Tectonics, climate and the diversification of the tropical African terrestrial flora and fauna

Abstract

Tropical Africa is home to an astonishing biodiversity occurring in a variety of ecosystems. Past climatic change and geological events have impacted the evolution and diversification of this biodiversity. During the last two decades, around 90 dated molecular phylogenies of different clades across animals and plants have been published leading to an increased understanding of the diversification and speciation processes generating tropical African biodiversity. In parallel, extended geological and palaeoclimatic records together with detailed numerical simulations have refined our understanding of past geological and climatic changes in Africa. To date, these important advances have not been reviewed within a common framework. Here, we critically review and synthesize African climate, tectonics and terrestrial biodiversity evolution throughout the Cenozoic to the mid-Pleistocene, drawing on recent advances in Earth and life sciences. We first review six major geo-climatic periods defining tropical African biodiversity diversification by synthesizing 89 dated molecular phylogeny studies. Two major geo-climatic factors impacting the diversification of the sub-Saharan biota are highlighted. First, Africa underwent numerous climatic fluctuations at ancient and more recent timescales, with tectonic, greenhouse gas, and orbital forcing stimulating diversification. Second, increased aridification since the Late Eocene led to important extinction events, but also provided unique diversification opportunities shaping the current tropical African biodiversity landscape. We then review diversification studies of tropical terrestrial animal and plant clades and discuss three major models of speciation: (i) geographic speciation via vicariance (allopatry); (ii) ecological speciation impacted by climate and geological changes, and (iii) genomic speciation via genome duplication. Geographic speciation has been the most widely documented to date and is a common speciation model across tropical Africa. We conclude with four important challenges faced by tropical African biodiversity research: (i) to increase knowledge by gathering basic and fundamental biodiversity information; (ii) to improve modelling of African geophysical evolution throughout the Cenozoic via better constraints and downscaling approaches; (iii) to increase the precision of phylogenetic reconstruction and molecular dating of tropical African clades by using next generation sequencing approaches together with better fossil calibrations; (iv) finally, as done here, to integrate data better from Earth and life sciences by focusing on the interdisciplinary study of the evolution of tropical African biodiversity in a wider geodiversity context.

Keywords: African geology; Cenozoic; dated molecular phylogenies; fossils; palaeoclimate models; speciation models; tropical Africa.

Fig 1

The modern geophysical, climatic and vegetation setting of tropical Africa. (A) Topography of tropical Africa, modified from Guillocheau et al. (2018). Topographic and bathymetric data taken from the GEBCO 2020 Grid (doi: 10.5285/a29c5465‐b138‐234de053‐6c86abc040b9). Scale on bottom left is altitude in meters. Numbers refer to major rivers: 1, Niger; 2, Benue; 3, Ogooué; 4, Ubangi; 5, Uele; 6, Congo; 7, Zambezi; 8, Shire; 9, White Nile; 10, Blue Nile; 11, Nile. (B) Summed annual rainfall amount (colour‐shading, in millimetres) and averaged surface wind velocity (vectors, in m/s); rainfall data retrieved from the 1961–1990 climatology from the Climate Research Unit data set, wind velocities are averages from the 1989–2010 ERA‐Interim reanalyses [data from New et al. (2002) and Dee et al. (2011)]. (C) Major vegetation types across Tropical Africa following Sayre et al. (2013). Major divisions are shown according to Sayre et al. (2013). Delimitation of biodiversity hotspots taken from https://zenodo.org/record/3261807#.Xvu69lVKiUk (doi: 10.5281/zenodo.3261807).

Fig 2

Geological evolution of Africa during the Cenozoic. The maps depict the geological setting for six periods of the Cenozoic: (A) Late Paleocene (59–56 Ma), (B) Middle Eocene (48–41 Ma), (C) Early Oligocene (34–28 Ma), (D) Early Miocene (23–16 Ma), (E) Late Miocene (11.5–5.5 Ma) and (F) Early Pliocene (5.5–3.5 Ma). These maps characterize the palaeotopography and the palaeohydrography (drainage divides, catchment areas and paths of the main rivers) of Africa. They also include data such as shorelines, deltas, depositional alluvial plains and lakes. Reconstruction of the palaeotopography was based on the restoration of the stepped planation surfaces constituting the plateaus (Guillocheau et al., 2018). These planation surfaces, mainly pediments and pediplains associated with weathering processes of laterite type, result from uplifts sometimes enhanced by climate (precipitation) changes. See Guillocheau et al. (2018) for details. The highest surfaces are the oldest (from Late Cretaceous to Middle Eocene) and the lowest are the youngest (Pliocene).

Fig 3

Geo‐climate evolution and biological diversification of tropical African biodiversity. (A) Global temperature change during the Cenozoic (Hansen et al., 2008) and major climate and tectonic events across Africa. KPB, Cretaceous–Paleogene Boundary; PETM, Paleocene–Eocene Thermal Maximum; EECO, Early Eocene Climatic Optimum; MECO, Mid‐Eocene Climatic Optimum; EOT, Eocene–Oligocene transition; MCO, Miocene Climatic Optimum; MCT, Miocene Climate Transition; PPT, Pliocene–Pleistocene Transition. (B) Temporal representation of major uplift and volcanic events in central and eastern Africa (Sepulchre et al., 2006; Guillocheau et al., 2015, 2018). (C) Origin of major mountain peaks, lakes and arid regions in Africa (Marzoli et al., 2000; Gehrke & Linder, 2014; Zhang et al., 2014). (D) Origin of extant species of plants and animals based on time‐calibrated molecular phylogenies (see Appendix S1). (E) Crown node mean age estimates of plant and animal genera based on time‐calibrated molecular phylogenies (see Appendix S1).

Fig 4

Schematic representations of three selected mechanisms of speciation relevant to tropical Africa. The fragmentation–refugia mechanism is an example of the geographic model, the ecotone speciation mechanism is an example of the ecological model, and the vanishing refugia mechanism has elements of both model types. The figure provides predictions in relation to rate of speciation, and the roles of ecology, phylogenetic niche conservatism and climate change in the speciation processes (see Table 1 for further details). The time axis is not equivalent between mechanisms.