Variations in visible genetic polymorphisms are assumed to decrease in populations on small islands because of intense founder effects, genetic drift and inbreeding. However, we have found evidence of a marked enhancement of colour polymorphisms within populations on small oceanic islands that were colonized from the mainland. The source populations on the mainland of the land snail Euhadra peliomphala in four oceanic islands were estimated by phylogenetic analysis of mitochondrial DNA sequences. Diversity of shell colour was higher in the island populations than in the source populations on the mainland. In addition, the shell colour morphs differed not only among populations from different islands but also between the island populations and the source populations on the mainland. By contrast, no mtDNA variations were found in any of the island populations, even though the source populations possessed high mtDNA diversity. Thus, components of colour morphs changed in the island populations after their colonization, and colour polymorphisms are enhanced in these islands despite the loss of genetic variation. The above findings suggest that ecological mechanisms such as morphological release owing to a release from competition may overcome the tendency toward reduced genetic variation in islands to enhance the colour polymorphism.

Exceptionally high levels of intraspecific variation of mitochondrial DNA (mtDNA) sequences encoding 16s ribosoinal RNA (16SrRNA) lvere detected in populations of the land snail Euhadra pdiomplinla from the eastern part of Japan. Geographical isolation has occurred within small areas and has created many genetically distinct local populations. These are morphologically segregated into two cgroups: the first consists of populations from the southern part of the Izu Peninsula. and thr second of populations from other areas within the peninsula. They are separated by mountains in thc middle of thc peninsula. Sequencc divergences between these h\-0 groups were 7.6-9.59'0. The sequcnce divergences and geologcal ages of the Izu peninsula and its mountains suggest that the divergence rate of mitochondria1 l6SrRNA sequences of this species is extremely rapid, approximately 10% per inillion years. 'This rapid evolutionary rate of nitDNA and vicariance events due to tectonic events and sea level changes has also contributed to creating exceptional geographical variation in the nitDNA of this species. 0 2000 I hr 1.innea11 Snrirt) of Loridon ADDITIONAL KEY WORDS:evolutionary rategenetic variationmitochondrial

BackgroundLong-distance dispersal events have the potential to shape species distributions and ecosystem diversity over large spatial scales, and to influence processes such as population persistence and the pace and scale of invasion. How such dispersal strategies have evolved and are maintained within species is, however, often unclear. We have studied long-distance dispersal in a range of pest-controlling terrestrial spiders that are important predators within agricultural ecosystems. These species persist in heterogeneous environments through their ability to re-colonise vacant habitat by repeated long-distance aerial dispersal (“ballooning”) using spun silk lines. Individuals are strictly terrestrial, are not thought to tolerate landing on water, and have no control over where they land once airborne. Their tendency to spread via aerial dispersal has thus been thought to be limited by the costs of encountering water, which is a frequent hazard in the landscape.ResultsIn our study we find that ballooning in a subset of individuals from two groups of widely-distributed and phylogenetically distinct terrestrial spiders (linyphiids and one tetragnathid) is associated with a hitherto undescribed ability of those same individuals to survive encounters with both fresh and marine water. Individuals that showed a high tendency to adopt ‘ballooning’ behaviour adopted elaborate postures to seemingly take advantage of the wind current whilst on the water surface.ConclusionsThe ability of individuals capable of long-distance aerial dispersal to survive encounters with water allows them to disperse repeatedly, thereby increasing the pace and spatial scale over which they can spread and subsequently exert an influence on the ecosystems into which they migrate. The potential for genetic connectivity between populations, which can influence the rate of localized adaptation, thus exists over much larger geographic scales than previously thought. Newly available habitat may be particularly influenced given the degree of ecosystem disturbance that is known to follow new predator introductions.