The Outer Hebridean fauna is almost entirely the result of the chance characteristics of opportunist colonizers modified by subsequent adaptation and sometimes later immigrants. A few primitive relicts may persist (e.g. possibly, the moth Nyssia zonarid), but the fauna is certainly not largely composed of early post-glacial invaders. These, traditional theory argues, are likely to have been eliminated on the mainland by later arrivals but to have been able to persist on the islands by the timely breaking of hypothetical land-bridges.

The influence of the original island colonizers is discernible through the persistence of similar traits in both ancestral and descendant populations despite massive genetical differences produced by stochastic sampling, and often by an apparently random pattern of differentiation between islands (e.g. field mice, cats, men, wrens). Adaptation is difficult to detect in the presence of unmeasurable founder effects, but is shown by large-scale clines (such as bridling in guillemots or colour phase frequencies in arctic skuas), by the existence of similar local races in different species (notably by melanism in Lepidoptera, especially Triphaena comes; but also possibly in bumble-bees and dragonflies), and by evidence of local selection (e.g. Cochlicella acuta).

In the light of these many pressures, simple equilibrium theories of island biogeography are woefully inadequate; every species has to be considered on its merits. This can be done for terrestrial vertebrates, and virtually all but the Pygmy Shrew prove to have been introduced by man.