A typological species can be defined on the basis of any trait occurring in at least some life stage of a member of the given species, where possible in all the individuals in the population. Simultaneously, theoretically any trait that can form the basis for differentiation of the members of a certain species from other species can be used as a diagnostic trait, i.e. a trait according to which membership in a particular species is recognized. The main criterion for a trait designated for definition of a typological species is its presence in the greatest possible number of individuals in the largest possible number of individuals. An ideal trait would occur in all the individuals in all the populations; however, it is frequently not an easy matter to find such a trait (Wiens & Servedio 2000). In contrast, the choice of a diagnostic trait is governed mainly by pragmatic considerations. Primarily a trait for which there is the lowest risk of a mistake in determining the species is chosen as a diagnostic trait. If, for example, it were necessary to decide whether to chose as the diagnostic trait morphological structure A, occurring only in the given species, in all the individuals, but which is difficult to distinguish, so that its presence is not found in 80% of individuals, or whether to chose morphological structure B, which occurs in only 90% of individuals, but whose presence or absence can be determined with 100% certainty, then obviously structure B is preferable. A side effect of the application of the definition of diagnostic traits is also that traits that do not have any relationship to the causes or mechanisms of evolutionary differentiation of the given species are employed as traits to differentiate the individual species in the vast majority of cases. If, for example, speciation occurred in a certain plant pest as a consequence of reorientation of part of the population to a new species of plant food, then the diagnostic trait could be chosen as the presence of a structure that is not in any way connected with the move to the new host species, for example a trait on the genitals rather than the ability to decompose a secondary metabolite of the new host species.