Changeable Hawk-Eagle - Systematics

Systematics

The Flores Hawk-Eagle has traditionally been treated as a subspecies of the Changeable Hawk-Eagle, but it is now often treated as a separate species, N. floris.

Two distinct groups exist in the Changeable Hawk-Eagle; one with crests and one without or with hardly visible crests. Dark morphs exist for some populations.

Changeable Hawk-Eagle

• Nisaetus cirrhatus cirrhatus

Gangetic plain southwards throughout India

Crested, no dark morph.

• Nisaetus cirrhatus ceylanensis

Sri Lanka (possibly also Travancore)

Smaller than nominate, crest proportionally longer on average. Apparently no dark morph.

Crestless Changeable Hawk-Eagle

• Nisaetus cirrhatus limnaeetus

Nepal, NE India, via Burma and Malay Peninsula along Wallace Line to Philippines

Much like nominate except crest. Dimorphic, with the dark morph chocolate-brown all over, tail base might appear lighter in flight.

• Nisaetus cirrhatus andamanensis

Andaman Islands

Similar to N. c. limnaeetus. Apparently no dark morph.

• Nisaetus cirrhatus vanheurni

Simeulue Island

Similar to N. c. limnaeetus. Apparently no dark morph.

Gamauf et al. (2005) analyzed mtDNA cytochrome b and control region sequence data of a considerable number of specimens of the Crested Hawk-Eagle and some relatives. Despite the large sample, even the most conspicuous dichotomy - that between the crested and crestless groups - was not as well resolved as it might have been expected to be.

The three small-island taxa (N. c. andamanensis, N. c. vanheurni, and N. floris) also appear as monophyletic lineages. Their placement is even more unresolved, with N. floris being apparently a very ancient lineage. The other two seem quite certainly to derive from N. c. limnaeetus. The latter taxon has a confusing phylogeny. Different lineages exist that are apparently not stable in space and time, are best described as polytomy, from which the similar island taxa derive.

Obviously, N. c. limnaeetus does not represent a monophyletic lineage. Neither the biological nor the phylogenetic species concepts, nor phylogenetic systematics can be applied to satisfaction. It appears as if the crested group is close to becoming a distinct species. The island taxa derived from N. c. limnaeetus appear to have undergone founder effects, which has restricted their genetic diversity. In the continental population, genetic diversity is considerable, and the evolutionary pattern of the 2 studied genes did not agree, and neither did the origin of specimens show clear structures. N. c. limnaeetus thus is best considered a metapopulation.

Gamauf et al. (2005) therefore suggest that the island taxa which are obviously at higher risk of extinction are, for conservation considered evolutionary significant units regardless of their systematic status. This case also demonstrates that a too rigid interpretation of cladistics and the desire for monophyletic taxa, as well as universal application of single species concept to all birds will undermine correct understanding of evolutionary relationships. It would even not be inconceivable to find mainland lineages to group closely with the western island taxa, if little genetic drift had occurred in the initial population. nonetheless, the divergence of this species' lineages seems to have taken place too recently to award them species status, as compared to the level of genetic divergence at which clades are usually considered distinct species.

N. c. limnaeetus appears for all that can be said with reasonable certainty basal pool of lineages in the crestless group that, despite not being monophyletic, should be considered a valid taxon as long as gene flow is possible through its range. In addition, as ancient DNA from museum specimens was used extensively, the possibility of ghost lineages must be considered. If it is assumed that all or most of the ancient lineages still exist today, considerable recombination must have taken place as the two genes' phylogenies do not agree much, indicating a healthy level of gene flow. Whether this still holds true today remains to be determined.