There is an intimate connection between sexual reproduction and the maintenance of mobile DNA sequences. If mobile DNAs are invariably harmful, then they will lower the fitness of their hosts. In a clonal population that contains some cells which lack transposable elements altogether, this genotype will inexorably spread, replacing all transposable-element-containing cells, however much the transposable element copy number increases in these cells (Cavalier-Smith 1980; Hickey 1982). Instead, the maintenance of selfish mobile DNAs in clonal organisms requires some horizontal spread of the mobile DNAs. The low rate of horizontal transfer seen in most bacteria is, however, probably insufficient to counter the loss of elements by selection against their harmful effects (Condit, Stewart, and Levin 1988). For these reasons, we must suspect that the maintenance of mobile DNAs in such primarily clonal organisms as the eubacteria is the result of selection. In the case of composite transposons this selection is for...