Tigers indeed fall into six genetically distinct groups, confirms a study that analysed the complete genomes of 32 representative specimens of the endangered animal.
These six subspecies include the Bengal tiger, Amur tiger, South China tiger, Sumatran tiger, Indochinese tiger, and Malayan tiger, first proposed in 2004, according to the study published in the journal Current Biology.
Three other tiger subspecies have already been lost to extinction, said researchers from Peking University in China.
Fewer than 4,000 free-ranging tigers remain in the wild. Efforts to protect these remaining tigers have also been stymied by uncertainty about whether they represent six, five or only two subspecies, they said.
"The lack of consensus over the number of tiger subspecies has partially hindered the global effort to recover the species from the brink of extinction, as both captive breeding and landscape intervention of wild populations increasingly requires an explicit delineation of the conservation management units," said Shu-Jin Luo from Peking University.
"This study is the first to reveal the tiger's natural history from a whole-genomic perspective. It provides robust, genome-wide evidence for the origin and evolution of this charismatic megafauna species," Luo said.
The researchers set out to expand their earlier genetic evidence on tiger's evolutionary history and population structure using a whole-genome approach.
They realised that genome-wide screening was also the only way to look for signals that distinct groups of tigers have undergone natural selection to adapt to the environments found in the distinct geographic regions they inhabit.
Fossil evidence shows that tigers go back two to three million years, researchers said.
However, the genomic evidence shows that all living tigers only trace back to a time about 110,000 years ago, when tigers suffered a historic population bottleneck, they said.
The genomic evidence shows that there is very little gene flow among tiger populations.
Despite the tiger's low genetic diversity, the pattern across groups is highly structured, offering evidence that these subspecies each have a unique evolutionary history.
That's quite unique among the big cats, the researchers say, noting that several other species, such as the jaguar, have shown much more evidence of intermixing across whole continents.
Tiger subspecies have distinct features. For example, Amur tigers are large with pale orange fur, while Sumatran tigers in the Sunda Islands tend to be smaller with darker, thickly striped fur.
In fact, despite the very recent common ancestor of all living tigers, the researchers were able to detect evidence of natural selection.
"In the end, we were quite amazed that, by performing a stepwise genome-wide scan, seven regions including 14 genes stood out as the potential regions subject for selection," Luo added.
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