One Species or Several?
The term "species" represents the basic lower unit of biological classification. For our purposes here, species may be defined as, "a population or series of populations of similar organisms that are capable of interbreeding freely with one another under natural conditions". It is generally agreed that there is only one, world-wide species of Great White Shark. But this does not mean that all individuals are exactly the same.
As with other species, Great Whites vary somewhat among individuals and from place to place. For example, Great Whites from southern Australia tend to have a more acutely pointed snout and are relatively slender in overall build compared those from South Australia or California. Although back and flank color in Australian Great Whites is highly variable, they tend to be shades of bronzy-grey. In contrast, those from other regions tend toward other shades. For example, the upper surfaces of Great Whites from Cape Waters of South Africa tend to be dun or olive in color while those from Californian waters are generally very dark — almost black — above. Such differences are too minor to merit separation into two or more species. However, the consistency of these regional differences suggests geographically discrete populations.
At the 2000 annual meeting of the American Elasmobranch Society, a team of researchers presented preliminary results from a study of population genetics in Great White Sharks. The team used a special type of genetic material known as mitochondrial DNA (mtDNA). In addition to its utility in molecular systematics, mtDNA has yet another handy feature: in sexually reproducing organisms — such as sharks — it is inherited only from the maternal line. Since each offspring can have only one mother, this peculiarity of mtDNA enables researchers to trace the genealogy of individual sharks with absolute confidence, thereby allowing them to determine whether Great Whites from different regions interbreed.
The researchers used a variable region of mtDNA extracted and amplified (cloned) from Great White tissue samples collected from seven regions around the globe. When analyzed, these mtDNA samples did not show a random variability, which one would expect from a single, interbreeding population. Instead, the researchers found that the individual Great Whites in their samples formed two major populations:
Pacific Ocean |
Atlantic & Indian Ocean |
|
California |
western North Atlantic |
|
Japan |
Brazil |
|
Australia |
South Africa |
|
New Zealand |
With one exception, the study found no evidence that individuals from the Pacific population interbreed with the Atlantic-Indian Ocean population
The research team also demonstrated that Great Whites from the same population interbreed, even among individuals separated by thousands of miles (kilometres) of open ocean. For example, using similarities in mtDNA, the researchers were able to determine that the Great Whites from Australia and New Zealand comprise the same, interbreeding population. That's not too surprising, because we've known for some time that the Great White, although primarily a coastal species, sporadically turns up at oceanic islands (such as the remote Hawaiian Islands) and tagging results have shown that even small species such as the School Shark (Galeorhinus galeus) have been known to travel between Australia and New Zealand. What is rather surprising is the study demonstrated a small but definite gene flow between Great Whites across entire ocean basins. For example, the analysis revealed good evidence of interbreeding between individuals from California and New Zealand, as well as between individuals from Tasmania and South Africa.
These findings strongly support the theory that there is only one, world-wide species of Great White Shark. They also demonstrate that Great Whites separated by large expanses of ocean may be part of the same breeding stock. Thus, the Great White Shark is not limited by our geopolitical boundaries and conserving this species will require a cooperative international effort.
