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Tuesday, November 23, 2010

Genetic boundaries between species

Our knowledge about living creatures has skyrocketed since the adoption of commonplace DNA sequencing. However, at the same time, that new information calls into question how species should be categorized. In other words, how should we define a species? Emma Vodoti from the University of Gothenburg has just completed a thesis that attempts to answer that question.

In the old days, pre 1980 or so, species were delineated purely by phenotype (physical or behavioral characteristics). If it looked different enough or if it displayed a unique behavior, particularly in mate attraction or selection, then it was labeled as a different species. With the advent of genome sequencing, biologists could compare organisms on the inside as well as on the outside. They’ve found some surprising things. It turns out that some organisms that look and act identical to one another are genetically unrelated. Case in point, the Atlantic horse mussel (Modiolus modiolus) was thought to be the same species as the identical looking Pacific variety. Upon finding that they are genetically distinct, one of the varieties will need a new scientific name.

Caption: Emma Vodoti conducts field studies.

Credit: Erik Boström

This brings us back to our question of how to define a species. Clearly, if two organisms have different enough genomes, they must be considered separate species. But what is ‘different enough’? This is the same question that was previously asked about physical appearance. When does a creature look different enough to be classified as a different species? There may never be an absolute dividing line. For one thing, as with phenotypic differences, some types of organisms can tolerate a greater degree of variety. Just look at dogs. For another, the delineations between species are manmade not natural, so there simply aren’t any sharp divisions between living creatures. Still, it’s an interesting exercise in cataloguing, particularly since genetic sequencing is bound to provide many more surprises.