The white-eyes Zosteropidae have long been known to be a highly speciose family, and also believed to have given rise to several aberrant taxa, traditionally separated in the genera Speirops, Rukia, Woodfordia and Chlorocharis.
Speirops is also a fine but lesser known example of endemic island adaptive radiation. Endemic to the Gulf of Guinea island chain - Bioko, Príncipe, São Tomé and Annobón - and neighbouring Cameroon, a number of species have evolved from just one or two common ancestors during one or two colonising events. More well-known examples of these adaptive radiations are the Hawaiian honeycreepers and Darwin's finches, and all are renowned for their rapid and very varied speciation.
A new study has underlined just how prone to speciation and how rapid such events can take place. Taking the example of the Gulf of Guinea white-eyes, which are traditionally thought to be the result of many colonisation events and are grouped into two genera, Zosterops and Speirops, the researchers used both nuclear and mitochondrial genes to track the evolution of these quite physically different genera and species. Together, the two genera match the entire amount of morphological variation within the Zosteropidae.
Instead of a large number of colonisations originating from neighbouring Cameroon, the birds' genes showed that only two independent colonisations had occurred, and that all five species found on the three most oceanic islands had derived from one common ancestor. Not only this, but the amount of physical differentiation - 'phenotypic divergence' - did not rely upon the amount of time that a particular species had been separate from its immediate ancestor. The 'aberrant' São Tomé Speirops birds derived from the most recent colonisation.
|Black-capped (São Tomé) Speirops and São Tomé White-eye, the latter now shown to be entirely separate from its Principe counterpart. Photo: Martim Melo.
The colonisations were both dated to well within the last three million years, making the two radiations much younger than the Gulf of Guinea islands themselves. The formerly near-endemic Speirops genus is firmly nested with Zosterops, and the implication is that it should be subsumed into the larger genus. The authors suggest that the different Speirops species evolved in sympatry (that is, geographically alongside) with a pre-existing Zosterops species, resulting in their widely varying plumage details - for instance São Tomé Speirops is dark brown with a prominent eye-ring, while Príncipe Speirops is nearly white with no eye-ring.
The Príncipe and São Tomé subspecies of Z ficedulinus, however, long thought to be sister subspecies of the same form, are in fact convergent and not each others' closest relatives - their genetic distance of c4.5 per cent makes them clear cryptic species. Sympatric species pairs on each of the islands have developed large and small forms in tandem, implying that adaptations to different ecological niches is a major driver and accelerator in the diversification of Zosterops in the islands. As their bills remain similar in structure, it is size that enables each species to specialise on different insect forms.
Other workers have recently shown that the three other 'aberrant' genera (see above) are also appropriate to be synonymised into Zosterops, making it both the most speciose bird genus and the most evolutionarily plastic.
Melo, M, Warren, B H and Jones, P J. 2011. Rapid parallel evolution of aberrant traits in the diversification of the Gulf of Guinea white-eyes (Aves, Zosteropidae). Molecular Ecology 20: no. doi: 10.1111/j.1365-294X.2011.05099.x