Genetic analysis of Herald, Trindade and Kermadec Petrels in Mauritius has revealed a breakdown of species boundaries.
The three closely related Pterodroma species all colonised Round Island, near Mauritius in the Indian Ocean, within the last 100 years. When confirmed in the 1940s, Round Island petrels were believed to be one species, Trindade Petrel, but Kermadec Petrels were discovered breeding there in the 1980s, and Herald Petrels in the 1990s. Currently all three are regarded as separate species, though no one character is diagnostic for any of them, leading some to classify them as subspecies of one superspecies.
A largely British team sequenced a gene fragment from Round Island and Pacific individuals of all three species, and used comparisons of this and their louse species to examine their relationships, along with Henderson Petrel, only recently split from Herald Petrel on the basis assortative mating, calls and DNA differences.
Plumage characerisitics were used to tentatively classify individuals to their currently accepted species designation, but many birds are present on the islands that have intermediate plumages and calls. Distinctive haplotypes - inherited combinations of genes - from dark-shafted birds assignable to Round Island Trindade Petrel were also found in Pacific Kermadec and Herald Petrels, as well as Round Island Kermadec Petrels. The different Atlantic Trindade and Pacific Kermadec haplotypes were differentiated, and Henderson Petrel haplotypes were shown to be likely to be ancestral.
Genetic differences do not show clear separation between the four 'species', though plumage, calls and parasite distribution, as well as the presence of some isolated haplotypes, does imply that these petrels are not the same species. Ancestors of the present-day birds may already have possessed significant polymorphism (that is, dark, intermediate or pale phases) or there may be a long history of hybridisation.
The simplest conclusion is that multi-species hybridisation between species still not completely isolated from each other has produced 'leakage' of genetic material between populations from both Atlantic and Pacific Oceans in the Indian Ocean. Hybrid pairs between birds showing 'specific' differences have also been recorded, reinforcing this conclusion.
Interestingly, the ability to colonise Round Island appears to have been facilitated by human habitat changes, as the original hardwood forests were destroyed by goats and rabbits, exposing the bare volcanic rock and opening it to breeding petrels, which prefer to nest in the bare rocks. Consequently, habitat degradation may well influence the evolution of other such species, resulting in the homogenisation of distinct species. It is also now apparent that some species are composed of mosaics of other species, and that this situation may be more widespread than previously realised.
Reference
Brown, R M, Jordan, W C, Faulkes, C G, Jones, C G, Bugoni, L, Tatayah, V, Palma, R L and Nichols, R A. 2011. Phylogenetic Relationships in Pterodroma Petrels Are Obscured by Recent Secondary Contact and Hybridization. PLoS ONE e20350: 1-8.
