Abstract
BackgroundThe major impact of Plio-Pleistocene climatic oscillations on the current genetic structure of many species is widely recognised but their importance in driving speciation remains a matter of controversies. In addition, since most studies focused on Europe and North America, the influence of many other biogeographic barriers such as the Sahara remains poorly understood. In this paper, climate-driven diversification was investigated by using a comparative phylogeographic approach in combination with phenotypic data in two avian species groups distributed on both sides of the deserts belt of Africa and Asia. In particular, we tested whether: 1) vicariance diversification events are concomitant with past climatic events; and 2) current ecological factors (using climate and competition as proxies) contribute to phenotypic divergence between allopatric populations.ResultsMitochondrial and nuclear sequence data indicated that the crested and Thekla lark species groups diverged in the early Pliocene and that subsequent speciation events were congruent with major late Pliocene and Pleistocene climatic events. In particular, steep increase in aridity in Africa near 2.8 and 1.7 million years ago were coincident with two north-south vicariance speciation events mediated by the Sahara. Subsequent glacial cycles of the last million years seem to have shaped patterns of genetic variation within the two widespread species (G. cristata and G. theklae). The Sahara appears to have allowed dispersal from the tropical areas during climatic optima but to have isolated populations north and south of it during more arid phases. Phenotypic variation did not correlate with the history of populations, but was strongly influenced by current ecological conditions. In particular, our results suggested that (i) desert-adapted plumage evolved at least three times and (ii) variation in body size was mainly driven by interspecific competition, but the response to competition was stronger in more arid areas.ConclusionClimatic fluctuations of the Plio-Pleistocene strongly impacted diversification patterns in the Galerida larks. Firstly, we found that cladogenesis coincides with major climatic changes, and the Sahara appears to have played a key role in driving speciation events. Secondly, we found that morphology and plumage were strongly determined by ecological factors (interspecific competition, climate) following vicariance.
Highlights
The major impact of Plio-Pleistocene climatic oscillations on the current genetic structure of many species is widely recognised but their importance in driving speciation remains a matter of controversies
Phylogeny As expected, two major groups broadly corresponding to the Thekla lark complex on one hand, and to the crested lark complex on the other hand were identified: for nuclear data, 6–7 mutations were fixed between the two species complexes, as compared to 0–1 mutation among sequences within groups, and for mtDNA, both groups were found to be monophyletic with strong bootstrap support (BS ≥ 94; Fig. 2; Table 1)
Periodic expansion of the Sahara appears to have played a major role in cladogenesis, by repeatedly isolating vicariant populations respectively north and south of the main desert landmass, but periods of aridification impacted populations of the Thekla lark complex living in the Horn of Africa, with (i) a split between populations in highland steppes and xeric grasslands that occurred ~1.8 million years ago (MYA); (ii) the separation of highland steppes populations by the Rift valley dated at ~1.1 MYA
Summary
The major impact of Plio-Pleistocene climatic oscillations on the current genetic structure of many species is widely recognised but their importance in driving speciation remains a matter of controversies. We tested whether: 1) vicariance diversification events are concomitant with past climatic events; and 2) current ecological factors (using climate and competition as proxies) contribute to phenotypic divergence between allopatric populations. In the standard allopatric model (in contrast to ecological speciation models), geographical isolation is viewed as a prerequisite for divergence [1]. While geographical isolation may be due to dispersal or vicariance (e.g. tectonics), the refuge theory argues that climate change is generally responsible for the fragmentation of a species range [2]. For populations that remain isolated a long time, climate-driven vicariance events may be sufficient for reproductive isolation to evolve by drift alone [3]. Climatic fluctuations of the Plio-Pleistocene (succession of glacial and interglacial events) may be expected to produce a stasis-punctuation model of evolution, where bursts of speciation are caused by major climatic changes [7,8,9,10] (hereafter, the climatedriven speciation hypothesis, from which the muchdebated "LPO" Late Pleistocene Origin model [11] is a derivative)
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