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The experimental populations used in this study were derived from four independent large (breeding size of~ 2400) laboratory populations of Drosophila melanogaster (DB1-4) which in turn trace their ancestry to four outbred populations called JB1-4. The detailed maintenance regime and ancestry of the JB1-4 populations has been described elsewhere (Sheeba et al. 1998). The maintenance regime of the DB1-4 populations are similar to the JB1-4, except that the former set of flies are introduced into population cages on the 12th day after egg collection From each DBi population (where iE [1, 4]), we derived two populations: VBi (short for ‘vagabond’, subjected to selection for dispersal) and VBCi (corresponding no-dispersal control). Thus VB and VBC populations that share a numerical subscript (eg say VB1 and VBC1) were related by ancestry (DB1 in this case), and hence were always assayed together and treated as blocks in statistical analyses.