June 25, 2011

Coconut scatter shows that, once established, a population structure is hard to alter

You may have heard of this by now:


I was a bit perplex at first because what I have read around is that this paper somehow demonstrates that coconuts only spread with human domestication and colonizing flows. This is a most extreme claim which hardly fits the nature of this plant, which is not truly a domesticate but a widely exploited wild plant in fact. It is a very hardy plant that grows primarily at the high tide line and is naturally transported across the oceans by mere drift.

Fig. 2 has the essence of the paper

It is evident from this paper that coconuts have at least two distinct ancestral populations: one seemingly originated in South Asia and the other from SE Asia/Pacific, that its dispersal to the Atlantic Ocean happened necessarily with human help and that the East African population while essentially the Indian variety, has some admixture from the SE Asian/Pacific variants.

Coconut germinating on Black Sand Beach, Island of Hawaii
Coconut germinating at a volcanic beach
This last element is argued by the authors to signify human influence by means of the Austronesian colonists of Madagascar. While this is plausible I see no definitive argument for this logic in fact. Similarly I fail to see the hand of Austronesians in the Pacific  scatter as something cast on iron, rather as just a possibility. 

The only clear case of human intervention are the Dwarf variants because they are self-pollinating and this is not a trait you typically find in wild plants. But the Dwarf component is relatively rare and is not even present in the alleged Austronesian-mediated arrivals to East Africa and South America (Panama variant). 

So I am not really persuaded of their thesis that most of this structure was caused by humans. It is possible but very far from demonstrated in fact. 

Regardless, what eventually brought me to write this entry was after all their other discovery, which is quite solid and obvious: that in spite of the palm being so widely exploited and moved around in the Modern Era, the original genetic structure has persisted almost unaffected. 

This is quite astonishing because copra (dried coconut flesh) and palm oil, as well as the fibre and the fresh fruit, so suitable as natural preserve for the long travels of sailors of not so long ago, make the coconut a clear candidate for extensive alteration of its ancestral genetic landscape, yet it has resisted all that almost impassible in all its range from Africa to South America. 

A lesson to be assimilated by all those who happily proclaim that established populations can easily be altered. It can happen indeed but it is not easy.

8 comments:

  1. Excellent writeup. Indeed your alternative explanation makes more sense (to me!), especially given the fact that on either coast of America the varieties are completely different.

    Somewhat related, Pacific Ocean sea snakes don't exist in the Caribbean, but they're found on the western side of Panama, and there's always been a concern that because of the Panama Canal, one day they will manage to cross over and it would be one of the worst cases ever of invasive species alteration of the environment.

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  2. That's a good observation because the authors seem to think "natural" that, in spite of the alleged human vector, the SE Asia/Pacific variant would be restricted to the Pacific coasts of America and never have crossed the Panama isthmus (or whatever other land route). I did not even think in that detail myself either. But it is quite a good argument against the model of human-mediated transport only.

    These are no chickens but nuts that are specializing in sailing with the currents. There's at least no particular reason for oceanic distribution not being spontaneous. Even one could imagine that the coconut could have traveled to the Atlantic across Africa (or even around Africa, Phoenicians for example circumnavigated it) long before Vasco de Gama.

    It is even possible that one of few coconuts drifted on their own to the Atlantic ocean causing a founder effect. There's no particular reason why humans should be involved except probably where domestication traits are salient (dwarf variants).

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  3. Thank you for commenting on our paper. We are not arguing against coconut being adapted to natural dissemination by sea currents. Our point is that human aided dissemination is much more efficient. And historical record abund.

    To concentrate on the isthmus of Panama. Coconut did cross it at some moment, westward. In some Panama populations about 10 of the genes come from the Atlantic (You can spot the orange sector in piechart K). But as you state correctly, once a population is well established, it is hard to alter its genetic structure significantly.

    Things changed in the 20th century. Due to the Lethal Yellowing disease, most of the coconuts grown in Jamaica are imported Panama Talls and their hybrids with the Malayan Dwarf.

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  4. Thanks for stopping by, Dr. Baudouin, I always feel honored by authors stopping by. It also says good things of such authors, who follow the impact that their brainchild may have caused, a clear sign that they have put love and not just work on it.

    Otherwise what can I say, I find that the paper fails to provide evidence in favor of human-mediated scatter in most cases, so I think it is a prejudice and not a sufficiently supported conclusion. (Don't worry: happens a lot and I tend to be highly critical, hopefully with good reason).

    While human-mediated scatter may have been advantageous in some aspects, it was also limited in time (the last few millennia). Instead natural dissemination may have been acting for a much longer time. Not sure for how long has been the tree existing as such, but since the coconut fruit evolved it has been necessarily floating by the currents of the Indian and Pacific oceans, and that may be even a million years or whatever (you're the expert: any estimate?)

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  5. Dear Maju,

    Thank you for the appreciation.

    Please note that this is a research paper, not a monography. We didn't attempt to prove already acquired results and referred to existing litterature. For example the absence of coconut in the Atlantic is well documented. If you are interested you can have a look at "Historia y dispersión de los frutales nativos del neotrópico" by Víctor Manuel Patiño. I didn't know when the paper was published. It is available as a Google book and contains a lot of information.

    Coconut has been cultivated for such a long period that it has become very difficult to find truly wild coconut. Yet it is thought that spontaneous populations still exist in remote places of the Philippines, in the Cocos-Keeling Islands, Palmyra, and possibly in the Seychelles.

    The origin of coconut as such seems to date back to the Tertiary with very suggestive fossiles in India, Australia and New Zealand.

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  6. One sentence of my previous message is not clear:

    I didn't know *about this book* when the paper was published.

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  7. Sure, Luc.

    Just to say that I did not challenge the attribution to humans for the Atlantic scatter of coconuts: I did not check the references but it is obvious that, because of climatic constraints, that was probably the case.

    The only thing I say is that the (post-)Neolithic attribution for the coconut scatter is, I understand, unproven in most cases in the Indian and Pacific oceans and that it can well be the product of natural dissemination since whenever the species exists. It is clear that coconut has that ability of using oceanic currents to self-disseminate its seeds and therefore does not need of any animal carrier, human or otherwise. It is like a corals or a mangroves in this aspect.

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  8. In any case, thanks for visiting and for your clarifications.

    If coconuts have been around since the Tertiary period and they have been able to scatter their seeds using oceanic currents all the time, it is very hard to argument that current distribution is of "Neolithic" human-mediated origin.

    It would be very interesting, I think, for you to study those presumably wild populations from Cocos island, Seychelles and other such remote places (what about Andaman islands?, North Australia?, Reunion and Mauritius? Indian Ocean British territory?...) and compare with semi-cultivated populations.

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