A polyphyletic group includes members who have characteristics of analogous similarity. Major branches give rise to less major ones. And, these give rise to even smaller clusters of branches. The recognition of this heirarchy of different levels of branching gave rise to the Linnaean system of classification. The species is a very special unit of classification because species seems to be a natural unit of classification.
Species act as individual evolutionary units.
One would suspect that there might be a continuous range of sub-species, say, as one species began to give rise to another. Imagine either allopatric or sympatric speciation. Once two populations of the original species are reproductively isolated, changes may occur. However, the process of speciation seems generally to be quite rapid, thus producing a distinct species in a short period of time. Thus, the species unit appears as a natural, most fundamental, unit for most organisms.