1. A botanical term designating the systematic study of populations and species, especially by means of ecological or experimental methods.
2. The study of the relationships among groups of species using criteria such as morphology, biochemistry, and DNA comparisons, especially to determine the evolutionary history of a species (used with a singular verb).
3. The area of systematics in which experimental taxonomic techniques are applied to investigate the relationships between taxa.

Such techniques include serological methods, biochemical analysis, breeding experiments, and cytological examination, in addition to the more established procedures of comparative anatomy.

Evidence from ecological studies may also be brought to bear.