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16 July 2007 @ 12:00 am
Mysterious Species distribution on the Indian sub-continent  
A little while after I got into serious wildlife-ing and birding I began to notice a strange pattern with respect to distribution of wet-zone (rain forest) birds. There were some species found only in the western ghats close the western coast of the peninsular India and over a thousand kilometers away in the North East Indian hills. It didn't seem all that strange at first, birds have wings after all and are known to embark upon long-distance migrations. And then, I heard about the distribution of the King Cobra -- again found only in the western ghats, the Himalayan foothills and North East India and beyond. That seemed strange because of the large distance between the two places. Just how did the distribution happen?

Apparently, I was not the first to notice that. That question has baffled biologists for centuries and there have been some excellent hypotheses put forth to explain the strange distribution. And with my limited knowledge about wildlife and biology, I had a handful of species belonging to this category, but there were hundreds who had this strange distribution pattern. After some reading and talking to some excellent people in this field, here are some of my findings.

The most widely accepted hypothesis until recently was the Satpura Hypothesis put forward by Indian ichthyologist Sunder Lal Hora in 1949. He noted that several hill stream fishes in the North Eastern India seemed to have distribution in the Western Ghats as well. So, he argued that there must have been a contiguous wet zone path all the way from the Eastern India to the Western Ghats. This dispersal of species must have happened recently(a few thousand years ago) since the species were still the same and not evolved into newer species. He gave enough proof of this and that hypothesis remained largely unchallenged for decades. This model gained support when it was proved that there were periods of alternating high and low rainfall and there could have existed good wet-zone rainforests paths from the Eastern Indian hills to the Western ghats during high-rainfall periods.

Probable species dispersal routes according to the Satpura hypothesis

While the Dispersal model always looked a strong contender, biologists thought about a new model - a vicariance model. This was based on allopatric speciation which occurs when a contiguous piece of land is broken down into multiple blocks seperated by an uncrossable natural barrier, in a way that the isolated species can no long inter-breed and thus evolve to become species in their own right. Scientists argued that the whole of the Indian land mass was once a massive rainforest and the forest cover had been gradually receding over the years. So, the species that are now in the western ghats are isolated and they could evolve differently from their North-East Indian relatives in the years to come.

18-11 million years ago 5-1 mya Today

So, the dispersal and vicariance models were widely accepted. However, science has evolved a lot since then, and new techniques are being employed to find out about species. Taxonomy in those days was all about morphological differences. It is much complicated now, with molecular bilogy and things like molecular phylogeny to find evolutionary relationships. It was proved that some of the fishes that were considered the same species were not so and suddenly the dispersal model looked weak. It was proved that the fishes probably had the same ancestors, but had embarked upon a journey of convergent speciation to look very similar to the North East Indian fishes. Convergent species are species which look very similar but are not really close relatives at all. A striking example of the convergent species are the vultures of the New World (Americas) and the Old World (Europe, Africa, Asia and Australia) -- though they look very similar they are not at all related. In fact, the new world vultures are more closely related to Storks (Ciconiiformes) than to the old world vultures (Falconiiformes). So, a new convergence model came up to explain the distribution of species. The Convergence models focuses on identifying evolutionary paths through molecular biology and hence prove that it was convergence and not geographical changes which brought about similar species.

It's strange, but all three models seem to have their own strengths and weaknesses. And at the end of reading up so many papers, there's still no perfect answer to explain the distribution. I'm just hoping they solve this one in my life time. :)

ex_neke on July 15th, 2007 03:14 pm (UTC)
Wow. That's really interesting. Thanks for sharing. :)
Shivakumar: lords_papushivakumar_l on July 15th, 2007 03:53 pm (UTC)
thats really a beautiful piece of information Yathin -- thanks a lot for that.
arucard2 on July 15th, 2007 10:23 pm (UTC)
thanks for that piece of info. fascinating stuff.
Have you read Guns, Germs and Steel?
Yathinyathin on July 15th, 2007 11:43 pm (UTC)
Nopes, I haven't read it. I did a quick search and amazon and seems to be a very interesting read and there seems to be a DVD too.
Anush Shettyanushsh on July 15th, 2007 11:56 pm (UTC)
Wonderful post with a lot of info
mamtanaidumamtanaidu on July 16th, 2007 12:39 am (UTC)
That's interesting information...
Preethifiveonehalf on July 16th, 2007 05:00 am (UTC)
Thanks for sharing the info.. it was really interesting :)