Weavers' tangled web

Madagascar Fody appears to not hybridise with other sympatric fody forms, apart from Forest Fody on its native Madagasacar. Photo: Drew Avery (commons.wikimedia.org).
Madagascar Fody appears to not hybridise with other sympatric fody forms, apart from Forest Fody on its native Madagasacar. Photo: Drew Avery (commons.wikimedia.org).

Two species of a radiation of finch-like weavers on the Indian Ocean islands have been found to be still hybridising, while the remaining island subspecies have been declared full species.

Madagascar Fody Foudia madagascariensis is a common species of weaver endemic to its nominal island, which it shares with another congeneric species, Forest Fody F omissa.

Madagascar Fody has also been introduced to several Indian Ocean islands, where there are also endemic fody taxa; in fact each island or island cluster has its own fody form with discrete differences in plumage. These introductions in essence bring the different fody forms into secondary contact after they have had the chance to evolve in isolation, thus creating the opportunity for a natural comparative experiment. The earliest introductions were about 235 years ago and Madagascar Fody appears not to be able to hybridise with these forms with which it has evolved in allopatry, that is with no geographical overlap.  

Researchers used mitochondrial and nuclear gene sequences from blood samples to investigate how isolated each form was, and specifically how much introgression (that is, gene transfer from one species to another) had occurred between them, if any.

The data collected indicated that speciation was more complete than that of the most famous example of seedeater radiation, that of Darwin's Finches on the Galápagos islands. All the forms away from Madagascar had no indication of introgression with the introduced species or each other, but a similar level of genetic divergence to the two mainland forms. Owing to the isolation and morphological difference between all island fodies, the authors suggest that all be declared full species.

However, despite Madagascar and Forest Fody both being as morphologically distinct - and as genetically distinct in their nuclear DNA - as any of the other forms, their mitochondrial DNA was remarkably similar. the simplest explanation for this is that the mixing of the two species at forest edges has encouraged hybridisation, and hybrids have indeed been discovered among mixed feeding flocks in that habitat. It is likely that both species are relatively young and have been forced into contact by the felling of most of Madagascar's forest cover.  

Therefore, on their islands of introduction the local fody forms were reproductively isolated and probably all deserved full species status. Contrastingly, widespread hybridisation had been taking place between the two Madagascan forms for over 10,000 years, underlining that isolation was not complete between the two, but ongoing.   

The full list of fody forms outside Madagascar is:

F sechellarum (granitic Seychelles)
F flavicans (Rodrigues);
F rubra (Mauritius);
F (eminentissima) consobrina (Grand Comore);
F (e) algondae (Mayotte);
F (e) anjuanensis (Anjouan);
F (e) eminentissima (Moheli);
F (e) aldabrana (Aldabra)

Warren, B H, Bermingham, E, Bourgeois, Y, Estep, L K, Prys-Jones, R P, Strasberg, D and Thébaud, C. 2011. Hybridization and barriers to gene flow in an island bird radiation. Evolution In press.