Mechanisms and tempo of evolution in the African Guineo–Congolian rainforest

Abstract

This paper reviews how and when African rainforest diversity arose, presenting evidence from both plant and animal studies. Preliminary investigations show that these African forests are an assemblage of species of varying age. Phylogenetic evidence, from both African rainforest angiosperms and vertebrates, suggest a Tertiary origin for the major lineages in some of these groups. In groups where savannah species are well represented and rainforest species are a minority, the latter appear to be relics of a Mid-Tertiary rainforest. By contrast, species that are primarily adapted to rainforest have arisen in the past 10 Myr with the main morphological innovations dating from the Late Miocene, and Quaternary speciation dominating in large, morphologically homogeneous groups. The small number of species-level phylogenies for African rainforest plants hinders a more incisive and detailed study into the historical assembly of these continental forests.

Introduction and synthesis: plant phylogeny and the origin of major biomes

“…Less attention has been paid to how we might calibrate these phylogenies by assigning ages to individual nodes, but calibration is potentially the largest source of error in the dating of phylogenetic trees. Two principal methods of calibration have been used, taking information from: The high frequency of long-distance dispersal events indicated for many plant groups by authors in this volume (Richardson et al 2004;Plana 2004;Renner 2004;Pennington & Dick 2004; see further discussion in x 6 below) highlights the danger of the use of geological events, especially old ones, as calibration criteria. For example, Renner, Plana and Lavin et al demonstrate that endemic radiations of Melastomataceae sensu stricto, Begonia and Leguminosae (Ormocarpopsis, Indigofera) on Madagascar date only from the Miocene.…”

“…The bulk of the remaining papers adopt the approach of reviewing what is known of the history of species-rich biomes, ranging from the Cape of Africa (Linder & Hardy 2004), Australia (Crisp et al 2004), the rainforests of west Africa (Plana 2004), the Amazon rainforest (Pennington & Dick 2004) to Northern Hemisphere temperate forests (Donoghue & Smith 2004). In the cases of Africa (Jacobs 2004), the Neotropics (Burnham & Johnson 2004) and Australia (Hill 2004), there are palaeontological reviews.…”

“…This was followed by suggestions that Pleistocene climate changes had similarly influenced the evolution of plant species (Prance 1987;Whitmore 1987), though these authors clearly envisaged a burst of speciation in the Pleistocene after older speciation events. Although the basis of 'refuge theory' has been heavily criticized, especially for the Neotropics (Colinvaux et al 1996(Colinvaux et al , 2001, it remains influential, particularly for Africa, where there is evidence that Pleistocene climatic changes were severe (reviewed by Plana 2004). Plana (2004) evaluates the evidence from the few available dated phylogenies of African rainforest plant species for Pleistocene species diversification.…”

“…8 Myr ago). This review provides a background and time-scale against which Plana (2004) evaluates evidence for the time-scale of species diversification in West African (Guineo-Congolian) rainforests, though her review also touches upon savannahs and East African rainforests.…”

“…This paper, and the others in this volume (e.g. Crisp et al 2004;Pennington & Dick 2004;Plana 2004;Burnham & Johnson 2004;Jacobs 2004) use biome in a geographically restricted sense (sub-biomes and eccoregions as defined above), which allows better incorporation of phylogenetic and taxonomic information.…”