Revising a Classical Model of Sympatric Speciation

A snail discovery is promising to shake up the age-old concept of species formation.

Northeastern University graduate student Meredith Doellman along with her advisors, Professors Geoff Trussell and Steve Vollmer and John Grahame – a collaborator from the University of Leeds in the UK – have found that a marine periwinkle snail species – also known as Littorina saxatilis – may have defied the simple textbook classifications of speciation by mixing both allopatric and sympatric speciation as it has evolved.

Sympatric Speciation
It is widely believed that new species most often arise when two populations become geographically isolated and diverge over time — a process known as allopatric speciation.

Another, and far less common, form of speciation is sympatric speciation. In this case, natural selection promotes divergence within a population that eventually leads to the creation of new species. Examples of sympatric speciation are rare, but garner special attention from scientists because of their novelty.

Challenging the Traditional Model
Until now, sympatric speciation was believed to only occur in species that inhabited the same geographic location in or around the same stretch of time.

The paper, published in the Proceedings of the Royal Society of London, Biological Series, raises new questions about this classic model system of sympatric speciation by showing that L. saxatilis and its periwinkle relatives have a much more complex evolutionary history than previously thought.

Researchers discovered that what was once thought to be one snail species may actually be different snail species inhabiting rocky shorelines on both sides of the North Atlantic. “The species L. saxatilis definitely needs to be redefined – it is probably two species,” said Trussell. “This also means that within this group there is evidence for multiple, repeated examples of sympatric speciation.”

Evolution in Action
This snail was found to have split into multiple morphological types (or species) in multiple locations in Europe, presumably due to differences in the intensity of crab predation. Essentially, high crab predation has caused the emergence of a thicker-shelled form of this species, which is less vulnerable to crab predation, and this species has evolved multiple times across Europe.

By analyzing DNA sequences from snails on both sides of the Atlantic, researchers were able to show that periwinkle snail types, which give live birth to baby snails, apparently originated not once, but twice in the past 640,000 years from their egg-laying relatives L. arcana and L. compressa.

Transatlantic Species
Both periwinkle lineages now co-exist on both sides of the Atlantic in a patchwork influenced by different colonization histories. They even show periwinkle snails that birth their young may be hybridizing with their egg-laying species.

“This study will not change how scientists view sympatric speciation, but it will change how they view speciation in this classic system,” Vollmer said.

This American-European study emerged as part of the NSF Research Coordination Network, “Coordinating Research on the North Atlantic” (CORONA) led by Dr. Cliff Cunnigham of Duke University.

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