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Insular Evolution: Large And Big-Footed Voles In An Outer Archipelago

August 4, 2010

According to evolutionary theory, natural selection favours traits that enhance dispersal of populations to new habitats. The empirical evidence supporting this theory, however, is relatively scarce. A study carried out by researchers from the Faculty of Biological and Environmental Sciences of the University of Helsinki, along with their Swedish colleagues, reports rapid evolution of traits facilitating dispersal in an outer archipelago. The results was published today in the Proceedings of the Royal Society B biological research journal.

Field research revealed that field voles (Microtus agrestis) in the outer archipelago of Stockholm are larger and have longer feet than those living on the mainland. The field voles included in the study were of the same descent, for their habitats emerged from the sea approximately 500 to 1000 years ago as a result of land elevation following the melting of the last glaciers. The islands were colonised by field voles that swam there from the mainland.

The researchers measured and weighed field voles in two mainland and six outer archipelago locations between 1983 and 1987. Then they reared descendants of field voles originating from the mainland and from the archipelago in a laboratory for three years. This was to ensure that the phenotypic differences (body size, foot length) were genetically determined and not caused by, for example, variability in food availability or other environmental conditions. The laboratory-reared descendants of insular voles also had large bodies and long feet. The rate of evolutionary change, measured in darwins, was remarkably rapid.

The results show that insular voles have genetically adapted, through natural selection, to survive in the harsh conditions of the outer archipelago. Their large body size and long feet facilitate dispersal by swimming. Their elongated hind feet enhance swimming ability by enabling better propulsion. Large individuals have better endurance and higher energy capacity for moving long distances in water and on land than small individuals. In addition, large individuals are less susceptible to hypothermia because of their body mass.

Field vole populations in the outer archipelago of Stockholm offered the researchers an excellent model system for testing theoretical predictions born out of evolutionary theory. Once again, the results illustrate and underline the significance of insular populations, such as those on the Galapagos Islands, to research in evolutionary biology. Divergent island populations are also valuable to biological diversity.

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