interstadial period. Although the theory is no longer accepted, the term Lusitanian is still used as a descriptive term for faunal elements such as the Kerry slug. Recently a better explanation of the occurrence of the Kerry slug and similar faunal elements in southwestern Ireland has been developed. This new theory is supported by two recent discoveries: the genetic similarity of much of Ireland's fauna to that of northern Spain, and the genetic similarity of much of Ireland's human population to that of northern Spain. Mascheretti et al. (2003) examined the genotypes of Eurasian pygmy shrew, a small mammal, across its range in Europe. The Irish population showed close genetic affinity to a population from Andorra but not to that of Britain or other places in Europe. The genetic structure of the population further showed that the entire Irish population of the Eurasian pygmy shrew had originated from a single founder event. The authors concluded that it had been introduced in the early (Palaeolithic) or middle (Mesolithic) Stone Age, by boat, probably from south-west Europe. This coincides with work on human populations, which found a strong genetic similarity in make-up between populations in western Ireland and in northern Spain. This would be explained by a human migration from Spain to Ireland in the late Paleolithic or early Mesolithic. It seems increasingly likely that much of Ireland's Lusitanian fauna is in reality an artefact of this era of human expansion in the early part of the Postglacial era. In other words, it seems likely that these species were introduced accidentally with trade items or goods brought by boat from Iberia. See also Geographical isolation Habitat Habitat fragmentation References Biogeography Biology terminology Habitat Ecology terminology