But there is a category beyond species that is often used, but barely defined. There is no set criteria for the subspecies category, other than varied genetic differences gained from a split in the population or some other factor leading to population mutation.
An example: Species and genus are fairly clear cut categories. They have criteria. The organisms in a certain genus are morphologically and genetically very similar, while organisms in a certain species are able to successfully interbreed.
The problem with subspecies is that it has no definite criteria like that. The mutations in a given population that makes them distinct could be literally anything: Succeptability to a disease or genetic disorder, slight differences in morphological traits, organ size, color differences, growth rate, food preference, etc. But the subspecies can still interbreed with other subspecies in the species category (species, species, species). Where will the resulting hybrids be classified? New subspecies?
Defining a population is another issue. They are never very clear cut, as E.O. Wilson observes in The Diversity of Life:
What exactly is a subspecies? The textbooks define it as a geographical race, a population with distinctive traits occupying part of the range of the species.
What then is a population? We are in immediate trouble. It is easy to say that a clearly defined population, one recognizable by everyone at a glance, occupies an exclusive part of the range of a species. And geneticists like to add, for purposes of mathematical clarity but not as an absolute requirement, that the population is a “deme”: its members interbreed at random, and any member is equally likely to mate with any other member in the population, regardless of its location._____snip_____
To the south, in the mountains of northern Georgia and Alabama, there is another generally recognized subspecies, Plethodon cinereus polycentratus, separated from P. cinereus cinereus by 80 kilometers of redback-free terrain. A third subspecies, P. cinereus serratus, occurs in several widely separated localities in the hill country of Arkansas, Oklahoma, and Louisiana. These two additional races offer the same difficulties as the main northern subspecies. Their triple names are a convenient shorthand, the statement of a rough truth. The classification works so long as we recognize that dicing up the whole species geographically is imprecise and to a large degree arbitrary. Depending on the criteria used, there could be one subspecies of P. cinereus, or there could be hundreds.
Another problem is more political than anything. If subspecies is indeed an arbitrary classification, then many endangered populations of organisms are not endangered after all. This is hard, since these populations do have unique characteristics, and conservationists (including myself) want to preserve what biodiversity we have on this planet, as best we can.
Recently there was debate over a certain population (subspecies) of the Sand Mountain blue butterfly, a subspecies of another blue butterfly in the area. One of the biggest challenges to protecting these animals was their arbitrary classification.
There is still controversy surrounding the subspecies category - some will tell you it is viable, most others will push it aside as artificial - but it presents a definite, divisive problem. How to categorize the temporal, instantaneous states of life? After all, in the grand scheme of things, the classification of species is looking at only one still of the entire movie.
The thing is, there are real morphologic and genetic differences between populations of a certain species, but subspecies is inadequate in supplying us with a sensible, useful taxonomic category as of yet.