Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Essentialist taxonomic systems were mostly monothetic, as they assumed that the presence or absence of a certain trait is decisive for inclusion of a species in a particular taxon. For example, monothetic systems classify angiosperm plants strictly on the basis of the number of individual flower components.
However, most modern systems are polythetic. Polythetic systems allow a taxon to be defined on the basis of a greater number of mutually interchangeable traits. In contrast to monothetic systems, polythetic systems are somewhat harder to use as classification schemes, i.e. as instruments for determination of organisms. On the other hand, only polythetic systems can be truly natural, i.e. can reflect the phylogenesis of the studied organisms.