Top models reveal all

Ok, the title is a shameless attempt to boost our visit count, but I did in fact want to talk about two modeling studies today — though of very different kinds. I already mentioned the first in a recent post. I have now got hold of the paper on the genetic modeling of domestication and can talk about it in a bit more detail.1

There’s a conflict in the data on crop domestication, as the authors of a recent PNAS paper see it.2 The conventional scenario divides the transition to agriculture in the Levant during the period of climate change around the Pleistocene-Holocene boundary into three steps: wild gathering, predomestication cultivation, and fixation of the domestication syndrome. Based on archaeology, field experiments and climatic considerations, each of these steps was thought to be fairly short: the transition was very rapid. The main genetic consequence of such a scenario should be monophyletic crops. And if you look at the number of mutant alleles connected with each bit of the domestication syndrome (the brittle rachis, for example), or the extent of narrowing of genetic diversity from wild relative to cultivated crop, or phylogenetic relationships based on molecular markers of different kinds, you do in fact get evidence that domestication happened only once in many crops, in a fairly restricted area (barley being an exception).

The problem is that archaeologists have now put a spanner in the works. They’ve changed their mind on the timescale, and in a pretty spectacular way. Rather than maybe 2,000 years, they are now saying the whole domestication process took closer to 12,000 years in the Levant, elongating each of its component stages quite considerably. This extended timeline means that the likelihood of independent, multiple, geographically dispersed domestications of a given crop — and indeed of the different crops making up the Neolithic Package — is much greater. That, however, would be expected to lead to polyphyletic crops.

So how do you reconcile a protracted domestication process with crops which genome-wide surveys suggest are monophyletic? Well, according to the authors, there is in fact nothing to reconcile.

They build an in silico model consisting of virtual plants with chromosomes carrying lots of biallelic markers, put them through one or more domestication bottlenecks and a subsequent expansion, with  varying possibilities for population amalgamation in the multiple domestication case, left the populations to cycle through a range of different numbers of generations, and then looked at the phylogenies for each chromosome.

The result was surprising, even counterintuitive. For multilocus systems, “multiple-origin crops are actually more likely to result in monophyly than single-origin ones.” All the simulations eventually led to monophyletic crops, the speed with which they did so depending on population size: by 2N generations, a crop was monophyletic whether it had been domesticated only once or multiple times.

What does it mean if the transition to agriculture was indeed as protracted as the archaeological evidence suggests — and as the genetic evidence can also be interpreted to suggest, at least based on this modeling study? Well, I suppose one thing that could be said is that the balance between artificial and natural selection may not have shifted as completely and suddenly as was thought. Which would perhaps strengthen the hand of those looking for ways of facilitating the use of large collections through provenance data.

The other kind of model I want to discuss is “climate envelopes.” We have also blogged about this before. The idea behind these things is simple. You dot-map the present distribution of a species. You then extract the climatic data for the places where the species has been observed. That’s your envelope for the species. You then say, ok, what’s going to happen in these places under climate change? Some places will change so much they will move out of the envelope. Other places which are nearby geographically but currently outside the envelope will move into it. The assumption is that, given no change in adaptation, the species will either migrate from the old to the new places, or go extinct. Apocalyptic estimates of possible extinctions due to climate change have been reached using these methods.

But it seems there may be some problems with such an approach, according to another PNAS paper3 We’ve always known that they omit things that are important in determining species distributions: soils, competitive effects, human interference. But there may be an even more fundamental flaw. The authors built climate envelopes for 100 European bird species both based on real data about species occurrence and also based on random collections of points “designed to mimic the spatial structure of the birds’ real distribution.”

The result?

For 68 of the 100 species, the five distributions that fitted their climate envelopes best were null distributions. So climate envelopes generated from real distribution data did not describe that data as well as some of the climate envelopes fitted to distribution data made up without any thought of climate.

Climate is no better than chance as a way of describing the distribution of many species. At least of European birds. Time to test if it is the same for crop wild relatives, say?

  1. LATER: Science Daily has now done a longish piece on the paper, with lots of quotes from the main author. []
  2. Allaby RG, Fuller DQ, Brown TA. 2008. The genetic expectations of a protracted model for the origins of domesticated crops. PNAS. []
  3. Beale, C. M., Lennon, J. J. & Gimona, A. 2008. PNAS. []

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