This distinctive species has been lumped with the Yellow Rosella in which the crimson red color is replaced by pale yellow. The two interbreed in their area of contact. In the past the hybrids were thought to be a separate variable species, the Adelaide Rosella in which the red color is replaced by blotchy orange. Recent studies have found that these color variants may confer resistance to the "beak and feather disease virus." This case has been cited as an example of the "Red Queen hypothesis" based on an episode in "Alice in Wonderland" in which the Red Queen advises Alice that she needs to keep running in order to stay in one place. Similarly, there is an evolutionary arms race in which the host evolves to escape from a disease while disease evolves to improve their success infecting the host. Thus both keep evolving to maintain the same equilibrium. Canon PowerShot SX60 HS.
References:
Collar, N. & Kirwan, G.M. (2019). Crimson Rosella (Platycercus elegans). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/54510 on 18 December 2019).
Joseph, L., Dolman, G., Donnellan, S., Saint, K.M., Berg, M.L. & Bennett, A.T.D. (2008) Where and when does a ring start and end? Testing the ring-species hypothesis in a species complex of Australian parrots. Proc. Roy. Soc. London (Ser. B Biol. Sci.) 275: 2431–2440.
Eastwood, J.R. et al. (2014) Phylogenetic analysis of beak and feather disease virus across a host ring-species complex Proceedings of the National Academy of Sciences 201403255; DOI: 10.1073/pnas.1403255111