Longstanding theory of how species evolve in the oceans challenged

January 3rd, 2009 - 4:50 pm ICT by ANI  

London, Jan 3 (ANI): New evidence, uncovered by oceanographers, is all set to challenge one of the most long-standing theories about how species evolve in the oceans.

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 are considered separate species.

But, research by Dr Philip Sexton and Dr Richard Norris, both of the Scripps Institution of Oceanography, San Diego, suggests that this mode of diversification may not be as prevalent for oceanic creatures as it is for land dwellers.

As part of the research, 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 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 Earths climate.

Further scrutiny of the sediments revealed that the second Atlantic appearance of this plankton species at 2.0 million years ago was pulsed, and each pulse lasted 19,000 years, corresponding to cyclic oscillations in Earths 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. (ANI)

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