Little ecological divergence associated with speciation in two African rain forest tree genera

Thomas Lp Couvreur,Lars W Chatrou,Holly Porter-Morgan,Jan J Wieringa

doi:10.1186/1471-2148-11-296

Abstract

BackgroundThe tropical rain forests (TRF) of Africa are the second largest block of this biome after the Amazon and exhibit high levels of plant endemism and diversity. Two main hypotheses have been advanced to explain speciation processes that have led to this high level of biodiversity: allopatric speciation linked to geographic isolation and ecological speciation linked to ecological gradients. Both these hypotheses rely on ecology: in the former conservation of ecological niches through time is implied, while in the latter adaptation via selection to alternative ecological niches would be a prerequisite. Here, we investigate the role of ecology in explaining present day species diversity in African TRF using a species level phylogeny and ecological niche modeling of two predominantly restricted TRF tree genera, Isolona and Monodora (Annonaceae). Both these genera, with 20 and 14 species, respectively, are widely distributed in African TRFs, with a few species occurring in slightly less humid regions such as in East Africa.ResultsA total of 11 sister species pairs were identified most of them occurring in allopatry or with little geographical overlap. Our results provide a mixed answer on the role of ecology in speciation. Although no sister species have identical niches, just under half of the tests suggest that sister species do have more similar niches than expected by chance. PCA analyses also support little ecological differences between sister species. Most speciation events within both genera predate the Pleistocene, occurring during the Late Miocene and Pliocene periods.ConclusionsEcology is almost always involved in speciation, however, it would seem to have had a little role in species generation within Isolona and Monodora at the scale analyzed here. This is consistent with the geographical speciation model for TRF diversification. These results contrast to other studies for non-TRF plant species where ecological speciation was found to be an important factor of diversification. The Pliocene period appears to be a vital time in the generation of African TRF diversity, whereas Pleistocene climatic fluctuations have had a smaller role on speciation than previously thought.Ecological niche modeling, species level phylogeny, ecological speciation, African tropics, Isolona, Monodora, Annonaceae

Highlights

The tropical rain forests (TRF) of Africa are the second largest block of this biome after the Amazon and exhibit high levels of plant endemism and diversity
Understanding the evolutionary processes responsible for high species richness of TRF has been a major focus of evolutionary biology [6], most studies have focused on the Amazon and Australian rain forests
Two main hypotheses have been advanced to explain speciation processes that have led to high levels of biodiversity in African TRF [7,8]: 1) geographic isolation between populations restricted to rain forest patches resulting in allopatric speciation; 2) divergent selection related to ecological gradients where genetic and/or geographic isolation is minor [7,14]

Summary

Introduction

The tropical rain forests (TRF) of Africa are the second largest block of this biome after the Amazon and exhibit high levels of plant endemism and diversity. Two main hypotheses have been advanced to explain speciation processes that have led to high levels of biodiversity in African TRF [7,8]: 1) geographic isolation between populations restricted to rain forest patches resulting in allopatric speciation (e.g. the refuge model, mountain speciation model, riverine barrier model [9,10,11,12,13]); 2) divergent selection related to ecological gradients where genetic and/or geographic isolation is minor [7,14]. Ecology is central to the processes of divergence and speciation [15,16] but in the two speciation models above (geographic isolation vs divergent selection) ecology plays contrasting roles [17] In the former, it would be expected that the variables that make up a niche have low rates of change over a phylogeny (niche conservatism [18,19] or niche stasis [20]). A possible first step in unraveling TRF diversification in Africa is to understand the role of ecology in explaining present day diversity

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