Typically, a well-marked cline does not even allow to distinguish subspecies, as it is impossible to draw clear dividing lines between populations in such a case. In population genetics, a cline could include a spectrum of subspecies, as allele and haplotype frequencies tend to vary over a larger space; moreover, in evolution genetic lineage sorting usually lags behind establishment of locally-different phenotypes. Regardless, the variation in neither case will yield different species as long as the geographically spread populations can interbreed with one another.
Ring species are a related concept, usually stretching over a wider range from one end to the other than clines within a single species. In ring species, the geographical distribution is circular in shape, so that the two ends of the cline overlap with one another. But these two populations rarely interbreed due to the cumulative effect of the many changes in phenotype along the cline. The populations elsewhere along the cline interbreed with their geographically adjacent populations as in a standard cline. Ring species present an interesting problem for those who seek to divide the living world into discrete species.