A species is an hypothesis. Even species that have been formally described and recognised for many years can still be subject to taxonomic changes given new data and analysis. In some of these cases, the species in question may be part of a complex of morphologically similar species. Working out the differences between the species in these complexes often takes a lot of time and study and sometimes the analysis of additional data, such as DNA.
Observations made by colleagues (Rob Davis and Kevin Thiele) and myself of consistent morphological differences among specimens of Ptilotus macrocephalus (commonly featherhead or green mulla mulla) from different parts of its range indicated that the species could consist of multiple species.
Ptilotus macrocephalus was originally described by Robert Brown in 1810 as Trichinium macrocephalum based on a specimen collected in Victoria. Many current species of Ptilotus were previously included in the genus Trichinium, but the two genera were combined due to a lack of strong characters to segregate them. Modern molecular phylogenies support the inclusion of Trichinium within Ptilotus.
Specimens identified under the name P. macrocephalus have been collected from Melbourne, Victoria to Cairns, Queensland and as far west as the Shark Bay, Western Australia. The distribution of the species covered most of the continent and occurred in the arid centre as well as higher rainfall areas of tropical and temperate eastern Australia.
Recognition of morphological differences between specimens in three regions of Australia (central and western, north-eastern, and south-eastern Australia) led us to hypothesise that specimens from these regions may be separate species.
In a study recently published in the journal Australian Systematic Botany, we examined hundreds of specimens from throughout the complete geographic range of P. macrocephalus. We found that there were discrete differences in the floral characters between specimens of these three regions, which supports the recognition of two new species as separate from P. macrocephalus.
As the name P. macrocephalus was applied to type specimens collected from south-eastern Australia, the species recognised from this region will keep the name. The two new species have been named Ptilotus psilorhachis (from eastern Queensland) and Ptilotus xerophilus (from central and western Australia).
We also analysed the distributions and environments of the areas where the three species occurred. This revealed that the species were partitioned based on climate as well as morphology. This pattern suggested to us that speciation could have been accompanied by shifts in climatic niches. Similar shifts in climatic niches have been observed elsewhere in Ptilotus. The topic of the ecological evolution of Ptilotus is the subject of a forthcoming research paper, which will provide insights into why the genus is so diverse in arid Australia.