Arable weeds as a case study in plant-human relationships beyond domestication

Arable weeds are plants that invade habitats created by people for the cultivation of other species. Though they are not the target of human cultivation, their growth in arable habitats means that they, like crops, are under human selection. Genetic studies of weedy crop relatives have documented traits (shattering/dehiscence, asynchronous flowering, seed dormancy etc.) that allow weeds to escape detection and eradication by farmers, and give them a competitive advantage over crops (Thurber et al. 2010, 2011; Qi et al. 2015). Selection of these traits in weedy crop relatives thus constitutes a (partial) reversal of the domestication syndrome (Hammer 1984), sometimes called ‘de-domestication’ (Ellstrand et al. 2010). But weeds can also adapt by taking on domestic traits (Hammer 1984); Harlan (1992: 66, 94) reports non-shattering populations of the weed Bromus secalinus and of weedy oats, for example. Moreover, genetic study of wild crop relatives and their domesticated counterparts has confirmed the importance of allele introgression from proximate wild populations (Song et al. 2014; Gutaker et al. 2017). The history of crops and weeds is thus deeply intertwined, the selection of traits in one shaping (directly and indirectly) the evolution of the other, in a particularly clear instance of mutual evolution through niche construction