First of all, it should be mentioned that any taxonomic category is a relative concept. Categories “family” and “genus” do not mean that there are several species included to them. There can be a single species. An example illustrating that is classifying of different pens. You have several pens of green color which are different by other characteristics and several pens of blue color which are also different by other features. You decided to identify color as the first criterion. Thus, you create two groups: green and blue. Then you divide each into smaller categories on the basis of some features of pens. However, later you discover a pen of red color. You cannot put it into any of the existing groups. Thus, you have to add group “red” with a single type of pen. There may be other red pens discovered later. Thus, you have to identify specific features in the red pen and assign further smaller categories for it. The same thing is with the species.