The taxonomy has taken a new and actual meaning since the most general acceptation of evolution through development of the modern synthesis between darwinism and mendelism. The concept of biological species was a consequence of that development but the mode by which a new species arises is still a matter of controversy, as it is the amount of genetic change necessary for speciation to take place. The kinds of variability and the methods currently used in Genetics to distinguish between allied species are also reported together with some representative examples. Finally, the origin of higher taxonomic categories and the present status of this problem is briefly discussed.