Wild plants live in uncertain environments.
A seed that germinates immediately after a single rainfall may encounter disaster if drought follows.
Many wild species therefore possess seed dormancy.
Seeds wait.
They may respond to temperature, day length, fire, abrasion or prolonged moisture before germinating.
Agriculture changes the calculation.
Farmers want synchronous germination
A cultivated crop works best when a large fraction of seeds germinate after sowing.
If half remain dormant until next year, the farmer loses much of the expected harvest.
Crop populations therefore often evolve reduced dormancy.
The evolutionary puzzle is how this happened.
Was reduced dormancy deliberately selected?
Some hypotheses proposed that early cultivators discovered rare non-dormant plants and deliberately propagated them.
If that were true, non-dormant forms should appear very early in archaeological cultivation.
But the evidence discussed in the paper does not support such a simple scenario.
Horsegram provides unusual evidence
One of the clearest archaeological cases comes from horsegram in southern India.
Researchers used X-ray tomography to examine ancient seed coats.
Dormancy in many legumes is associated with thick seed coats.
Through time, horsegram seed coats became progressively thinner.
Importantly, the change was gradual and stepped.
This pattern is much more compatible with prolonged evolutionary selection than with farmers finding a single miraculous non-dormant mutant and immediately propagating it.
Cultivation changes the reproductive lottery
Under repeated sowing, seeds that germinate promptly contribute disproportionately to the current harvest.
Those plants therefore have more descendants among the seeds that farmers collect and resow.
Dormant seeds may survive in the soil, but they contribute less frequently to the human-managed reproductive cycle.
Generation after generation, the balance shifts.
The result is another domestication trait that can emerge without intentional plant breeding.
The farmer changes the schedule.
Evolution gradually synchronises the plant to it.
################################################################################
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
################################################################################
No comments:
Post a Comment