It fell to the study of Brown & Toft (1999) to compare the previously available data with their mitochondrial 12S rRNA sequence research and resolve the issue. Ignorance of this fact, however, led to attempts to resolve the evolutionary history and prehistoric biogeography of the cockatoos, which ultimately proved fruitless because they were based on invalid assumptions to start with. In consequence, the ancestors of the galah, the cockatiel and Major Mitchell's cockatoo were thought to have diverged from the main white cockatoo line at some stage prior to that group's main radiation this was indeed correct except for the placement of the cockatiel. Early DNA studies allied the galah with the cockatiel or placed it close to some Cacatua species of completely different appearance. Obvious morphological similarities are shared between the galah and the white cockatoos that make up the genus Cacatua and indeed the galah was initially described as Cacatua roseicapilla. It was separated in the monotypic genus Eolophus, but the further relationships were not clear. The classification of the galah was difficult. The changes brought by European settlement, which have been disastrous for many species, have been highly beneficial for the galah, because of the clearing of forests in fertile areas and the provision of stock-watering points in arid zones. While it is mostly found in inland areas, the galah is rapidly colonising coastal regions. It is common in all habitats in its range except for dense forests, especially those with high rainfall. It is common in metropolitan areas such as Adelaide, Perth, and Melbourne, and abundant in open habitats that offer at least some scattered trees for shelter. A large population expansion occurred in the 1960s following many escapees from captivity. The galah has been introduced to Tasmania through anthropogenic means and there were no sightings prior to 1848. The galah can be found throughout Australia, and is absent only from the driest areas and the far north of Cape York Peninsula.
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