Geographical patterns of genetic subdivision in the cellar spider Pholcus phalangioides (Araneae).

Geographical patterns of gene flow and drift were analysed in the commensal cellar spider Pholcus phalangioides to get insight into the causes affecting genetic variation in this species strictly associated with man. Our sampling consisted of 23 subpopulations collected over five urban regions in central Europe (distances ranged from 920 km to sites within the same building complex). Five variable allozyme loci showed significant interpopulation subdivision (theta=0.146) and isolation by distance over the area studied. On a regional scale (up to 70 km) significant differentiation was found, but the genetic pattern did not correlate with distance. Moreover, significant two-locus disequilibria were detected and a recent reduction in the effective population size was indicated within six sites. These results suggest that in P. phalangioides a high potential of dispersal and strong effects of drift within small, demographically unstable mating units seem to cause significant, but unpredictable genetic differentiation patterns at lower geographical scales. Our study documents strong effects of drift in a strictly commensal species outside the murine rodents.