Principles of Plant Genetics and Breeding

crops. Primary crops are those whose wild progenitors
were deliberately cultivated by humans, genetic changes
occurring in their new environments. Secondary crops
are those that evolved from weeds that arose in cultivated
fields. For example, the common oat (Avena
sativa) evolved from the hexaploid wild oats (A. sterilis
and A. fatua). The domestication of vegetables, root
and tuber crops, and most fruit trees is described as
gradual domestication. This is because it is difficult to
use a single characteristic to differentiate between wild
and cultivated species of these horticultural plants.
These crops are commonly vegetatively propagated,
hence evolution under cultivation would occur mainly
from variation originating from somatic mutations.
Seed crops have the advantage of genetic recombination
through sexual reproduction to create new variability
more rapidly.
Centers of plant domestication
Centers of plant domestication are of interest to
researchers from different disciplines including botany,
genetics, archeology, anthropology, and plant breeding.
Plant breeders are interested in centers of plant domestication
as regions of genetic diversity, variability being
critical to the success of crop improvement. De
Candolle was the first to suggest in 1886 that a crop
plant originates from the area where its wild progenitor
occurs. He considered archeological evidence to be the
direct proof of the ancient existence of a crop species in a
geographic area.
Several scientists, notably N. Vavilov of Russia and
J. R. Harlan of the USA have provided the two most
enduring views of plant domestication. Vavilov, on his
plant explorations around the world in the 1920s and
1930s, noticed that extensive genetic variability within
a crop species occurred in clusters within small geographic
regions separated by geographic features such
as mountains, rivers, and deserts. For example, whereas
he found different forms of diploid, tetraploid, and
hexaploid species of wheat in the Middle East, he
observed that only hexaploid cultivars were grown in
Europe and Asia. Vavilov proposed the concept of
centers of diversity to summarize his observations.
He defined the center of origin of a crop plant as
the geographic area(s) where it exhibits maximum
diversity (i.e., where the greatest number of races and
botanical varieties occur). He identified eight major
centers of diversity, some of which were subdivided
(subcenters). These centers, with examples of associated
plants, were:
THE ART AND SCIENCE OF PLANT BREEDING 19
1 China (e.g., lettuce, rhubarb, soybean, turnip).
2 India (e.g., cucumber, mango, rice, oriental cotton).
2a Indochina (e.g., banana, coconut, rice).
3 Central Asia (north India, Afghanistan, Turkmenistan)
(e.g., almond, flax, lentil).
4 The Near East (e.g., alfalfa, apple, cabbage, rye).
5 Mediterranean Sea, coastal and adjacent regions
(e.g., celery, chickpea, durum wheat).
6 Ethiopia (e.g., coffee, grain sorghum, pearl millet).
7 Southern Mexico and Middle America (e.g., lima
bean, maize, papaya, upland cotton).
8 Northeastern South America, Bolivia, Ecuador, and
Peru (e.g., Egyptian cotton, potato, tomato).
8a Isles of Chile (e.g., potato).
Furthermore, he associated over 500 Old World crops
and about 100 New World crops with these centers.
Most (over 400) of the Old World crops were located in
Southern Asia.
Vavilov noticed that even though one species or one
genus was associated with a center of diversity, often it
occurred also at a few other centers. However, whenever
this was the case, the types were often distinguishable
from place to place. He called the centers where maximum
diversity occurs primary centers, and the places where
types migrate to, the secondary centers. For example,
the primary center of corn is Mexico, but China is a secondary
center of waxy types of corn. Vavilov associated
these centers of diversity with the centers of origin of
these crops, proposing that the variability was predominantly
caused by mutations and their accumulation in the
species over a long period of time. These variations were
preserved through the domestication process.
Other scientists of that era, notably Jack Harlan, disagreed
with the association of centers of diversity with
the centers of crop origin. He argued that the origin of
a cultivated plant was diffuse both in time and space.
This opposing view was arrived at from his observations
that plant diversity appeared to exhibit hybrid features,
indicating they likely arose from recombination (i.e.,
centers of recombination). He proposed the new concept
of centers and non-centers as summarized below:
Centers Non-centers
(Temperate and (Corresponding
geographically restricted) tropical areas)
A1 Near East A2 Africa
B1 North China B2 South East Asia
C1 Mesoamerica C2 South America
Each center had a corresponding non-center. The centers
contained wild relatives of many crop plants, whose