Erosive processes after tectonic uplift stimulate vicariant and adaptive speciation: evolution in an Afrotemperate-endemic paper daisy genus

Abstract

BackgroundThe role of tectonic uplift in stimulating speciation in South Africa’s only alpine zone, the Drakensberg, has not been explicitly examined. Tectonic processes may influence speciation both through the creation of novel habitats and by physically isolating plant populations. We use the Afrotemperate endemic daisy genus Macowania to explore the timing and mode (geographic versus adaptive) of speciation in this region. Between sister species pairs we expect high morphological divergence where speciation has happened in sympatry (adaptive) while with geographic (vicariant) speciation we may expect to find less morphological divergence and a greater degree of allopatry. A dated molecular phylogenetic hypothesis for Macowania elucidates species’ relationships and is used to address the potential impact of uplift on diversification. Morphological divergence of a small sample of reproductive and vegetative characters, used as a proxy for adaptive divergence, is measured against species’ range distributions to estimate mode of speciation across two subclades in the genus.ResultsThe Macowania crown age is consistent with the hypothesis of post-uplift diversification, and we find evidence for both vicariant and adaptive speciation between the two subclades within Macowania. Both subclades exhibit strong signals of range allopatry, suggesting that geographic isolation was important in speciation. One subclade, associated with dry, rocky environments at high altitudes, shows very little morphological and ecological differentiation but high range allopatry. The other subclade occupies a greater variety of habitats and exhibits far greater morphological differentiation, but contains species with overlapping distribution ranges.ConclusionsSpecies in Macowania are likely to have diversified in response to tectonic uplift, and we invoke uplift and uplift-mediated erosion as the main drivers of speciation. The greater relative morphological divergence in sympatric species of Macowania indicates that speciation in the non-sympatric taxa may not have required obvious adaptive differences, implying that simple geographic isolation was the driving force for speciation (‘neutral speciation’).

The evolution of African plant diversity

“…The most recent specialization might be to the subalpine conditions along the summits of the Drakensberg, subsequent to the Pliocene uplift of ca. 900 m (Partridge and Maud, 2000), with erosional fragmentation leading to allopatric differentiation, for example in the daisy genus Macowania (Bentley et al, 2014). This scenario envisages a series of local radiations of a widespread southern African flora in response to new, extreme habitats, as these become available, and sees the iconic Cape flora as a part of this Austro-temperate Flora.…”

Historical biogeography of Boraginales: West Gondwanan vicariance followed by long‐distance dispersal?

“…The age of this corridor is consistent with the age of African disjunct lineages, such as Macowania Oliv. (Asteraceae; Bentley et al, 2014) and Thamnosma Torr. & Fr em.…”

Diversification of the cold-adapted butterfly genus Oeneis related to Holarctic biogeography and climatic niche shifts

“…Vila et al, 2011;Condamine et al, 2012;Bentley et al, 2014). A clade is rarely the pure product of allopatry (Muller and Beharegaray, 2010;Imada et al, 2011).…”