Longstanding Theory Of Origin Of Species In Oceans Challenged

Most scientists believe that allopatric speciation, where different species arise from an ancestral species only after breeding populations have become physically isolated from each other, is the dominant mode of speciation both on land and in the sea. The key to this theory is the existence of some kind of physical barrier that operates to restrict interbreeding (gene flow) between populations so that, given enough time, such populations diverge until they’re considered separate species.

For example, finches that were blown by storms from South America to the Galapagos Islands (and were studied by Charles Darwin) were consequently isolated from their host populations and these isolated breeding colonies evolved separately from each other until they became separate species.

Research by Dr Philip Sexton formerly of the National Oceanography Centre, Southampton (now at the Scripps Institution of Oceanography, San Diego) and Dr Richard Norris (also of Scripps) suggests, however, that this mode of diversification may not be as prevalent for oceanic creatures as it is for land dwellers and somewhat controversially, they assert that the above model of speciation may actually be very rare in the world’s oceans.

The oceans are not as uniform as one would think, but rather are made up of regional water masses that are distinct in their temperature and salinity. It has been theorized that the boundaries between these water masses act as barriers to the movement of plankton, which are organisms that cannot actively swim against currents, but instead drift with them. The existence of these supposed ‘barriers’ has resulted in the general assumption amongst scientists that allopatric speciation is the dominant mode of plankton diversification throughout the oceans. However, the new work published in the journal Geology suggests an altogether different picture.

Sexton and Norris examined the fossils of Truncorotalia truncatulinoides (a species of microscopic plankton and part of the group called ‘foraminifera’) buried in sediment layers beneath the seabed. By looking at different sediment layers from around the world containing these fossils, they were able to track the spread of this species from its ancestral home to its current distribution.

Previous work on this species had indicated that it first appeared about 2.8 million years ago in the Southwest Pacific and took until 2.0 million years ago to spread into other oceans. In line with the popular theory of allopatric speciation, previous thinking had been that the confinement of T. truncatulinoides to the Southwest Pacific for 800,000 years demonstrated that some kind of barrier (caused by the particular pattern of ocean currents) had restricted its range for that entire interval.

However, a detailed examination of sediment layers at two sites in the Atlantic revealed that T. truncatulinoides made a brief appearance in the Atlantic roughly 2.5 million years ago before disappearing again. Crucially, this appearance and subsequent disappearance exactly coincided with a major change in Earth’s climate. Further scrutiny of the sediments revealed that the second Atlantic appearance of this plankton species at 2.0 million years ago was ‘pulsed’; each pulse lasted 19,000 years, corresponding to cyclic ‘oscillations’ in Earth’s solar orbit associated with the Ice Ages.

Sexton and Norris propose that it was the climate, and its role in determining the availability of favourable oceanic habitat, that restricted the distribution of T. truncatulinoides, rather than the presence of physical ocean barriers. In this new view, plankton are freely dispersed throughout the ocean but local conditions determine whether or not the species can ‘take hold’ and thrive. An analogy is that of coconuts, which sometimes wash up on the shore of Britain; cold temperatures prevent coconuts from germinating, but should the climate suddenly shift to a subtropical state, coconut trees might become a common sight lining Britain’s shores.

This new idea that there are few, if any, barriers to the free dispersal of plankton throughout the world’s oceans has been corroborated by genetic research showing that rates of gene flow throughout the oceans are remarkably high. Furthermore, distributions of a number of larger ocean-dwellers such as tuna and molluscs show that, despite having regions of favoured habitat, small numbers of them are regularly found outside of their ‘core range’. These observations suggest that species’ distributions are more controlled by habitat availability rather than by an inability to disperse.

Sexton and Norris’ findings augment a growing body of evidence which support the idea that sympatric speciation, where different species arise from a parent species without the presence of physical barriers, is more common than previously thought. In this mode of speciation, the necessary isolation might instead be achieved through shifts in the timing or depth of reproduction. However, until more research offers a clearer picture of how speciation occurs in the oceans, Sexton and Norris’ contention that sympatric and other similar processes are the “prevalent modes of marine speciation” will, no doubt, remain at odds with prevailing theories.

Sexton, Philip F. & Norris, Richard D. Dispersal and biogeography of marine plankton: long-distance dispersal of the foraminifer Truncorotalia truncatulinoides. Geology, 36 (11), 899-902 (2008).