The area of study focusing on the development of a classification system, or taxonomy, for plants based on their evolutionary relationships (phylogeny). The assumption is that if classification reflects phylogeny, reference to the classification will help researchers focus their work in a more accurate manner. This principle is illustrated by the example of a mother and daughter, with a great deal being known about the genetic traits of the mother. The genetic traits exhibited by the daughter can be largely predicted in this case, whereas for two people of unknown relationships no conjecture is possible regarding their genetic similarity. The task is to make phylogeny reconstruction as accurate as possible. How this phylogenetic system of classification is to be constructed has been subject to varying opinions over time. The basic unit of classification is generally accepted to be the species, but how a species should be recognized has been intensely debated. A historical perspective is useful in explaining the nature of this controversy.