The genus problem – Eucalyptus as a model system for minimising taxonomic disruption

The ongoing reclassification of higher plant genera (their division, combining and recircumscription) is the cause of extensive modifications to binomials. Many genus reclassifications have been far more taxonomically disruptive than necessary to achieve monophyletic genera. We suggest six rules that should be followed when considering genus reclassifications to minimise any taxonomic disruption and maintain the stability of genus‐species binomials without compromising their scientific validity. Here, we apply these rules to the eucalypts (Myrtaceae), a well‐known group of 840 tree and shrub species that dominate the Australian continent and have worldwide economic importance. Traditionally, most eucalypts have been placed in the genus Eucalyptus. However, the genus‐level classification of the eucalypts has been in a state of flux since 1995, when the segregate genus Corymbia was described, resulting in new binomials for over 100 species, including a number of well‐known species. More recently, the segregate genus Blakella has been described, again resulting in numerous new binomials, and again including a number of well‐known and widely grown species. Here, we demonstrate that the genus‐level classification of Brooker provided far less taxonomic disruption compared to the three‐genus classification of Hill and Johnson and the four‐genus classification of Crisp & al., while still achieving genus‐level monophyly (as understood at the time). We also demonstrate that a one‐genus classification will minimise any future genus‐level taxonomic disruption that would otherwise occur and provides the greatest taxonomic utility for the range of users dealing with this important group of plants as we continue to gain a deeper understanding of the relationships between taxa.