Abstract
Evolutionary “bet-hedging” refers to situations in which organisms sacrifice mean fitness for a reduction in fitness variance over time. Germination heteromorphism is the quintessential and most well understood bet-hedging strategy. It has evolved in many different plants, including the wild progenitors of some crops. Erect knotweed (Polygonum erectum L.), an annual seed crop, was cultivated in Eastern North America between c. 3000–600 BP. By c. 900 BP, cultivation had produced a domesticated subspecies with greatly reduced germination heteromorphism. Field observations and greenhouse experiments suggest that cultivation eliminated the selective pressures that maintain the bet-hedging strategy in erect knotweed, while humans also directly selected for seeds that germinated reliably and for seedlings with rapid early growth. The protection provided to erect knotweed under cultivation explains the domestication syndrome that has been observed in some archaeological assemblages. Dormancy provides seeds a means of escaping adverse conditions in time, while dispersal provides an escape in space. Farmers relaxed selective pressures that maintained dormancy in erect knotweed by acting as seed dispersers, spreading disturbance-adapted plants to predictable and protected environments, and by saving and exchanging seed stock. Experimental data also indicate that adaptive transgenerational plasticity may have been working against the expression of domestication syndrome in this case.
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Notes
When Asch and Asch (1985a:145) made the first report of domesticated erect knotweed in the Hill Creek site, they also reported measurements of achenes from several harvests of erect knotweed made in Kansas and Illinois in 1977–1982. From one harvest – on November 9, 1977 – they measured 103 smooth morphs and only 2 tubercled morphs. However, it is unclear if this was the proportion of the sample, or if they disproportionately picked smooth morphs to increase the sample size for their morphometric analysis. The table includes an ambiguous footnote that reads: “Number of achenes measured for each type does not necessarily reflect the relative frequency of types in a collection.” All of the other sample proportions reported in their table are within the range of variation observed in this study, although many of the sample sizes are too small to be considered accurate reflections of population proportion.
It is important to note that by this time several other crops had already been cultivated in this area for almost a millennium; see Smith and Yarnell 2009.
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Acknowledgements
I would like to acknowledge and thank the many people who contributed to this project by helping me access collections or populations under their care: Alan Brant, Paul and Nancy Krautman, Michelle Berg Vogel, Jim Solomon, George Yatskievych, Rusty Russell, Bruce Smith, Mary Suter, Mary Simon, George Crothers, David Pollack, Jack Rosen, Jane Buikstra, Jason King, Kris Gremillion, Wayna Adams, Neal Lopinot, Michael Meinkoth, Dee Ann Watt, John Kelly, Bill Green, Caitlin Rankin; and by sharing their knowledge with me and discussing these ideas: Michael Dyer, Gayle Fritz, Logan Kistler, Liz Horton, Kelsey Nordine, and Andrew Flachs. I would also like to acknowledge Nancy Asch Sidell, David Asch, and Sonia Sultan, three researchers whom I have never met but whose meticulous research inspired and greatly improved this project, and Marti Mueller for her careful editing and thoughtful questions. Support for this project was provided by the National Science Foundation, the Wenner-Gren Foundation, and the Lynne Cooper Harvey Fellowship in American Culture Studies at Washington University.
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This research was funded by US National Science Foundation Doctoral Dissertation Improvement Grant #58292.
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Mueller, N.G. Evolutionary “Bet-Hedgers” under Cultivation: Investigating the Domestication of Erect Knotweed (Polygonum erectum L.) using Growth Experiments. Hum Ecol 45, 189–203 (2017). https://doi.org/10.1007/s10745-016-9881-2
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DOI: https://doi.org/10.1007/s10745-016-9881-2