While the usual methods for taxonomy rely heavily on phenotype (physical characteristics) as markers distinguishing between species, a polyphyletic group, defined in this paper as a group including species with similar characteristics that were not inherited from a common ancestor, can defy these methods of categorization. But because other characteristics are not easily observed, the identification of polyphyletic groups takes time. Often only vigorous scientific discussion can prompt further investigation into a group's taxonomy; for instance, lively controversies about the classification of the Felimida complex of nudibranch species have resulted in genetic analyses of several specimens in an attempt to determine their origins. Such investigations may overturn decades of accepted scientific knowledge. Whereas the Felimida group had been classified based on phenotypes of four species, two of the “species” are in fact subgroups of two other species, while another, new species has been established based on the new species' lack of a common ancestor with the rest of Felimida. The usefulness of phenotype as a criterion for classification, then, appears to depend on several factors, including the phenotypic diversity of the genus as well as its interspecies interactions (for example, predator-prey interactions heavily influence phenotypic expression) and its geographic range (since similar phenotypes might evolve independently in similar but distant environments).