Abstract
Domesticated grain crops evolved from wild plants under human cultivation, losing natural dispersal mechanisms to become dependent upon humans, and showing changes in a suite of other traits, including increasing seed size. There is tendency for seed enlargement during domestication to be viewed as the result of deliberate selection for large seeds by early farmers. However, like some other domestication traits, large seeds may have evolved through natural selection from the activities of people as they gathered plants from the wild, or brought them into cultivation in anthropogenic settings. Alternatively, larger seeds could have arisen via pleiotropic effects or genetic linkage, without foresight from early farmers, and driven by selection that acted on other organs or favored larger plants. We have separated these unconscious selection effects on seed enlargement from those of deliberate selection, by comparing the wild and domesticated forms of vegetable crops. Vegetables are propagated by planting seeds, cuttings, or tubers, but harvested for their edible leaves, stems, or roots, so that seed size is not a direct determinant of yield. We find that landrace varieties of seven vegetable crops have seeds that are 20% to 2.5‐times larger than those of their closest wild relatives. These domestication effect sizes fall completely within the equivalent range of 14% to 15.2‐times for grain crops, although domestication had a significantly larger overall effect in grain than vegetable crops. Seed enlargement in vegetable crops that are propagated vegetatively must arise from natural selection for larger seeds on the occasions when plants recruit from seed and are integrated into the crop gene pool, or via a genetic link to selection for larger plants or organs. If similar mechanisms operate across all species, then unconscious selection during domestication could have exerted stronger effects on the seed size of our staple crops than previously realized.
Highlights
The Neolithic origins of agriculture in multiple regions across the globe transformed human history, marking the transition from hunter-gatherer subsistence to agricultural economies exploiting domesticated animals and crop plants (Purugganan and Fuller 2009; Larson et al 2014)
Domesticated grain crops evolved from wild plants under human cultivation, losing natural dispersal mechanisms to become dependent upon humans, and changing across a range of other traits, including loss of seed dormancy and increase in seed size (Harlan et al 1973; Hammer 1984)
Archaeobotanical evidence shows that seed enlargement during domestication was gradual, and occurred in different crops before or after the loss of dispersal (Tanno and Willcox 2006; Fuller 2007; Brown et al 2009; Purugganan and Fuller 2011). It is widely accepted among archaeologists and biologists that at least some domestication traits evolved under unconscious selection
Summary
The Neolithic origins of agriculture in multiple regions across the globe transformed human history, marking the transition from hunter-gatherer subsistence to agricultural economies exploiting domesticated animals and crop plants (Purugganan and Fuller 2009; Larson et al 2014). Domestication traits may evolve under selection from sowing into cultivated soil, competition within crop stands and from the methods used for harvesting (Darlington 1956; Harlan et al 1973; Rindos 1984; Ladizinsky 1987; Zohary 2004; Tanno and Willcox 2006; Weiss et al 2006; Purugganan and Fuller 2009, 2011) We show evidence of seed enlargement in domesticated vegetable crops, whose magnitude is comparable with that in cereals and pulses, and is likely explained by unconscious selection
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