Most taxonomists will agree that the species level of classification holds the most biological relevance. Distinction at the species level theoretically reflects a group of closely related individuals. There are many criteria a taxonomist can use to characterize species. One method is to group individuals together that are capable of sharing a pool of genes through interbreeding. This concept lies at the heart of a "biological" species.
Comparative morphology has been a tool of plant taxonomy for centuries, classically providing the data from which phylogenetic hypotheses were reconstructed. Stated very simply, morphological classification uses similarity in physical characteristics (or characters) to establish species boundaries, as well as relationships between species. Characters can be qualitative in nature (e.g., presence or absence) or quantitative (e.g., size or number). By evaluating a large number of morphological characters, organisms sharing a greater number of characters can be classified as closely related. Certain criteria must be met for a character to be considered taxonomically useful.