This blog was originally supposed to be an interesting article on Thanksgiving foods; specifically, a discussion about crops (i.e. squashes, onion, corn, carrots, parsnips, etc...) that traditionally accompany our Thursday turkey, as well as their weedy relatives.
However, I stumbled upon more thought-provoking article that I've chosen to share, instead.
The paper, 'Crops gone wild: evolution of weeds and invasives from domesticated ancestors', which was published in the journal 'Evolutionary Applications' (Norman C. Ellstrand (UC-Riverside) et al., 3:494-504) in 2010, can be accessed here.
The authors, who are interested in the development of weediness and invasiveness, chose to examine pest plants that had purportedly descended, either directly or through hybridization, from cultivated crops. They reasoned that the evolutionary changes correlated with weed development might be best studied by comparing problematic species to their more well-characterized (i.e. genetically, physiologically, morphologically, etc...) progenitors. To quote the authors: "Our original motivation was to understand how natural selection works on the descendants of domesticated species so that they are able to become weedy or invasive."
With this intention in mind, Ellstrand et al. (2010) conducted a literature search to identify weedy descendants of domesticated species. They considered four criteria when choosing examples to explore further:
1. "...cases involving ancestors that are well-domesticated taxa" (At least 100 years)
2. "...required genetic or historical evidence that the problematic lineage has evolved from a domesticated taxon"
3. "...asked whether evidence existed that the feral lineage is indeed problematic"
4. "...required evidence that the lineage is more problematic than its crop progenitor (in the case of endoferals) or each of its progenitors (in the case of exoferals)"
As a result, 13 species were determined to have evolved from domesticated species (in italics), including: artichoke thistle (Cynara cardunculus var. scolymus), semi-wild wheat (Triticum aestivum), weedy finger millet (Eleusine coracana subsp. coracana), Johnsongrass (Sorghum bicolor), Columbus grass (Sorghum bicolor), forrageira (Raphanus sativus), California wild radish (Raphanus sativus), weedy rice (Oryza sativa japonica, Oryza sativa japonica x Oryza sativa indica), 'blackhull' weedy rice (Oryza sativa indica), 'strawhull' weedy rice (Oryza sativa indica), weedy rye (Secale cereale), and weedy beet (Beta vulgaris subsp. vulgaris).
Important results highlighted in the article with respect to de-domestication include: the involvement of at least one parent that is primarily outcrossing; the most common adaptations were related to increased seed production and easier seed dispersal (i.e.shattering), relative to the crop progenitors; increased seed dormancy, relative to ancestors, was observed in only three cases; other traits associated with the evolution of a weedy pest from a domesticate included the production of rhizomes, woody roots, the transition to an annual life-cycle, and the development of resistance to the ALS-inhibiting herbicides.
Ellstrand et al. (2010) point out that crops and weeds are both adapted to growth and development in disturbed habitats and are often grown/grow in dense monoculture (or else very simple communities); as such, crops and weeds are not ecologically dissimilar. The authors conclude their manuscript with several intriguing questions, including:
"...with regard to crops and their weedy derivatives, we note that both grow in exactly the same location, but they are subjected to different selection regimes. How do weedy crop derivatives end up perceiving different selection pressures so that diverge in sympatry?"
"...how do [weedy descendants] diverge given that they are likely to be swamped by gene flow from the initially more abundant crop?
"If humans select crops that grow densely in monospecific stands, are those plants only a few allele changes away from becoming invasives or weeds?"
To quote the authors: "Genetic and genomic approaches, often used in concert with ethnobotanical data, have been successful in illuminating the evolution of crops from wild species under domestication.... These approaches may prove to be equally powerful in investigating evolution in the other direction, the evolution of sustainable feral populations from domesticated species."