Most classic domestication research has focused on cereals.
That creates a hidden bias.
Wheat, barley, rice and maize reproduce mainly through seeds and preserve relatively well archaeologically.
But enormous human populations have historically relied on crops that reproduce very differently.
Potato.
Cassava.
Sweet potato.
Taro.
Yams.
These plants force us to rethink what domestication even means.
Vegetative propagation changes evolution
Many underground crops are propagated using pieces of the plant itself.
A tuber, corm, rhizome, sucker or cutting can generate a new individual.
That means successful plants can effectively be copied.
This drastically changes evolutionary dynamics.
Instead of waiting for sexual reproduction to recombine genes, cultivators can preserve a useful phenotype immediately.
Find a particularly tasty yam?
Replant part of it.
Find a cassava plant with useful characteristics?
Clone it.
Plasticity comes first
Many vegetatively propagated crops display strong phenotypic plasticity.
The same genotype can produce different forms depending on soil, shade, moisture and cultivation.
The article proposes that early cultivators may initially have manipulated these phenotypes through environmental management.
Over long periods, characteristics produced plastically could become increasingly genetically assimilated.
Domestication therefore may have begun not by selecting mutations, but by repeatedly creating environments that induced useful forms.
Domestication can reduce sexual reproduction
The evolutionary trajectory can even run opposite to cereals.
Seed crops often become tightly adapted to sexual reproduction through human-controlled sowing.
Vegetative crops may become increasingly dependent on clonal reproduction.
Some cultivated forms eventually become sterile.
Triploid cultivars provide famous examples.
Once such plants arise, humans become essential to their propagation.
Sweet potato shows that clonality is not absolute
Even clonally propagated crops can retain sexual reproduction.
Sweet potato is normally propagated vegetatively.
Yet sexually produced seedlings can occasionally appear within fields or feral populations.
Farmers may recognise useful new plants and incorporate them back into the clonal stock.
Crop evolution therefore becomes a fascinating mixture of cloning, mutation, sexual reproduction and human selection.
Why archaeology struggles with tubers
The archaeological record strongly favours hard seeds.
Tubers are soft.
They rot.
Their remains are often fragmentary.
Researchers therefore depend on approaches such as:
phytolith analysis,
starch-grain analysis,
microscopy,
microCT,
charred parenchyma analysis,
and potentially ancient DNA or molecular residue techniques.
Page 6 and page 7 of the paper emphasise how much more difficult this evidence is to interpret.
No vegetatively propagated crop yet has an archaeological domestication sequence comparable to the beautifully documented non-shattering trajectories of some cereals.
That gap is not merely inconvenient.
It means a major part of humanity's agricultural history remains partly invisible.
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Further reading: This post is inspired by and draws on Dorian Q. Fuller, Tim Denham, and Robin Allaby, “Plant domestication and agricultural ecologies,” Current Biology 33, no. 11 (2023): R636–R649, doi:10.1016/j.cub.2023.04.038. Read the original article in Current Biology
For a classic and highly readable perspective on why plant domestication arose in some regions but not others, see Jared Diamond, Chapter 8, “Apples or Indians: Why Did Peoples of Some Regions Fail to Domesticate Plants?”, in Guns, Germs, and Steel: The Fates of Human Societies (W. W. Norton, 1997), pp. 131–156. Read “Apples or Indians” View Guns, Germs, and Steel on Google Books
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