Warwick, United Kingdom
September 19, 2008
Until recently researchers say the
story of the origin of agriculture was one of a relatively
sudden appearance of plant cultivation in the Near East around
10,000 years ago spreading quickly into Europe and dovetailing
conveniently with ideas about how quickly language and
population genes spread from the Near East to Europe. Initially,
genetics appeared to support this idea but now cracks are
beginning to appear in the evidence underpinning that model
Now a team led by Dr Robin Allaby from the
University of Warwick
have developed a new mathematical model that shows how plant
agriculture actually began much earlier than first thought, well
before the Younger Dryas (the last "big freeze" with glacial
conditions in the higher latitudes of the Northern Hemisphere).
It also shows that useful gene types could have actually taken
thousands of years to become stable.
Up till now researchers believed in a rapid establishment of
efficient agriculture which came about as artificial selection
was easily able to dominate natural plant selection, and,
crucially, as a consequence they thought most crops came from a
single location and single domestication event.
However recent archaeological evidence has already begun to
undermine this model pushing back the date of the first
appearance of plant agriculture. The best example of this being
the archaeological site Ohalo II in Syria where more than 90,000
plant fragments from 23,000 years ago show that wild cereals
were being gathered over 10,000 years earlier than previously
thought, and before the last glacial maximum (18,000-15,000
years ago).
The field of Archaeobotany is also producing further evidence to
undermine the quick development model. The tough rachis mutant
is caused by a single recessive allele (one gene on a pair or
group of genes) , and this mutant is easily identifiable in the
archaeological specimens as a jagged scar on the chaff of the
plant noting an abscission (shedding of a body part) as opposed
to the smooth abscission scar associated with the wild type
brittle rachis.
Simply counting the proportion of chaff types in a sample gives
a direct measure of frequency of the two different gene types in
this plant. That study has shown that the tough rachis mutant
appeared some 9,250 years ago and had not reached fixation over
3,000 years later even after the spread of agriculture into
Europe was well underway. Studies like these have shown that the
rise of the domestication syndrome was a slow process and that
plant traits appeared in slow sequence, not together over a
short period of time.
Genome wide surveys of crops such as einkorn and barley that in
the past that have suggested a single origin from a narrow
geographical range, supporting the rapid establishment view,
have long been in conflict with other gene studies. The most
notable conflict is in the case of barley for which there is a
large body of evidence that suggests more than one common
ancestor was used in its development.
These challenges to the fast model of agricultural development
need a new model to explain how and why the development was so
slow and demonstrate why artificial selection of just one plant
type does not have the expected quick result. This computer
model has now been provided by Dr Robin Allaby and his team at
the University of Warwick, the Institute of Archaeology,
University College London, and Manchester Interdisciplinary
Biocentre has outlined the new mathematical model in a paper
published in Proceedings of the National Academy of Sciences USA
2008 and in a summary article in the Biologist (2008 55:94-99).
Their paper entitled The genetic expectations of a protracted
model for the origins of domesticated crops used computer
simulations that showed that over time a cultivated population
will become monophyletic (settle into one stable species) at a
rate proportional to its population size as compared various
gene variations in the wild populations. They found this rate of
change matched closely the 3000 years it took the tough rachis
mutant to become established.
Ironically, this process is actually accelerated if there is
more than one wild source population (in other words if attempts
at domestication happen more than once) because any resulting
hybrid between those domesticated populations then has a
heightened differentiation compared with either one of the wild
populations of the two parent plants.
This mathematical model also more supportive of a longer complex
origin of plants through cross breeding of a number of attempts
at domestication rather than a single plant type being
selectively bred and from a single useful mutation that is
selectively grown quickly out paces the benefits natural
selection
Dr Robin Allaby says: "This picture of protracted development of
crops has major implications for the understanding of the
biology of the domestication process and these strike chords
with other areas of evolutionary biology."
"This lengthy development should favour the close linkage of
domestication syndrome trait genes which may become much more
important because linked genes will not be broken up by gene
flow – and this makes trait selection and retention easier.
Interestingly, as more crop genomes become mapped, the close
linkage of two or more domestication syndrome genes has been
reported on several occasions."
"This process has similarities to the evolution of ‘supergenes’
in which many genes cluster around a single locus to contribute
to one overall purpose."
"We now need to move this research area to a new level.
Domestication was a complex process and can now be viewed more
legitimately as the paragon of evolutionary process that Darwin
originally recognized. There are many interacting factors
involved that we know about operating on a wide range of levels
from the gene to the farmer and climate – the challenge is to
integrate them into a single story." |
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