Darwin's “Unconscious Selection” Explains a Huge Part of Domestication

Charles Darwin introduced an extremely useful distinction when discussing domesticated organisms.

Selection does not need to be either completely natural or carefully planned.

Between these lies unconscious selection.

Humans repeatedly alter environments and reproductive opportunities. Organisms then evolve in response, even when humans are not deliberately selecting particular genes.

Plant domestication is full of such cases.

Different farming activities create different selective pressures

The article distinguishes several operations that matter:

tillage,

sowing,

harvesting,

and propagation.

Each changes plant fitness differently.

Harvesting may favour plants whose seeds remain attached.

Sowing may favour rapid germination.

Tillage may alter competition among seedlings.

Vegetative propagation may preserve particular clones.

These pressures accumulate.

Environmental selection

Non-shattering provides a good example.

Wild cereal plants release their seeds naturally.

A harvesting system disproportionately collects grains still attached to the plant.

Those grains become next year's seed stock.

The harvesting environment therefore creates a reproductive threshold.

Plants whose seeds remain attached gain an enormous advantage inside the agricultural system.

This is environmental selection.

Competitive selection

Seed size follows a different dynamic.

Larger seedlings may compete more successfully for soil resources and sunlight.

As the population changes, the competitive environment changes too.

Selection can therefore accelerate or proceed in episodes.

The paper suggests that archaeological patterns of grain enlargement in several species fit this type of process.

Weak selection can be powerful

Another important idea is that domestication may often have involved relatively weak selection.

At first this sounds paradoxical.

How can weak selection transform wild plants into crops?

Time.

Thousands of generations allow small fitness differences to accumulate.

Weak selection may actually have been advantageous to early human communities because extremely strong selection could reduce the available food population.

Domesticated populations were not laboratory experiments. They were dinner.

People could not discard 90% of their food supply simply because the plants lacked a desired allele.

Food security itself may therefore have constrained the speed of selection.

This helps explain why early domestication looks so different from modern crop breeding.

Modern breeders can deliberately isolate particular traits.

Early cultivators were maintaining entire food-producing populations while evolution unfolded within them.

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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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