Speciation is the formation of new and distinct species, whereby a single evolutionary line splits into two or more independent ones. It is one of the key evolutionary processes and is responsible for the diversity of life that exists on Earth. For example, humans, chimpanzees and bonobos are close relatives by sharing the common ancestors and similar genetics, while, according to the analysis of genetic drift and recombination, humans and chimpazees speciated apart 4.1 million years ago.
The influencing factors of speciation are various and debated. Besides of natural events, speciation may also be induced artificially, through animal husbandry or laboratory experiments. To date, geological events and major climatic shifts are the common accepted driving forces of species vicariance, dispersant and ecological divergence.
The earliest frog fossils indicated their appearances about 125-213 millions years ago. Now, as the most diverse groups of vertebrates, more than 5,000 frog species have been described, and they are distributing from tropic to subarctic regions. Because of their obvious advantages, such as strict habitat specificity and physiological requirements, highly genetic structured population and significantly high resolution signals of historical responses to environmental perturbations, frogs are adopted as ideal model to study biological processes and to test biogeographic hypotheses.
East Asian brown frogs of the Rana chensinensis species complex provide an excellent system for studying speciation. This small complex includes three species, R. chensinensis, Rana kukunoris and Rana huanrensis and they spread from the eastern edge of the Qinghai–Tibetan Plateau (QTP) to Korea. Uplift of the QTP largely re-shaped the climate pattern and landscape features of central and eastern Asia, which have been hypothesized as the driving force of vicariant speciation in several species. Based on the facts, Ph.D candidate ZHOU Weiwei (Kunming Institute of Zoology, the CAS) and his colleagues have conducted series experiments to try to reveal the speciation mechanisms of Rana chensinensis species complex.
According to ZHOU, they have collected samples across the entire range of the complex and have three objectives there: (1) to reconstruct the evolutionary relationships of species in the R. chensinensis complex and reveal any existing cryptic divergence; (2) to explore the influence of orogenesis on the R. chensinensis species complex and gain insights into the relative roles played by vicariance and ecological selection in speciation; and (3) to test the macroevolutionary null hypothesis of no gene flow between species, and if gene flow is found, to explore its influence on speciation.
As the results, by using both mitochondrial DNA (mtDNA) and multiple nuclear DNA (nuDNA) markers, their analyses support a four-species hypothesis within the Rana chensinensis species complex including an un-described cryptic species. Uplift of the QTP and its consequential Late Cenozoic dramatic environmental changes are largely responsible for re-shaping the physiography and climate patterns of Asia. These events may be responsible for driving vicariant speciation within the R. chensinensis species complex, especially for R. kukunoris. Adaptation to diverging habitats probably leads the subsequent ecological divergence of the clades. Such divergence among-populations restricted organisms’ migration ability thus intensified the isolation among populations and kept them from mixing after re-contact. Hybridization occurs in contact zones, yet introgression and gene flow appear to have had little influence on speciation.
The main findings of this study have been published on Molecular Ecology (http://onlinelibrary.wiley.com/doi/10.1111/j.1365-294X.2011.05411.x/full).
