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How do new species form?  One key process is by genetic divergence following geographical isolation – allopatric speciation.  This can happen when different populations of a single species are separated, cease to have contact over time and no longer interbreed.  This separation, divergence and formation of new species will often be attributed to changes in genetic makeup as a result of adaptation to different environments or ecosystems, or simply to accumulated genetic changes - genetic drift.

When it's difficult for individuals from the population to cross geographical barriers, it's possible to explain how isolation of populations occurs, and therefore why speciation has happened. An example would be the different but related species found on islands separated from the mainland, where a few individuals managed to cross the water barrier and form a new population that eventually became a distinct new species.  Charles Darwin collected specimens of mockingbirds on the Galapagos, for example, that are related to mainland species but which have diverged from the parent population to become a separate species, living in a new and different environment.

In the sea, however, many animals have pelagic larvae – free-floating planktonic forms - that can be carried for many hundreds of kilometres in currents, even though the adults have limited mobility on the sea bed.  This pelagic mobility means that closely related species from different places are potentially connected over distances of 1,000 km or more, so it is unclear how allopatric speciation is achieved – the populations appear to be capable of connection in geographical terms.

Zoology PhD student Martine Claremont, together with her Museum supervisors Drs Suzanne Williams and David Reid, and university supervisor Professor Tim Barraclough, sampled populations of the intertidal muricid gastropod genus Stramonita (a marine snail) throughout the Atlantic Ocean and used statistical analysis of DNA sequences to identify the number of distinct species, their distributions and relationships.

 

For species in which the larvae spend only a short time in the plankton, it is possible for populations to be clearly isolated geographically by currents, island chains or other factors such as the immense flow of fresh water flowing from the mouth of the Amazon. However, Stramonita spends 2-3 months as a planktonic larval form, theoretically permitting genetic contact across the entire ocean basin, which might lead to expectations that a single population would be found around the Atlantic. 

 

cropFig8 small.JPG

Stramonita brasiliensis, the new species described in the work (E, Plymouth, Tobago, BMNH acc. no. 2341; F, holotype, Sao Paulo, Brazil, BMNH 20100324)


However, Martine and her supervisors found five distinct species in the Atlantic (one of which is described as new).  They suggest that this speciation might be attributed in part to past changes or interruptions in ocean currents, preventing free circulation and isolating populations for sufficient time to enable speciation.  Other factors that seem to be of importance are the ancient separation of the Caribbean and Gulf of Mexico and the development of ecological specialization.

 

Claremont, M., Williams, S.T., Barraclough, T.G., Reid, D.G. (2011) The geographic scale of speciation in a marine snail with high dispersal potential. Journal of Biogeography, 38: 1016–1032.