How many generations does it take to domesticate plants?

Question

Modern crops have diverged significantly from their wild ancestors over thousands of years of selective breeding. Not all of this was deliberate, e.g. an early adaptation of wheat was for seeds that cling to the stalk instead of falling off; this was a consequence of farmers plowing under the fallen seeds and planting the clinging ones.

Suppose you have to start from scratch with wild ancestral plants, but with modern knowledge. No bio lab for genetic engineering, just selective breeding, but you know what you are doing, and are running a project to carefully, deliberately select the best individual plants in each generation. I think that should be a lot quicker than the thousands of years it took the first time round.

How many generations would it take to produce high-yield wheat, plump apples and strawberries etc. from wild ancestors?

To take a specific example, consider this ancestral strawberry: https://en.wikipedia.org/wiki/Fragaria_virginiana

How many generations of selective breeding would it take to double the yield of strawberries, in terms of kilograms per hectare?

Answer 1

Assume:

• as many individuals per generation as needed to show all possible phenotypes
• the desired trait depends on a single 1-1 phenotype to genotype relationship - there's two genes and they each correspond to a single phenotype when expressed A-small fruit and b-big fruit
• one gene, undesirable, has complete dominance over the other, the desirable trait. That is, if there's ANY A in the offspring, it will show small fruit. Big fruit only shows when both genes are b.
• each progenitor (first generation parent) is a mixed genotype (therefore would have small fruits, but carry the potential for making big fruits)
• and there's no significant genetic mutations (there's ALWAYS mutations, most don't make much of a difference)
• complete knowledge and control over pollen exchange

IF all of the above is accepted, then you can change the phenotype to big fruits that breed true in only one generation, because all plants that exhibit the BigFruit phenotype would have double recessive, bb, genes. None of the above assumptions are very likely, so this result is pretty unhelpful except to show how simple simple would have to be in order to have a clear and consistent answer.

As others have said, luck, perception, scientific knowledge, the genetic complexity of the desired trait, environmental factors contributing to gene expression, methods the plants use for moving genetic material (eg pollen), the people's ability to manipulate the plants and environment (not like in a lab, but like, can you keep bugs away? can you block the wind, etc) to control the pollination, the people's knowledge of how this genetic stuff even works - do they know what pollen is? do they actively intervene in pollination, or are they selecting seed based on whether the plant exhibits the desired trait? Can they vegetatively clone the result once they have one they like? Sometimes crops become crops specifically because they respond to breeding efforts.

The answer is somewhere between 0 and impossible. There are plants and animals that are, as far as we know, undomesticatable. Some plants (peas) are used to teach genetics and breeding due to the relative simplicity of some genotype/phenotype relationships, their usual tendency to self-pollinate (making controlling pollination relatively simple) etc.

tl;dr - This is an incredibly complex question. It depends on so many factors that it is unanswerable except by "one or more generations, if it's even possible".

Now, if you have a situation where you want it to take, say, 10 plant generations, and you want to know some things that would make it so that that's coherent with the traits of the plant, that might be more answerable.

But, with only the information you have provided, and my knowledge (I work with horticulturalists and germplasm specialists, but am not trained as such myself), I do not believe there is a "correct" answer.

Hope that's helpful, sorry it's not a more concrete answer.

These resources, however, may be interesting to your worldbuilding:

This one allows you to search crop plants by desirable traits and, for many, get a genetic lineage for them: https://npgsweb.ars-grin.gov/gringlobal/descriptors

This one allows you to search wild crop ancestors and find out what traits have been introduced when bred back into domestic lines: https://npgsweb.ars-grin.gov/gringlobal/taxon/taxonomysearchcwr

Sources: years of trying to make sense of working between genetic researchers, germplasm resources, and farmers on the ground trying to solve crop issues with more training in communication than biology.

Answer 2

Plump strawberries take exactly one generation.

They are basically hybrids of the European strawberry with the American strawberry. This improved their size, color, and flavor significantly.

Apples and wheat might be better examples. Apples take about 5-7 years before you can test whether the new plant is worthwhile. This is similar to most other fruit trees. But, with fruit trees, you're able to keep a specific variety once you've discovered it through grafting. Grafting is basically caveman cloning (the ancient Romans did it) where you take a branch of a variety you like, and cut it into the bark of another tree where it will start growing again as a sort of Frankentree. Once the branch is growing, you can once again take pieces of that branch, and do it again.

In this way, some "trees" have been growing for centuries, and I believe there are a handful that have been growing for over a millennium.

However, that said, that's just the end product. Apples started out as tiny little fruits, pithy, filled with seeds, and in many cases sour or even bitter. From what I've read, modern apple trees are under 1200 generations away from their undomesticated wild/crab apple ancestors. That much is agreed upon. But the estimates vary considerably, and they may in fact be as few as 700-800 generations ago.

Wheat is different, in that in some places in the world you can have more than one generation per year, but this doesn't make it any quicker really (ancient people weren't experts on domestication, they sort of just stumbled their way through the process, so they were still likely limited to one crop/generation per year). We might look at another grain, teosinte... which eventually becomes world-famous corn/maize. The differences in the two are startling. Few people recognize their relation to look at them, in that corn cobs are so much larger than the heads of seeds in teosinte, so different in every way.

The estimates I'm pulling off of Google, say that it's in the low (single-digit) thousands of generations.

But again, ancient people weren't experts at plant breeding, not like we are today.

The various experiments that have been conducted on such things suggests that as few as 100 generations can make changes significant enough that we'd label them "domestication", if the breeder is sophisticated, goal-oriented, and well-funded. And this seems to hold true for both plants and animals (the experiments with domesticated foxes, for instance).

Answer 3

3-4 plant generations.

In strawberry you can double the yield in 3-4 generations, domesticated strawberry are so large due to cold shock polyploidy, they area hybrid between two octoploid wild varieties. That is they have 8 copies of all their chromosomes, and cold shock can trigger chromosome duplication in plants. You can run both ancestral lines in parallel.

1. 2 sets of chromosomes / cold shock

2. 4 sets of chromosomes / cold shock

3. 8 sets of chromosomes / hybridize

4. the big strawberries you want.

So you can recreate them in four generations of plants at most. You may even find wild varieties that are already tetraploid or octoploid, so you get down to two generations.

However this is not a good indicator of how fast you can do it with most other plants. Corn for instance could take you thousands of generations. However something similar will work for some other fruits which are often also polyploidy.

https://www.nature.com/articles/s41588-019-0356-4

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4315931/