Molecular Systematics and Evolutionary History of Akalats (Genus Sheppardia): A Pre-Pleistocene Radiation in a Group of African Forest Birds

摘要

The African forest robins (akalats) of the genus Sheppardia are represented by eight forest-dwelling species with restricted distributions in tropical Africa. All but three are strictly montane, inhabiting isolated highland areas in eastern and western Africa. Due to their subtle plumage variation and almost wholly allopatric distribution, determining systematic relationships based upon morphology has proven difficult. However, akalats, due to their distribution, offer an ideal opportunity to test models of speciation within tropical forests. We therefore investigated the phylogeny of species of this genus from presently sampled regions of their distribution using mtDNA sequence analysis. We found that the monophyly of described species, even from disjunct populations, is well supported. However, relationships among species is generally poorly resolved, with support given only to the paired relationships S. montana/S. lowei, S. bocagei/S. gunningi, and S. aequatorialis/S. cyornithopsis. This dataset lent support to S. montana and S. lowei representing a superspecies. All species appear to have evolved rapidly from a common ancestor around the Miocene/Pliocene transition, a time of the last uplift of East Africa's montane region. Surprisingly, intraspecific radiations suggest a far more recent population expansion in the upper Pleistocene, concordant with major climatic variation and vegetational changes. We discuss the implications of our results in the light of previous discussions of the montane speciation model.