HIERARCHICAL GENETIC-STRUCTURE AND EFFECTIVE POPULATION SIZES IN PHALACRUS-SUBSTRIATUS

In this paper we present an analysis of the hierarchical population st ructure of the mycophagous beetle Phalacrus substriatus. The analysis showed that P. substriatus is substructured at both hierarchical level s studied, among islands and among local populations within islands. T he level of differentiation among local populations was about three ti mes as high as among different islands (F-PL = 0.043 and F-LT = 0.013, respectively). This stands in marked contrast to the patterns expecte d, based on the dispersal of P. substriatus, as the average dispersal distance of individual beetles is less than a metre per generation. Se veral explanations are discussed which can explain the observed patter ns. We also estimated the effective population size for both hierarchi cal levels. The results show that the N-e/N ratios are well below unit y, both for local populations and for islands. The aver age N,IN ratio for local populations was only 0.210 (geometric mean, 0.172), whereas for islands the observed N-e/N ratios ranged from 0.75 to 0.98. Popul ation subdivision is expected to increase the global (island) effectiv e population size under equilibrium situations. However, random extinc tions and recolonizations can lead to significant reductions in the gl obal effective population size. Because population turnover is a commo nly occurring phenomenon in P. substriatus, we argue that this is, at least partly, responsible for the low N-e/N ratios observed in this sp ecies. The low effective population sizes, both for local populations and for islands, will result in rapid erosion of a large proportion of the genetic variation present. The present study thus highlights the need to take random processes, such as extinction-recolonization dynam ics, into account when studying effects of spatial subdivision.