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2 Posts tagged with the species tag

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.


Understanding the diversity of life is central to the mission of the Natural History Museum. Science sees diversity in many ways: populations, species, ecosystems, individuals or genes and the Museum's collections of more than 70 million items are used by scientists for research on many aspects of diversity.  The collections have developed over the past 250 years with a very strong emphasis on the idea of the species, but reflect diversity within species as well - the differences between populations from different areas, for example.


What separates one species from another is not always an easy question: it is a key question for the science of taxonomy and has important practical implications.  The established biological species concept defines two species as two groups of organisms that cannot interbreed to produce fertile young when in the same location.  When different species are present in the same location, this can be observed in theory.  However, when two groups of similar organisms are geographically separate, are they different populations, different subspecies, or different species? This will be the case for many thousands of species and has led to heated debate among scientists who have taken different views.


Beyond science, this is of importance because the species is often used in practical policy-making and economic activity.  There needs to be accurate definition for biodiversity conservation, pest control in agriculture, human health and other activities.


A group of collaborating scientists from Oxford and Cambridge Universities and from BirdLife International have used the Museum's bird collections to try to define a reliable standard for species. They aimed to define how much genetic, morphological and behavioural distance there was between known species and subspecies, and within species.


The scientists looked at pairs of 58 closely-related species and subspecies, including European swallows and linnets, North American blackbirds and tyrant flycatchers and African Illadopsis. They examined more than 2,000 specimens from the NHM bird collections and more than 140 from Louisana State University for morphological data and plumage, and looked also at song, ecological and behavioural differences. The intention was to use this suite of characters to define a reliable and objective difference between species.


Tobias et. al (2010) published their results in the journal Ibis, concluding that this is a reliable way of confirming species separations and propose that this could be used increasingly to improve the reliability of understanding of bird diversity. An article in Nature (Brooks and Helgen, 2010), commenting on the paper, suggested that there could be very interesting possibilities in applying similar techniques to other groups of organisms and with DNA data.


Thousands of visiting scientists routinely use the Museum's collections as a research resource: the collection represents a body of evidence to address new questions and test established knowledge of natural diversity, and continues to develop as research interests expand.


TOBIAS, J. A., SEDDON, N., SPOTTISWOODE, C. N., PILGRIM, J. D.,  FISHPOOL, L. D. C. and COLLAR, N. J. (2010), Quantitative criteria for  species delimitation. Ibis, 152: 724–746.  doi: 10.1111/j.1474-919X.2010.01051.x


Brooks, T. M. and K. M. Helgen (2010). "Biodiversity: A standard for species." Nature 467(7315): 540-541.