PLB143 - Lecture 08

What is a crop?

The domestication syndrome

© Paul Gepts 2012

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PLB143: Readings - Lecture 08

What is Domestication?

Harlan 1992; Davies and Hillman 1992

  • Process which:
    • Occurs under cultivation in populations of early wild-type crops (sown from seed gathered from wild stands)
    • Selectively advantages rare mutant plants lacking features necessary for survival in the wild (or having features necessary for survival in the field)
    • Continues until the mutant phenotypes dominate the crop population
  • Important aspects:
    • Cultivation (necessary but not sufficient condition); selection --> allele frequency changes; evolutionary process: gradation within and between species

Domesticated plants: taxonomic distribution

Harlan 1992

  • Number of species:
    • Leguminosae: 41 (bean, pea, lentil)
    • Gramineae: 29: maize, wheat, barley, rice,
      sorghum  (from U. of Wisconsin Botany image collection )
    • Solanaceae: 18 (tomato, pepper, potato)
    • Cruciferae: 13 (canola , radish)
    • Cucurbitaceae: 13 (cucumber, melon , squash)
    • Rosaceae: 11 (apple, Prunus, strawberry)
    • Liliaceae: 11 ( onion )
    • Umbelliferae: 9 (carrots, fennel, dill)
    • Araceae: 8 (taro , , from Nation of Hawai`i's Canoe Plants )
    • and other families such as Compositae
      (lettuce, sunflower , from Texas A & M)
  • Large number of vicarious domestications:
    • Old vs. New World: cotton, lupin, amaranth, Solanum
    • Mesoamerica vs. South America: Phaseolus, Capsicum
  • Trend: species actually contributing to nutrition of humanity
    • small and declining number
    • increased emphasis on cereals
  • Sorghum

  • Canola

  • Onion

  • Carrot

  • Taro

  • Sunflower

Some related sites of interest

  • Taro
  • Sunflower
  • Cabbages
    • Wild Brassica taxonomy

Major crops in the world


Million tons Annual, perennial Ecology* Self, cross-fert., or veget. Ploidy
Wheat 468 A M S 2,4,6
Maize 429 A S C 2
Rice 330 A S S 2
Barley 160 A M S 2
Soybean 88 A W S 2
Sugar cane 67 P R V (C) many
Sorghum 60 A S S 2
Potato 54 A H V (C) 2,4,6
Oat 43 A M S 2,4,6
Cassava 41 P S V (C) 4
Sweet Potato 35 A S V (C) 6
Sugar beet 34 A C C 2,3,4
Rye 29 A M C 2
Millets 26 A S C,S 2,4
Rapeseed 19 A M C 4,6
Bean 14 A S S 2
* M: Mediterranean; S: savanna; W: woodlands; R: tropical forest; H: highlands; C: coastal

World's leading food crops in terms of production

Harlan 1992

world crops The predominance of cereals over other crops

Selection pressures exercised during domestication 

Selection was exercised at different stages of the growth cycle. Each of these are underlined in the next section, starting with harvesting

Harvesting: Increase in seed yield (I)

  • Increase in seed recovered:
    • elimination or reduction of seed dispersal
      • non-shattering: e.g., cereals: teosinte (wild maize) 
      • non-dehiscence
    • additional traits affecting seed dispersal: example of Triticum monococcum
    • more compact growth habit
      • reduction in branching: e.g., teosinte 
      • synchronous tillering
      • climbing --> bush

  • Seed dispersal in teosinte, the ancestor of maize:

  • Additional traits affecting seed dispersal in cereals:

    (from Davies and Hillman 1992, © Cambridge University Press)
  • Reduction in branching:
    (from Doebley et al. 1990)

Recovery rate of seeds under different harvesting methods

Davies and Hillman 1992: For further explanations, see also Lecture 16: Experiments of Hillman and Davies

Harvest method

Fragile rachis (wild)

Tough rachis (domesticated)


44-84 % (single - multiple pass)

5 %

Sickle reaping

40 %

100 %

Evolution from climbing to bush growth habit in legumes

Smartt 1978

  • Wild beans are vines, i.e. plants that climb on top of other plants used as physical support
    • Hence, they have long branches: long internodes and many nodes; many branches; "twining" ability;
  • Domesticated beans:
    • Show a a wide range of growth habits from climbing to bush
    • The most "advanced" or modified, compared to the original climbing habit, is the bush habit , which combines the following traits:
      • Reduction in internode length
      • Reduction in the number of nodes
      • Suppression of twining response
      • Determinacy
      • Reduced branching
      • Conversion from diageotropic branching to negative geotropic branching + change in angle of branching
    • Climbing show only part of these traits or a reduction in intensity of their expression
  • Beans in their native habitat:

Harvesting: Increase in seed yield (II)

  • Increase in number of seeds produced
    • Reduction in daylength sensitivity:
      • Adaptive value of sensitivity
    • Reversal of reduction to sterility
      • Example: barley: 2-ranked vs. 6-ranked
    • Increase in inflorescence size:
      • Examples: maize, sorghum, pearl millet, foxtail millet (from B. Smith, the Emergence of Agriculture, © Scientific American Library 1995)
    • Increase in the number of inflorescences:
      • Examples: wheat, barley, rye
  • Foxtail millet

Evolution of Zea mays female inflorescence or "ear"

Harlan 1992

  • From teosinte to domesticated maize ear. How?  
  • Teosinte: axil of several leaves: several small, two-ranked (two rows) fragile ears, each in a husk
  • Early maize: axil of several leaves: small, four-ranked (8 rows), non-fragile ears, each in a husk
  • Modern maize: 1 (2) large many-ranked non-fragile ear surrounded by several husks

  • Teosinte "ear" (inflorescence):

  • Maize ear:

  • Evolution of ear from teosinte to maize

    (from Doebley et al. 1990)

Planting: Increase in seedling vigor

  • Larger seeds:
    • Increase reserves for germination
    • Conflict with number of seeds
    • How? carbohydrates!
  • Non-dormant seeds
    • Adaptive value of dormancy
    • Example: Wild oat, einkorn, emmer: 2 seeds/spikelet:
      1. larger, without dormancy
      2. smaller, dormancy > 1 yr.
  • Conflict: premature germination
  • Correlated response: reduced chaff

Reproductive system

  • Outcrossing --> selfing
    • Reasons?
  • Reduction or absence of sexual reproduction
    • Examples: bananas, plantains; navel orange
  • Vegetatively reproduced crops:
    • Vegetative propagation --> instant domestication
    • Grafting technique: extend advantages of vegetative propagation to crops propagated by seed

Human environment: Adaptation to human utilization and tastes

  • Farmer:
    • Growth habit & cropping system
    • Phenology
  • Consumer:
    • Color (e.g., common bean), flavor, texture, storage quality, cooking time, etc
    • Reduction in toxic or unpleasant compounds:
      • cyanogenic glucoside: e.g., cassava, lima bean
      • bitterness: e.g., squash: wild vs. cultivated
    • Other examples:
      • maize: popping, boiling, off-the-cob, flour (from National Geographic Society)
      • rice: glutinous vs. non-glutinous, long-grained vs. short-grained, aromatic, etc.
      • barley: food, livestock feed, beer

  • Common bean seeds
    © Paul Gepts
  • Wild squash
    © Paul Gepts
    Domesticated squash
    © Paul Gepts
    (a farmer's harvest in Oaxaca, Mexico)
  • Maize types

© National Geographic Society

Summary of the major differences between wild and cultivated common-bean (Phaseolus vulgaris)


General Wild Domesticated
attribute Trait (G12873) cv. Midas)

Seed Pod suture fibers Present Absent
dispersal Pod wall fibers Present Absent

Seed Germination 70.5% 100%

Growth habit Determinacy Indeterminate Determinate Twining Twining Non-twining Number of nodes on the main stem 22.5 7.5 Number of pods 43.2 13.9 Internode length 1.6 cm 2.9 cm Gigantism Pod length 5.7 cm 9.8 cm

weight 3.5 g 19.5 g

Earliness Number of days to
flowering under
12 h days 69 46

Number of days to
maturity 107 80

Photoperiod Delay in flowering
sensitivity under 16h days >90 days 0 days

Harvest Seed yield/biomass 0.42 0.62

Seed Presence vs. Present Absent
pigmentation absence

Less than full domestication

Example: Oil palm (Elaeis guineensis)
  • fruit: 50% oil in mesocarp; cooking, soap
  • thrives at forest edges; favored by slash-and-burn and by religious beliefs
  • three types: durra (traditional, thick shell, single homozygous dominant gene); tenera (thinner shell, heterozygous); pisifera (no shell, homozygous recessive, female sterile)
  • tenera and pisifera are preferred for oil production

Some related sites of interest

Oil palm tree

oil palm tree
from Centre for Remote Imaging, Sensing and Processing (CRISP)

"Trial-and-error" during domestication

  • Example 1: Mesoamerica (Callen 1967)
    • Setaria (foxtail) seeds increased in size; suggested domestication
    • abandoned when maize was introduced
  • Example 2: China (Li 1969)
    • Malva sylvestris: most important green vegetable in Ancient China replaced by Brassica chinensis (Chinese cabbage)
  • Example 3: Near East
    • Medicago sativa : abundant in earliest sites in Near East, disappears afterward upon introduction of other legumes (pea, lentil, etc.)
  • Alfalfa

    (from Southwest School of Botanical Medecine, Michael Moore, Medicinal Plant Images)

The domestication syndrome: conclusion

  • Fully domesticated plants are characterized by a similar set of traits that confer adaptation to the human environment
  • The two most important traits of the syndrome are loss of seed dispersal and seed dormancy
  • The specific details for each trait will depend for each crop
  • Fully domesticated plants have most of the traits of the syndrome but generally not all of them; within a crop, differences in the degree of domestication can be observed

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