Abstract
Allopatric speciation requiring an unbroken period of geographical isolation has been the standard model of neo-Darwinism. While doubts have been repeatedly raised, strict allopatry without any gene flow remains a plausible mechanism in most cases. To rigorously reject strict allopatry, genomic sequences superimposed on the geological records of a well-delineated geographical barrier are necessary. The Strait of Malacca, narrowly connecting the Pacific and Indian Ocean coasts, serves at different times either as a geographical barrier or a conduit of gene flow for coastal/marine species. We surveyed 1700 plants from 29 populations of 5 common mangrove species by large-scale DNA sequencing and added several whole-genome assemblies. Speciation between the two oceans is driven by cycles of isolation and gene flow due to the fluctuations in sea level leading to the opening/closing of the Strait to ocean currents. Because the time required for speciation in mangroves is longer than the isolation phases, speciation in these mangroves has proceeded through many cycles of mixing-isolation-mixing, or MIM, cycles. The MIM mechanism, by relaxing the condition of no gene flow, can promote speciation in many more geographical features than strict allopatry can. Finally, the MIM mechanism of speciation is also efficient, potentially yielding mn (m > 1) species after n cycles.
Highlights
Speciation driven by geographical isolation with no possibility of gene flow, or strict allopatric speciation, has been the standard model of neo-Darwinism [1,2]
We analyse the divergence in five distantly related taxa of mangroves, which are woody plants that independently invaded the intersection between land and sea within the last 100 million years (Myrs) [21,24,25,26]
Because mangroves occupy a narrow band along tropical coasts, their distributions are essentially one-dimensional, making it easier to identify geographical barriers between species
Summary
Speciation driven by geographical isolation with no possibility of gene flow, or strict allopatric speciation, has been the standard model of neo-Darwinism [1,2]. Occasional exceptions are acceptable to this view [3,4,5], extensive violations of strict allopatry would contradict many of its central tenets. One of these tenets is the nature of species as defined by the Biological Species Concept [2]. The argument for strict allopatry has usually been that gene flow would homogenize diverging populations and retard speciation [2]. The stringent requirement for complete geographical isolation, is not without difficulties. Chief among them is the paucity of geographical features that can fully stop gene flow to sustain
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