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Published online by Cambridge University Press: 26 July 2012
Fifty years ago James Ritchie declared that “in recent times we are looking in Scotland upon Evolution in its course”. The basis for Ritchie's remark was the large number of distinct taxa which have been described on Scottish islands, mainly on the Outer Hebrides. The most effective ingredient in producing taxonomic distinctiveness is isolation, particularly when the isolated form is founded by a small group of organisms which will almost inevitably differ genetically from its ancestral population. Subsequent adaptation may produce divergence between two related but isolated groups, but the process of differentiation will be fairly slow, since natural selection must depend upon the inherited variation present, never mind consideration of evolutionary ‘cost’, etc.
The Inner Hebrides are ecologically very diverse, but the gene-flow between animals and plants living on them and on the mainland of Scotland must be much greater than is the case for Outer Hebridean populations. Notwithstanding, subspecifically distinct races of field mice, voles, shrews, and stoats have been described, and clinal variation shows that adaptive changes have taken place in several bird and lepidopteran species.
The Inner Hebrides offer excellent experimental opportunities to measure gene-flow between different islands, and between islands and mainland. It should be possible to estimate some of the evolutionary forces involved in producing species differentiation, as well as in local extinctions.