In a gene cloning experiment, first a recombinant DNA molecule is constructed, where the gene of interest is ligated to the vector, [The step would not be affected] and introduced inside the host cell (transformation). Since, not all the cells get transformed with the recombinant / plasmid DNA, in the absence of selectable marker, it will be difficult to distinguish between transformants and non-transformant, because role of selectable marker is in the selection of transformants.