Cultivation as slow evolutionary entanglement: comparative data on rate and sequence of domestication

Recent studies have suggested that domestication was a slower evolutionary process than was previously thought. We address this issue by quantifying rates of phenotypic change in crops undergoing domestication, including five crops from the Near East (Triticum monococcum, T. dicoccum, Hordeum vulgare, Pisum sativum, Lens culinaris) and six crops from other regions (Oryza sativa, Pennisetum glaucum, Vigna radiata, Cucumis melo, Helianthus annus, Iva annua). We calculate rates using the metrics of darwin units and haldane units, which have been used in evolutionary biology, and apply this to data on non-shattering cereal spikelets and seed size. Rates are calculated by considering data over a 4,000-year period from archaeological sites in the region of origin, although we discuss the likelihood that a shorter period of domestication (1,000-2,000) years may be more appropriate for some crops, such as pulses. We report broadly comparable rates of change across all the crops and traits considered, and find that these are close to the averages and median values reported in various evolutionary biological studies. Nevertheless, there is still variation in rates between domesticates, such as melon seeds increasing at twice the rate of cereals, and between traits, such as non-shattering evolving faster than grain size. Such comparisons underline the utility of a quantitative approach to domestication rates, and the need to develop larger datasets for comparisons between crops and across regions.