Why are crop seeds often larger than those of their wild relatives?
The obvious answer seems to be human preference.
People preferred bigger seeds because bigger seeds provided more food, so farmers consciously selected them.
The article argues that this explanation is often too simple.
Seed enlargement may initially have been an unintended consequence of cultivation.
A seedling arms race
Imagine a cultivated plot.
People disturb the soil and sow seeds into comparatively favourable conditions.
Some seeds germinate slightly earlier.
Some seedlings grow slightly faster.
Some seeds contain larger nutrient reserves.
Those seedlings gain early access to sunlight, water and nutrients.
Once competition begins, small differences matter.
Larger seeds often produce stronger seedlings.
Those seedlings can suppress smaller neighbours.
If farmers harvest the successful plants and sow their descendants, the cultivated population gradually shifts toward larger seeds.
Nobody needs to deliberately choose “large-seed genes.”
Agricultural ecology generates the selection pressure.
Competitive selection
The paper describes this as competitive selection.
The important feature is that fitness depends partly on what neighbouring plants are doing.
Suppose the average seed size in a population increases.
A seed size that was previously adequate may now be disadvantageous because surrounding seedlings have become stronger competitors.
The evolutionary race can therefore accelerate.
This differs from a simple constant selection pressure.
Archaeology can see the change
Seed size is particularly useful archaeologically because charred grains often preserve their dimensions.
Researchers can measure ancient seeds and compare populations through time.
The lower portion of Figure 1 in the article charts increasing grain size for numerous crops, including wheat, barley, lentils, peas, rice, millet, soybean, chickpea and horsegram.
Many show prolonged trajectories rather than instantaneous jumps.
Bigger food was not necessarily the target
This leads to a delicious evolutionary irony.
Humans ultimately benefited from larger edible seeds.
But the original selection pressure may have been less “farmers choosing bigger dinner” and more “seedlings competing inside environments created by farmers.”
Cultivation changed the competitive arena.
Evolution responded.
Humans later inherited the culinary consequences.
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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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