What vegetable seeds tell us about the intentions of ancient farmers

A new study pub­lished in Evol­u­tion Let­ters, avail­able now via Early View, has giv­en us an inter­est­ing insight into the his­tory of crop domest­ic­a­tion. The work, by research­ers at The Uni­ver­sity of Shef­field, UK, and the Uni­ver­sity of Toulouse, France, shows that seed enlarge­ment prob­ably evolved without the delib­er­ate inten­tion of early crop farm­ers. Humans have been arti­fi­cially select­ing for spe­cif­ic traits in domest­ic plants and anim­als for hun­dreds of years, select­ing only the best indi­vidu­als to breed and reject­ing those with undesir­able traits. How­ever, this study shows that dur­ing the course of domest­ic­a­tion, unin­ten­ded changes have also occurred with dra­mat­ic effects.

Here Pro­fess­or Colin Osborne, the Prin­cip­al Invest­ig­at­or on the study, explains how his research team deduced the motives of farm­ers thou­sands of years ago, and tells us what veget­able seed size has in com­mon with your pet dog’s floppy ears…

Carrotdime
Domest­ic crop seeds, like these car­rot seeds, have become much lar­ger than their wild rel­at­ives.  Photo by John Alan Elson (commons.wikimedia.org/wiki/File:Carrotdime.jpg)

EL: It’s fas­cin­at­ing to learn that early farm­ers had such a huge impact on crop evol­u­tion without know­ing what they were doing. How were people able to influ­ence seed size so strongly, without delib­er­ately select­ing for large seeds?

CO: Archae­olo­gists have long sus­pec­ted that early farm­ers unin­ten­tion­ally changed their crop plants, and this is often referred to as uncon­scious selec­tion. For example, har­vest­ing cer­eal crops using stone sickles could select for seeds which stay on the plant and don’t dis­perse by fall­ing off. This ser­i­ously impedes repro­duc­tion in a wild plant, so it’s not favoured in nature, but it’s bene­fi­cial for a crop if people are col­lect­ing and sow­ing the seeds. 

Seed size could be influ­enced in a sim­il­ar way if lar­ger seeds are more likely to sur­vive plant­ing or if big­ger seed­lings are bet­ter com­pet­it­ors. Anoth­er pos­sib­il­ity is that people played a more act­ive role, thin­ning out small plants but leav­ing the big­ger ones, or delib­er­ately choos­ing to sow the seeds from large plants next sea­son. If there is a genet­ic link between plant size and seed size, this could lead to lar­ger seeds. In this case, plant size would be the tar­get of selec­tion (unin­ten­tion­al or inten­tion­al), but seed size would change unintentionally.

So there are a num­ber of pos­sib­il­it­ies, but none of them involves a delib­er­ate ploy by early farm­ers to breed crops with lar­ger seeds.

EL: Your study looks at seed size in veget­ables like pota­toes and car­rots. Why did you choose to focus on veget­ables, instead of grain crops like wheat and barley?

CO: You’re right that grain crops are the import­ant ones – today 60% of human cal­or­ie intake comes from wheat, rice and maize. And we know that seed size in these spe­cies is influ­enced by domest­ic­a­tion – for example, in our study, maize seeds are 15 times lar­ger than their wild rel­at­ives, rice seeds are 90% lar­ger, and wheat 20%. But it’s tricky in these crops to dis­tin­guish inten­tion­al from unin­ten­tion­al effects, because the seeds are also the grains that people har­vest and eat. It’s plaus­ible they would have bred lar­ger grains.

This is why we also chose to look at the seeds of veget­able crops. Farm­ers grow these plants to har­vest oth­er parts, like their roots (e.g. car­rots) or leaves (e.g. lettuce). We expect that, although farm­ers might be inter­ested in breed­ing big­ger car­rot roots or lettuce leaves, they would not delib­er­ately breed lar­ger seeds in these crops. So by look­ing at veget­ables, we can assume that any effects of domest­ic­a­tion on seed size are prob­ably unintentional. 

Ipomoea_batatas_006.JPG
Farm­ers do not typ­ic­ally grow sweet pota­toes from seed, but their seeds have still enlarged with domest­ic­a­tion. Photo by Llez (commons.wikimedia.org/wiki/File:Ipomoea_batatas_006.JPG)

We also included root crops like potato, cas­sava and sweet potato. These are even more use­ful for our work because farm­ers don’t usu­ally sow seeds at all, but plant tubers (e.g. pota­toes) or stem cut­tings (e.g. cas­sava) in the soil. In these crops, any seed enlarge­ment must come from genet­ic links to plant or tuber size. 

Our res­ults show that domest­ic­ated veget­ables have lar­ger seeds than their wild rel­at­ives. We see this in veget­ables like car­rot, where seeds are planted, and potato, where tubers are planted (and seeds are not). Unin­ten­tion­al selec­tion is the most likely explan­a­tion for this enlarge­ment. The effect for veget­able seeds falls with­in the range we see for grain crops, so we can also say that unin­ten­tion­al selec­tion would have been suf­fi­cient to drive the seed enlarge­ment we see in grain spe­cies too. But it’s prob­ably not all that happened – domest­ic­a­tion over­all has a lar­ger effect in grains than in veget­ables, which means that grain domest­ic­a­tion was prob­ably caused by a com­bin­a­tion of unin­ten­tion­al selec­tion and delib­er­ate breeding.

EL: How import­ant is seed size for plants? What impact might unin­ten­tion­al selec­tion for seed enlarge­ment have had?

CO: Seed size in wild plants is bal­anced on one hand by the need to dis­perse – smal­ler seeds tend to fall fur­ther from their par­ent – versus the bene­fits for sur­viv­al of being large. Lar­ger domest­ic­ated seeds won’t dis­perse as far as their wild rel­at­ives, but this isn’t an issue for many crops, since they don’t release their seeds any­how. Instead they are har­ves­ted and sown by farm­ers – prob­lem solved – and this allows seeds to get lar­ger without the pen­alty of reduced dispersal. 

Some people think that sow­ing seeds in the soil could select for lar­ger size because it improves sur­viv­al, but in a pre­vi­ous study we’ve looked at this and it doesn’t work as a gen­er­al explan­a­tion. How­ever, we do know that lar­ger seeds make big­ger seed­lings, and we’re cur­rently work­ing on the idea that this makes them bet­ter com­pet­it­ors with weeds and smal­ler crop seeds.

EL: To what extent do you think unin­ten­tion­al selec­tion might have influ­enced oth­er traits in the plants you looked at, such as leaf or root size?

CO: Based on our work, we can’t really say. If there is a gen­er­al effect of domest­ic­a­tion on plant size, then we might expect oth­er parts of the plant to get lar­ger too. Recent pub­lished work does sug­gest this – for example, crop plants have lar­ger leaves than their wild rel­at­ives. In fact, there is a gen­er­al tend­ency for gigant­ism in crop plants – they are lar­ger, more robust and flesh­i­er than the wild spe­cies they evolved from. How­ever, wheth­er this was the res­ult of delib­er­ate breed­ing or unin­ten­tion­al selec­tion is an unanswered ques­tion for anoth­er day.

EL: How gen­er­al do you think the effects of unin­ten­tion­al selec­tion could be across all domest­ic plants and anim­als? Could some of the traits we see in our garden flowers, or even our pet dogs, have come about unin­ten­tion­ally dur­ing their domestication?

CO: Unin­ten­tion­al selec­tion has prob­ably been import­ant for lots of domest­ic­ated anim­als and plants. Dogs are a great example. It seems pretty unlikely that people delib­er­ately tamed wolves. Instead, the first dogs were prob­ably camp fol­low­ers, scav­en­ging food from people. Over time, nat­ur­al selec­tion would have favoured tamer wolves who were less afraid of people, even­tu­ally res­ult­ing in anim­als more like dogs. That’s a story, but there is also exper­i­ment­al evid­ence from Siberi­an foxes about how this tam­ing might have led to domest­ic­ated anim­als.  

1024px-Vulpes_vulpes_standing_in_snow.jpg
Unin­ten­tion­al selec­tion may explain traits like floppy ears and curly tails in domest­ic dogs, and foxes bred for tame­ness. Photo by V. Bern (commons.wikimedia.org/wiki/File:Vulpes_vulpes_standing_in_snow.jpg)

Rus­si­an sci­ent­ists have car­ried out select­ive breed­ing of foxes using only the tamest anim­als every gen­er­a­tion, and over fifty years they have got some remark­able res­ults. The foxes are now tame like dogs, but they also resemble dogs phys­ic­ally – with white patches of fur, floppy ears, short­er muzzles and curly tails. None of these char­ac­ter­ist­ics were delib­er­ately bred, but emerged unin­ten­tion­ally, per­haps because they are pro­duced by the same devel­op­ment­al or hor­mon­al path­ways as tame­ness. This is dir­ectly com­par­able to what we think happened with the seeds of crop plants – one char­ac­ter was selec­ted for, and oth­ers were dragged along unintentionally.

EL: Mod­ern farm­ing prac­tices are sci­en­tific­ally and tech­no­lo­gic­ally advanced. How likely is it that crops are still being sub­jec­ted to unin­ten­tion­al selec­tion today?

CO: Pretty likely, I’d say. Crop breed­ing hap­pens out­doors in fields, and so the plants are exposed to any changes in cli­mate or new pests and dis­eases. The breed­ers don’t con­trol these, because they want new crops to do well in lots of con­di­tions, so they select the best per­form­ing lines across lots of fields. In the case of pests and dis­eases, they can see the symp­toms, but for changes in cli­mate the effects are less obvi­ous. I’d expect crops to be adapt­ing to these without us noticing.

For example, every year at the moment, human emis­sions cause car­bon diox­ide to increase in the air by about 1%. That may not sound like much, but new crops are bred over 10 or 20 years, which amounts to a large car­bon diox­ide increase. More car­bon diox­ide leads to more pho­to­syn­thes­is, so we might expect that breed­ers have unin­ten­tion­ally bred crops that are bet­ter able to use this extra car­bon. It’s an intriguing idea, but weirdly it seems that mod­ern crops are less able to use the extra car­bon than old vari­et­ies. We don’t know why – and that’s a story for anoth­er day…

You can read more about the study in this art­icle by Colin in The Con­ver­sa­tion. The ori­gin­al paper is avail­able open access, via Evol­u­tion Let­ters Early View.

Leave a Reply