Biological classification is also known as taxonomy . It is a science, and like most sciences has evolved over time. At various times different principles were adopted, and it is not rare for different scientists to use different methods. Since the early 20th century, groupings are supposed to fit the Darwinian principle of common descent . These days, molecular evolution studies, which use DNA sequence analysis as data, are popular. This is often called "phylogenetics", a branch or form of cladism . This approach creates an evolutionary Tree of life (biology) and uses characters ( traits ) to decide on the branches of the taxonomy .
A truly scientific classification should express the system of laws, inherent to the aspect of reality depicted in it, that determines the properties and relationships of the objects fixed in the classification. The systematization of these laws must take into account that in nature there are no strict delimitations and that transitions from one class to another are an inseparable property of reality. This demand upon classification is reflected in such special procedures as the use in library classifications of cross-references (“see” and “see also”) and the location of the same concept in different places of the classification.