51genetic variation than their diploid progenitors. It is possible that polyploidy enabled
genome plasticity that in turn enabled accelerated evolution to take place as proposed
in wheat (Dubcovsky and Dvorak 2007 a, b) and as was recently shown in experimen-
tal evolution studies in yeast (Selmecki et al. 2015 ). It might be also that recurrent
hybridization with the diploid progenitors occurred during the evolutionary history of
allopolyploids as discussed below. The diploid species have undergone divergent
evolution and consequently, have diverse genomes that are isolated genetically from
one another. They exhibit a relatively limited morphological and molecular variation,
specialization in their ear structure, occupation of few well-defi ned ecological habi-
tats, and distribution throughout relatively small geographical areas (Zhukovsky
1928 ; Eig 1929a , b ; Zohary and Feldman 1962 ; Kimber and Feldman 1987 ; Van
Slageren 1994 ). The allopolyploids show wider morphological variation, occupy
a greater diversity of ecological habitats, and are distributed over larger geographical
area than their diploid progenitors (Zhukovsky 1928 ; Eig 1929a , b ; Zohary and
Feldman 1962 ; Kimber and Feldman 1987 ; Van Slageren 1994 ). The distribution
areas of most of the tetraploids overlap, completely or partly, with that of their two
diploid parents and extend beyond it. They grow well in a very wide array of edaphic
and climatic conditions and so do not show the marked ecological specifi city of the
diploids. Their weedy nature is refl ected in their ability to colonize rapidly and
effi ciently a variety of newly disturbed and secondary habitats. Undoubtedly, the
expansion of agriculture and the opening up of many new habitats played a key role
in the massive distribution of these allopolyploid species throughout the range of the
group (Zohary and Feldman 1962 ; Kimber and Feldman 1987 ).
Therefore, students of wheat evolution have been fascinated by this paradox and
have dedicated themselves to unraveling the processes and mechanisms that con-
tributed to the buildup of genetic and morphological diversity of the allopolyploids
and to their great evolutionary success in term of proliferation and adaptation to new
habitats, including under domestication.
In clear contrast to the rarity of inter-specifi c hybridization at the diploid level,
hybridization between allotetraploid species, particularly between those sharing a
common genome , is a frequent phenomenon (Zohary and Feldman 1962 ; Feldman
1965a ). Such allotetraploid species tend to grow in mixed stands, and many F 1
hybrids as well as backcrossed progeny were repeatedly found in many localities in
Israel, Turkey and Greece (Zohary and Feldman 1962 ; Feldman 1965a ). Actually, a
range of morphological intermediates between allotetraploids growing together in
one mixed stand is a common phenomenon. Additional evidence for the existence
of introgressed genomes in allopolyploid Aegilops were obtained from C-banding
analysis (Badaeva et al. 2004 ). An introgression of a DNA sequence from allopoly-
ploid common wheat to the allotetraploid Aegilops species, Ae. peregrina, was
recently described (Weissmann et al. 2005 ). Hence, hybridization between allotetra-
ploids, particularly between those sharing one common genome, and, to a lesser
extent, between allotetraploids and diploids, facilitates a rapid buildup of genetic
variability at the tetraploid level. The differential genomes that are isolated from one
another at the diploid level genetically and geographically where emphasis is on
divergence and specialization are brought together and allowed to recombine at the
2 Origin and Evolution of Wheat and Related Triticeae Species