Tuesday, March 1, 2016

Vavilovian Mimicry

Vavilovian mimicry is a form of mimicry where an agricultural weed begins to take on characteristics of a domesticated agricultural crop due to the selective forces present in the agricultural system.

The first example is involving wheat. Well, really, it involves both wheat and barley. During the early phases of domestication of these grains, they weren't distinguished as separate plants. Initially, the large seeded grasses were collected from the wild and planted closer to home. The now-farmers would then collect the large seeds from the grasses they grew, eat most and protect some, and then plant what was left the following year. You could say that wheat and barley are Vavilovian mimics of each other, because we really don't know which was the first grass to enter domestication.

At some point while the wheat/barley agricultural system was developing, other weedy grasses invaded the prime growing habitat found in the fields. Two of these weeds evolved to become what we call "rye" and "oats". They developed larger, non-shattering seed heads and an annual life-cycle. This allowed their seeds to be collected, saved, and planted as contaminants to the main crop. Both rye and oats are more tolerant of cold conditions and poor soils. Because they had become mimics (and contaminants) of the major crop, when farmers tried to establish the crop system in marginal conditions, these mimics can come to predominate as the major crop.
Wheat/Barley -> Wheat (Triticum spp.)
Wheat/Barley -> Barley (Hordeum vulgare)
Wheat/Barley -> Rye (Secale cereale)
Wheat/Barley -> Oats (Avena sterilis)

Another often-cited case of Vavilovian mimicry is found in the agriculture of lentils (Lens culinaris). A common weed in lentil fields is the Common Vetch (Vicia sativa). The Common Vetch seeds are bitter, so farmers are able to sell their crop for less if there is too much vetch contamination. As farmers have increased the selection pressure on the vetch by mechanical (and computer-vision) assisted seed sorting, strains of the vetch have evolved so that their seeds mimic the lentils in color and size, as well as the characteristic flattened lens-shape.

A. Lens culinaris. B. Vicia sativa, wild and mimic.
(from: www4.ncsu.edu/~fgould/pdfs/Gould1991.pdf)
Lentil (Lens culinaris) -> Common-Vetch (Vicia sativa)
Lentil (Lens culinaris) -> Black-Pod-Vetch (Vicia sativa subsp. nigra)
If farmers could impart some selective force on the mimic vetches such that they would lose their bitter flavor, they would have effectively created a new crop. This new crop might grow better in some conditions where lentils don't thrive, thus spreading the useful area of agriculture.

The selection force involved in the development of Vavilovian mimicry can be mechanical (as in Flax weeds) or manual (as in Rice weeds). What is key is that the selection force separating weeds from the crop has to progressively get more and more stringent over time. This allows the weed population to always have some individuals that will escape the selection force applied to them.
Flax -> False-Flax (Camelina sativa linicola)
Flax -> Flax-Dodder (Cuscuta epilinum)
Rice (Oryza sativa) -> Early-Baryard-Grass (Echinochloa oryzoides)

An interesting case that I think is related to Vavilovian mimicry is the complex of Andean tuber crops. I don't know which crop was first domesticated in this region, but since before modern history, five species of tuberous crops have been traditionally grown together in fields. Growing several different crops together in this agricultural system mean that there will always be production, even if any given plant doesn't produce in some year (due to weather, disease, or other factors).
Potato (Solanum tuberosum) -> Maca (Lepidium meyenii)
Potato (Solanum tuberosum) -> Oca (Oxalis tuberosa)
Potato (Solanum tuberosum) -> Mashua (Tropanolum tuberosum)
Potato (Solanum tuberosum) -> Ullucus (Ullucus tuberosus)
Though I doubt any of these species entered the agricultural system as weeds, I expect that each species will undoubtedly have evolved towards a set of traits similar to those of the most common plant grown in the fields. Any individual plants that didn't prosper in the agricultural system would have contributed less to the next generation and the species would shift to a form that did prosper. This shifting of the traits of one species to align with another, due to the selection forces favoring the majority plant species, is a characteristic common between Vavilovian mimicry and whatever this case should be referred to as.