One possible evolutionary scenario that could explain this observation is the following: when an overlapping region is 'created'for example by the elongation of one of the genes involved, then it is likely to initially be under nonnegative selection. Since the organism survived both with and without the overlap, it might be expected to be able to evolve without detrimental effects. A thus possible behaviour would be for the newly coding region to be encouraged to evolve quickly, whilst the other gene remains under negative selection as before. The estimated selection strengths may subsequently help deduce which overlaps are the 'newer' regions -for example our study suggests that the pol gene extended itself both onto the gag and the vif gene. The effect would essentially be similar to that noted on selection occurring on duplicated genes, where the duplicated gene reaches fixation in the population due to initially being under positive selection [24] .