59The economic advantage of growing various genotypes, and even species, as a
mixture in one fi eld was yield stability—an economic consideration which was
much more important than occasional high yield (Zeven 1980 ). The traditional farm-
ers preferred a “safe”, average yield each year, rather than a high yield for several
years that might have been followed by a crop failure. The lack of suitable means for
long storage and for large-scale wheat-import rendered a crop failure in any year to
have drastic consequences. Hence, yield stability was of utmost importance.
In such polymorphic fi elds inter-genotypic competition played a decisive role.
Plants with horizontal leaves, which shade weeds and competitors, had an advan-
tage over genotypes with erect leaves. Also high tillering and vigorous vegetative
growth were traits with a high adaptive value.
The main evolutionary advantage in such fi elds was the possibility for occasional
hybridization between genotypes and species. Because of the self-pollinating sys-
tem, hybridizations resulted in numerous homozygous recombinants, thereby con-
stantly providing the farmer with new stable genotypes for selection.
Since farmers selected and planted grains that were most desirable for their spe-
cifi c needs, selection pressures were exerted consistently but in different directions
in various locales. These efforts resulted in increased plant height, increased tiller-
ing, development of canopy with wide horizontal leaves, larger seed size, increased
grain number per spikelet, better fl our quality, improved seed retention (non shatter-
ing), increased competitiveness with other wheat genotypes and weeds, and better
adaptation to a wider range or climatic and farming regimes.
Under scientifi cally planned modern breeding , starting at the end of the nine-
teenth century, the wheat fi eld has become genetically uniform and no longer con-
ducive to spontaneous gene exchange. On the other hand, large-scale gene migration
has been promoted by worldwide-introduction services. Massive scientifi c screen-
ing has aided in revealing desirable genes, and modern methods for manipulating
and transferring these genes from one genetic background to another became avail-
able. Hybridizations have been confi ned mainly to intra-specifi c crosses. Lately,
however, some inter-specifi c and inter-generic crosses are also being performed.
Individual genotypes, rather than mixtures, became the unit of selection. Selection
has been made mostly for traits that improve wheat performance in dense stands,
such as minimum intra-genotypic competition, upright leaves to improve light pen-
etration and prevent shadowing of neighboring plants, low tillering, and a higher
number of seminal roots whose development is non-dependent on tillering, enhanced
response to fertilizers and agrichemicals, increased resistance to diseases, pests, and
lodging, improved harvest index, as well as improved baking and bread-making
quality. The general goal was to achieve highest possible yields per area the main
limiting factor (globally) is water availability. Plants were selected to address this
goal and constraint.
Modern high-yielding cultivars are semi-dwarf (90–120 cm) or dwarf (60–
90 cm), which have replaced the conventional taller (120–140 cm) cultivars. These
cultivars, containing genes for short stature, the Rht genes (Gale and Youssefi an
1985 ), respond well to new agrotechnical practices, particularly to high application
rates of fertilizers, without lodging. The high performance of these cultivars results
2 Origin and Evolution of Wheat and Related Triticeae Species