Forest refugia in Western and Central Africa as 'museums' of Mesozoic biodiversity

Abstract

The refugial speciation model, or 'species pump', is widely accepted in the context of tropical biogeography and has been advocated as an explanation for present species distributions in tropical Western and Central Africa. In order to test this hypothesis, a phylogeny of the cryptic arachnid order Ricinulei, based on four nuclear and mitochondrial DNA markers, was inferred. This ancient clade of litter-dwelling arthropods, endemic to the primary forests of Western and Central Africa and the Neotropics, might provide insights into the mode and tempo of evolution in Africa. Twenty-six African ricinuleid specimens were sampled from eight countries spanning the distribution of Ricinulei on the continent, and analysed together with Neotropical samples plus other arachnid outgroups. The phylogenetic and molecular dating results suggest that Ricinulei diversified in association with the fragmentation of Gondwana. The early diversification of Ricinoides in Western and Central Africa around 88 (±33) Ma fits old palaeogeographical events better than recent climatic fluctuations. Unlike most recent molecular studies, these results agree with fossil evidence, suggesting that refugia may have acted as 'museums' conserving ancient diversity rather than as engines generating diversity during successive episodes of climatic fluctuation in Africa.

Figure 1.

Map of Western and Central Africa showing localities of the African Ricinulei samples from which DNA was sequenced for the present study. Land cover map after Mayaux et al. [15]. Hypothetical refugia at the Last Glacial Maximum after Maley [1].

Figure 2.

Maximum credibility tree for African Ricinulei and outgroup taxa, obtained by Bayesian inference in Beast [19]. 95% Confidence intervals for ages are represented by bars at nodes. Clade posterior probabilities above 50% are indicated above nodes and maximum-likelihood bootstrap frequencies below nodes. Posterior distributions for nodes of interest are also depicted below the tree. Palaeogeographic maps from Ron Blakey, NAU Geology at http://jan.ucc.nau.edu/~rcb7/.