GEOGRAPHICAL PATTERNS OF OLD AND YOUNG SPECIES IN AFRICAN FOREST BIOTA - THE SIGNIFICANCE OF SPECIFIC MONTANE AREAS AS EVOLUTIONARY CENTERS

A widely accepted paradigm for speciation in tropical forests, the ref uge theory, requires periodic habitat fragmentation driven by global c limatic fluctuations to provide conditions for allopatric speciation. This implies that comparative species richness in refugia is due to lo ss of diverse communities in areas affected by climatic cycles. In thi s study we compare distribution patterns of bird and plant taxa which we consider to be of either deep phylogenetic lineages or recent radia tions, It is demonstrated that lowland areas which have been postulate d as Pleistocene refugia are dominated by species which represent line ages of pre-Pleistocene age. Since variations in species richness with in these forest tracts reflect currently apparent environmental variab les which might be considered to determine carrying capacity, we do no t need to postulate that richness is the result of changes in forest c over in the past. Recently diversified taxa of plants and birds are fo und mainly at the periphery of the main rain forest blocks and in habi tat islands outside them. Here, peak concentrations of young restricte d-range species are often congruent with clusters of old and biogeogra phically relictual species. It is suggested that this reflects special intrinsic environmental properties of these areas, in the form of lon g-term environmental stability caused mainly by persistent orographic rain or mist. In this case, richness is not necessarily due to extinct ion outside these areas. Stability not only enables survival of relict ual taxa, but also promotes morphological differentiation of radiating taxa, leading to aggregates of taxa of restricted distribution.