individuals living on the east end of the island having higher levels of observed heterozygosity than those in the west. While inbreeding was indicated for subdivisions I, II and III, some outbreeding was found in IV, the easternmost subdivision—of the sampled horses, those at the east end of the island had ten percent greater heterozygosity than those in the west. The reasons for this difference are unclear, and further research is required. Among factors to be considered are distribution and movements of horses on the island. The island’s shape may to some degree function as a corridor that reduces individual and group movements, and limits dispersal of offspring, thus restricting island-wide gene flow. Another factor may be differing life histories with some individuals potentially having greater local influence.