The exact relationship between these two species is controversial, but it is accepted that the relationships between the three polecats and the domesticated ferret are extremely close.
Splitters, those zoologists who tend to emphasize the importance of small differences between species, argue there are three species of polecat: the European polecat (Mustela putorius), the steppe polecat (M. eversmannii), and the black-footed ferret (M. nigripes). The domesticated ferret is either assigned a subspecies position (M. putorius furo), or given species rank (M. furo).
Lumpers, those zoologists who tend to emphasize the similarities between species, argue that the polecat group probably represents a single species that inhabits the entire northern hemisphere (M. putorius putorius, M. putorius eversmannii, and M. putorius nigripes). If this is ultimately accepted, the domesticated ferret will almost certainly be classified as M. putorius furo.
Currently, genetic studies have failed to resolve the wild ancestor of the ferret, so the use of M. furo for the domesticated form is appropriate. The relationship of the domesticated ferret to the black-footed ferret is extremely close Ñ they can all breed and form fertile hybrids. Since genetic studies show that the European polecat and steppe polecat are sister species (they share a common ancestor that is not shared by other species) and many zoologists are arguing the black-footed ferret is a subspecies of the steppe polecat, perhaps one could say that domesticated and black-footed ferrets are, ahem, “steppe”-sisters.