The world's oceans are host to an enormous diversity of drifting, microscopic organisms, known as plankton. How this biodiversity has arisen has puzzled biologists for decades. An international team of researchers has now succeeded in elucidating how new planktonic species are formed, providing an explanation for the large biodiversity seen today.
The findings have been published in the Proceedings of the National Academy of Sciences.
Speciation is the evolutionary process by which new species arise. One of the most common theories states that species are formed when a population is separated into two groups by some geographic barrier. Once populations are isolated, gene flow is interrupted and each group goes its own evolutionary way, which may lead to speciation. For animals and plants on land, such barriers are easy to imagine, for example a mountain range or the sea between two islands.
For marine microbial organisms, speciation by geographic isolation seems unlikely. In the first place because oceans are interconnected, therefore lacking physical barriers. Secondly, planktonic microorganisms form gigantic populations, which are easily dispersed around the globe by ocean currents, resulting in an assumed uninterrupted gene flow. So how are new planktonic species formed and how did this large diversity arise in the sea?
By analysing microsatellites (short, hypervariable fragments of DNA) in a globally distributed planktonic diatom species, the researchers showed that, in contrast to the current idea, the dispersal of marine microorganisms is limited. They found that gene flow between population of distant coasts is interrupted, resulting in a clear geographic genetic structuring.
Furthermore, the study showed that under appropriate geographic and environmental circumstances, like the pronounced climatic changes in the Pleistocene (2.6 million to 12,000 years before present) this population structuring may lead to speciation and hence may play an important role in diversification of marine planktonic microorganisms.
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