In his landmark work on cultivated plants, N.I. Vavilov (1926) used the cen- tres of diversity of native crop species to predict where they were initially domesticated. This system works well for a high percentage of crops, but a number, such as wheat, sorghum, pearl millet and beans, do not have a true centre of diversity and others, such as barley and rice, were domesti- cated far from their centres of diversity (Harlan, 1992). To adjust for this problem, Vavilov developed the concept of secondary centres to describe those cases where centres of diversity and origin were not the same. This approach helps with several crops, including wheat, barley and rice, but there are still a number of crop species that appear to have been domesti- cated completely outside their native ranges. These transdomestications might have resulted from long-range oceanic drift (bottle gourd outside Africa, sweet potato in Polynesia), dispersal by migratory birds (perhaps tomato in Mexico, arabica coffee in Arabia), human trade of wild material (cotton from Africa to India, perhaps pepo gourds in eastern North America) or original movement as a weed (rye and oats).
Harlan (1975, 1976) has classified domestication patterns into five classes: (i) endemic – the domesticant occupies a well defined, small geo- graphical region (guinea millet); (ii) semi-endemic – the domesticant occu- pies a small range with some dispersal out of it (African rice and tef f);
(iii) monocentric – the domesticant has a wide distribution with a dis- cernible centre of origin (the later plantation crops, such as coffee, rubber,
cacao and oil-palm); (iv) oligocentric – domesticant has a wide distribution and two or more centres of diversity (our major food crops, such as wheat, barley, pea, lentil, chickpea, flax, maize and lima bean); (v) non-centric – domesticant has a wide distribution, but no discernible centres of diversity (American beans, radish, sorghum, pearl millet, cole crops and bottle gourd).
While people in developed countries are familiar with only a few dozen oligocentric and non-centric crops, in reality, hundreds of endemic and semi-endemic crops were domesticated. In his book Crops and Man, Harlan (1992) provides what he calls a ‘short list’ of world crops, which encompasses 11 pages of text. Most of these crops are unknown outside their region of origin. In the publication Lost Crops of the Incas (Anon., 1989) over 30 species are described that were domesticated by the Andean peoples but are little grown outside South America (Table 7.7);
many of these are restricted to specific elevational gradients (Fig. 7.7).
Ethiopia provided the world with coffee, but also a unique assemblage of endemic crops, including the cereal teff, the oil crop noog, the mild nar- cotic chat, and enset, a relative of banana whose stem base rather than fruit is eaten (Harlan, 1992). Thousands of plant species have been uti- lized by somebody somewhere, but very few of them have attracted wide- spread attention.
Summary
The transition from hunter–gatherer to farmer was a gradual one that took thousands of years. The earliest grains and legumes were not domesticated until about 10,000 years ago, and it took another 5000 years for the rest of our major crops to emerge. The earliest group of domesticants were all herbaceous annual species and most were selfing, which provided quick harvests after planting and true-to-type seed. Once plants were domesti- cated, they were dramatically altered by humans through conscious and unconscious selection. The simple act of harvesting resulted in the selec- tion of non-shattering traits, more determinate growth, more uniform ripening and higher seed production. The seedling competition that occurred after seed were scattered probably increased seedling vigour and rate of germination. Many of the traits associated with plant domestication are regulated by only a few genes, making rapid evolutionary change pos- sible. Domestication often resulted in reduced levels of variability, but hybridization between crops and their wild progenitors occasionally increased their local adaptations and expanded their geographical range.
Transfers of genes between wild and domesticated populations also led to the evolution of weeds that mimic the crop in such a way that their removal becomes difficult.
Table 7.7. Little-known plants domesticated by the Incas in South America (source – National Research Council, 1989, Lost Crops of the Incas, National Academy Press, Washington, DC).
Type of crop Common name Species Distinctive properties
Roots and Achira Canna edulis Staple with unusually large starch grains tubers Ahipa Pachyrhizus ahipa Crisp like apples, addition to salads
Arracacha Arracacia Has flavours of celery, cabbage and xanthorrhiza chestnut
Maca Lepidium meyenii Sweet tangy flavour; can be stored for years Mashua Tropaeolum tuberosum Starchy staple; very easy to grow Mauka Mirabilis expansa ‘Cassava’ of the highlands Oca Oxalis tuberosa Second most important staple in the
highlands (to potatoes)
Potatoes Many other than Most important staple; huge diversity Solanum tuberosum
Ulluco Ullucus tuberosus Very brightly coloured; staple in some areas
Yacon Polymnia sonchifolia Sweet and juicy, but almost calorie free Grains Kaniwa Chenopodium Very nutritious and extremely hardy
pallidicaule
Kiwicha Amaranthus Protein quality is equivalent to milk caudatus
Quinoa Chenopodium quinoa Excellent source of protein; extremely hardy
Vegetables Basul Erythrina edulis Tree with edible seeds; staple in some areas
Nuñas Phaseolus vulgaris Dropped into hot oil and popped Tarwi Lupinus mutabilis Extremely rich in protein and oil Peppers ManyCapsicum Huge range in pungency and taste Squashes Many Cucurbita Unusually robust and productive Fruits Mora de Castilla Rubus glaucus Superior in flavour and size to other
raspberries
Ugni Myrtus ugni Sprightly flavour; blueberry relative Capuli cherry Prunus capuli A large, sweet black cherry
Cherimoya Annona cherimola Has flavours of pawpaw, pineapple and banana
Goldenberry Physalis peruviana Yellow fruits are excellent in jam; very hardy
Highland pawpaws Caricaspecies Unusually cold adapted
Lucuma Pouteria lucuma Used as both a staple crop and a fresh fruit; a tree can feed a family Naranjilla (lulo) Solanum quitoense Like tomato; has particularly refreshing
juice
Pacay Ingaspecies Long pods filled with soft white pulp (called ice-cream beans)
Passion-fruits Passifloraspecies High quality and huge variabiliy Pepino Solanum muricatum Tastes like sweet melon Tamarillo Cyphomandra betacea A tree with a tomato-like fruit Nuts Quito palm Parajubaea cocoides Nut tastes like tiny coconuts
Bitter potatoes
5000
4000
3000
2000
1000
Arracacha Achira Yacón Ajipa Mauka Native potatoes Ulluco Oca Mashua Maca
Fig. 7.7. Elevational gradients of Andean crops (from CIP Circular, September 1994, Centro Internacional de Papas, Lima, Peru).