Geographic mosaics and changing rates of cereal domestication

Abstract

Domestication is the process by which plants or animals evolved to fit a human-managed environment, and it is marked by innovations in plant morphology and anatomy that are in turn correlated with new human behaviours and technologies for harvesting, storage and field preparation. Archaeobotanical evidence has revealed that domestication was a protracted process taking thousands of plant generations. Within this protracted process there were changes in the selection pressures for domestication traits as well as variation across a geographic mosaic of wild and cultivated populations. Quantitative data allow us to estimate the changing selection coefficients for the evolution of non-shattering (domestic-type seed dispersal) in Asian rice (Oryza sativa L.), barley (Hordeum vulgare L.), emmer wheat (Triticum dicoccon (Shrank) Schübl.) and einkorn wheat (T. monococcum L.). These data indicate that selection coefficients tended to be low, but also that there were inflection points at which selection increased considerably. For rice, selection coefficients on the order of 0.001 prior to 5500 BC shifted to > 0.003 between 5000 and 4500 BC, before falling again as the domestication process ended 4000-3500 BC. In barley and the two wheats selection was strongest between 8500 and 7500 BC. The slow start of domestication may indicate that initial selection began in the Pleistocene Glacial era.

A re‐evaluation of the domestication bottleneck from archaeogenomic evidence

“…The process of domestication cannot be pinned down to a specific point in time as different traits of the domestication syndrome were selected for at different times through changing behaviors (Fuller, Allaby, & Stevens, ), and even within single traits, selection pressures were dynamic over time (Allaby et al., ). Furthermore, a common theme from investigations in the cereals is the weakness of the selection pressures involved (Hufford et al., ; Purugganan & Fuller, ).…”

“…The lack of a dip in genetic diversity that could be associated with a distinct bottleneck episode suggests a disconnect between an intensification of selection, such as from sickle technologies (Allaby et al, 2017), and meaningful loss of genetic diversity during the rise of domesticated types. Rather, the drift-and selection-associated processes responsible for the removal of diversity have acted continuously.…”

“…In the case of cereals such as rice, wheat, and barley, there has been a shift from a rapid to a protracted transition paradigm in which the emergence of domesticated forms was slow (Allaby, 2010;Allaby et al, 2008). Archaeological data show that selection for domestication syndrome traits such as loss of seed shattering and seed size was weak-comparable to that of natural selection (Purugganan & Fuller, 2009;-and that the onset of selection likely preceded domesticated forms by over ten thousand years in the case of the cereals of the Near East (Allaby, Stevens, Lucas, Maeda, & Fuller, 2017).…”

Domestication and crop evolution of wheat and barley: Genes, genomics, and future directions

“…Recent work suggests that, like emmer wheat, barley possesses contributions from multiple wild populations (Poets et al ; Pankin et al ). In addition to the evidence suggesting multiple wild populations contributed to the genomes of modern wheat and barley, it has been proposed that domestication was not a rapid process (Allaby et al ) and that the rate of domestication was likely not constant (Allaby et al ). These claims are controversial and, as others have pointed out, the hypothesis of a protracted domestication is based on strong assumptions that are difficult to verify.…”

Innovation: an emerging focus from cells to societies

“…Niche construction is therefore an extension of the phenotype, subjected to selection, and manifests in gene -culture coevolution [159]. Allaby et al [44] review one of the most fascinating forms of niche construction: domestication. Briefly, one species selects for sought-after traits in another, especially predictability, value and yield, and in so doing the former evolves through the development of management techniques and technologies.…”