Phenotypic evolution of dispersal-enhancing traits in insular voles

Abstract

Evolutionary theory predicts that in metapopulations subject to rapid extinction-recolonization dynamics, natural selection should favour evolution of traits that enhance dispersal and recolonization ability. Metapopulations of field voles (Microtus agrestis) on islands in the Stockholm archipelago, Sweden, are characterized by frequent local extinction and recolonization of subpopulations. Here, we show that voles on the islands were larger and had longer feet than expected for their body size, compared with voles from the mainland; that body size and size-specific foot length increased with increasing geographical isolation and distance from mainland; and that the differences in body size and size-specific foot length were genetically based. These findings provide rare evidence for relatively recent (less than 1000 years) and rapid (corresponding to 100-250 darwins) evolution of traits facilitating dispersal and recolonization in island metapopulations.

Figure 1.

Map showing (a) study area indicated by the square. (b) The two mainland localities G = Granskär, U = Uppsala. (c) The six groups of islands in the Baltic Sea archipelago. S = Själsten, H = Inre Hamnskär, N = Norrpada, R = Rödlöga, A = Ängskär, F = Svenska Högarna. Circled plus symbol indicates the centre of each island group. Scale bar, (b) 30 km.

Figure 2.

(a,d) Mean (± s.e.) structural body size (measured as the first principal component based on body length and nine different skull traits); (b,e) body mass in g; and (c,f) size-specific hind foot length (expressed as least squares means (l.s.m.) from linear regression of hind foot length on body length). (a,b,c) data for populations of Microtus agrestis from two mainland six insular localities in the Stockholm archipelago. (d,e,f) data for descendants to mainland and insular voles reared in the laboratory using a common garden experimental design.

Figure 3.

(a) Variation in mean body size and (b) size-specific hind foot length among insular populations of Microtus agrestis as a function of inter-island fragmentation. Body size measured as first principal component. Hind foot length measured as least squares means (l.s.m.) from linear regression of hind foot length on body length. Error bars depict ± s.e.