Abstract
BackgroundThe Eastern Arc Mountains of Africa have become one of the focal systems with which to explore the patterns and mechanisms of diversification among montane species and populations. One unresolved question is the extent to which populations inhabiting montane forest interact with those of adjacent lowland forest abutting the coast of eastern Africa. The Tiny Greenbul (Phyllastephus debilis) represents the only described bird species within the Eastern Arc/coastal forest mosaic, which is polytypic across an altitudinal gradient: the subspecies albigula (green head) is distributed in the montane Usambara and Nguru Mountains whereas the subspecies rabai (grey head) is found in Tanzanian lowland and foothill forest. Using a combination of morphological and genetic data, we aim to establish if the pattern of morphological differentiation in the Tiny Greenbul (Phyllastrephus debilis) is the result of disruptive selection along an altitudinal gradient or a consequence of secondary contact following population expansion of two differentiated lineages.ResultsWe found significant biometric differences between the lowland (rabai) and montane (albigula) populations in Tanzania. The differences in shape are coupled with discrete differences in the coloration of the underparts. Using multi-locus data gathered from 124 individuals, we show that lowland and montane birds form two distinct genetic lineages. The divergence between the two forms occurred between 2.4 and 3.1 Myrs ago.Our coalescent analyses suggest that limited gene flow, mostly from the subspecies rabai to albigula, is taking place at three mid-altitude localities, where lowland and montane rainforest directly abut. The extent of this introgression appears to be limited and is likely a consequence of the recent expansion of rabai further inland.ConclusionThe clear altitudinal segregation in morphology found within the Tiny Greenbul is the result of secondary contact of two highly differentiated lineages rather than disruptive selection in plumage pattern across an altitudinal gradient. Based on our results, we recommend albigula be elevated to species rank.
Highlights
The Eastern Arc Mountains of Africa have become one of the focal systems with which to explore the patterns and mechanisms of diversification among montane species and populations
The Tiny Greenbul (Phyllastephus debilis) represents the only described bird species within the Eastern Arc/ coastal forest mosaic, which is polytypic across an altitudinal gradient
Morphological variation Birds from the lowland coastal forests of East Africa (n = 49), subspecies rabai, are generally small (Male: weight 14.2 ± 0.57 g, wing 68.3 ± 2.4 mm, bill 13.7 ± 0.82 mm, tail 64.0 ± 2.7 mm, tarsus 17.1 ± 0.94; Female: weight 14.4 ± 0.48 g, wing 66.1 ± 3.1 mm, bill 13.5 ± 0.74 mm, tail 61.1 ± 3.7 mm, tarsus 17.7 ± 0.69) with a mouse grey crown, pale grey underparts with bright yellow streaks (Picric Yellow lateral feather edges) contrasting with a very pale grey throat and extensive pale yellow belly
Summary
The Eastern Arc Mountains of Africa have become one of the focal systems with which to explore the patterns and mechanisms of diversification among montane species and populations. The Tiny Greenbul (Phyllastephus debilis) represents the only described bird species within the Eastern Arc/coastal forest mosaic, which is polytypic across an altitudinal gradient: the subspecies albigula (green head) is distributed in the montane Usambara and Nguru Mountains whereas the subspecies rabai (grey head) is found in Tanzanian lowland and foothill forest. Extensive field research within the Eastern Arc has taken place over the past 15 years which has led to the description of several new species and the development of novel hypotheses concerning patterns of differentiation among species and populations distributed in East Africa [4,8,9] Despite this accumulating body of knowledge, it still remains to be determined to what extent populations inhabiting montane forest interact with populations distributed in adjacent foothills and along a narrow ribbon of lowland forest abutting the coast of eastern Africa [e.g. The existence of some greenish feather edges on the nape and crown of some lowland birds could lead to the alternate interpretation that gene flow occurs, plumage is under directional selection, with greener plumage being favored in the wetter high-altitude habitats
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
