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| Neolithic Revolution is the shift from hunter-gatherers to keeping animals and growing crops (cultivation). Image courtesy of http://historymadeeveryday.files.wordpress.com/2013/09/neolithic-age.jpg?w=640 |
Darwin’s
use of artificial selection as an analogy to evolution (by natural selection)
is much debated.
Species evolution by human association such as plant domestication is presumed to hold strong selection pressures with rapid evolution of
cultivated species in as little as a few hundred years. Humans (Homo sapiens) began domesticating plant and animal species during the Neolithic
agricultural revolution approx. 10,000-13,000 years ago. A change from
hunter-gatherer into sedentary agricultural groups shows a transformation in
human behavioural ecology which ultimately led to the co-evolutionary
origins of domesticated crop and livestock species.
Seed crops are the most successful of
domesticated plant species whose rates of evolution may be representative of
other domesticated plant taxa. Purugganan & Fuller (2010) researched the rate of evolution in
phenotypic traits during the domestication process and compared them to the
rates experienced by wild species under natural selection.
Archaeobotanical records provided
quantitative information on ‘non-shattering’ (the retention of seed after
harvesting) and grain size (which increases under domestication) from in five
regions in Asia, Africa, and North America across 11 crop species that date
from the Neolithic period. The evolution and fixation of the non-shattering trait
is regarded as a hallmark of domestication, due to the reduced ability for
natural seed dispersal, therefore detrimental in wild populations, making the
cultivated species dependent on humans for continued reproduction.
It was found that phenotypic evolution in multiple crop species is significantly
slower than rates observed in wild species. This indicates
that the rates of evolution during the domestication process, including the
strength of selection, may be similar to those measured for wild species. Domestication
may be driven by unconscious selection pressures similar to that observed for
natural selection; proving artificial selection
to be a valid model for the study of evolutionary change. Some proposed reasons for the unexpected similarities
are:
- The domestication process is in principle simply natural selection in an environment established by human agriculture. It could be considered as a form of animal/plant coevolution.
- Early cultivation of proto-domesticates would have been established alongside wild plants resulting in continued gene flow, slowing the fixation of selected alleles.
- Deleterious mutations may segregate at higher frequencies in the population bottlenecks associated with the establishment of crop plants. These may lead to decreased selection efficiency and a slower pace of phenotypic evolution.
Interesting post this week. I am a little confused. You say that the rate of phenotypic evolution in crops is slower than the rates observed in wild species, then state the rates of evolution during the domestication process may be similar to that of wild species. This seems to be contradictory. Perhaps you could clarify?
ReplyDeleteHey Tasmin, of course! Their study across those particular crop species looked at only two traits, those of which had a slow rate of evolution compared to their wild counterparts. The next was a generalised, broad statement saying they have found a contradiction to the common assumption that domestication leads to rapid evolution. Realistically across species, the rate of evolution varies little (is more similar than different) under both models (artifical vs natural selection). The main area of difference between natural selection and domestication is not the rate at which traits evolve but the actual traits selected for. Within species traits have different responses - fast or slow evolution - irrespective of the model under which pressure is applied.
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