Meaning of Evolution:
A divergent process which increases genetic diversity and leads to change in allelic frequencies in a population is known as evolution. In other words, a process which leads to significant deviation in the characteristic features of existing individuals as compared to their pre-existing individuals is termed evolution. Thus, evolution leads to change in genetic composition of a population or individuals of population.
The evolution is of two types, viz:
(1) Natural evolution, and
(2) Man-made evolution.
In case of natural evolution, natural selection operates, while in man-made evolution human selection operates. Thus, both natural and human selections play significant role in the process of evolution. Plant breeding is considered as current phase of crop evolution.
Changes which are brought out as a result of evolution are measured in terms of morphological, anatomical, embryological, physiological, biochemical and genetic modifications in the present forms of individuals as compared with their past forms.
Genetic Basis of Evolution:
Selection plays a key role in the process of evolution. Selection either by nature or by human has been responsible for evolution of various crop plants. However, selection is effective in changing the features of species only when vast variability exists in the population of that species.
Three genetic factors or forces, viz:
(1) Polyploidy,
(2) Introgression, and
(3) Mutations have played significant role in the evolution of various crop plants.
These three factors aid in the process of evolution by way of inducing additional genetic variability, which is a basic requirement for selection to operate.
The role of above three factors in evolution of crop plants is briefly described below:
1. Polyploidy:
Polyploidy refers to numerical change in the genome (A basic set of chromosomes).
Polyploidy is of two types, viz:
(a) Autopolyploidy, and
(b) Allopolyploidy.
Role of each type of polyploidy in evolution is discussed below:
a. Autopolyploidy:
This is also known as simple polyploidy or single species polyploidy, because the increase in chromosome number relates to the same species. This type of polyploidy can occur in nature as well as can be induced by colchicine treatment. Increase in chromosome from diploid to tetraploid state leads to increase in vigour and size of flowers and fruits over diploid forms.
However, polyploid plants exhibit slow growth rate and reduced fertility due to chromosomal imbalance. Autopolyploidy has been used in crops like banana, apples, sugarbeet, watermelon, potato, oranges, tulips, etc. The commercial banana is autotriploid (3n), which has seedless and larger fruits than diploid forms.
Some varieties of apples in USA are triploids, which are propagated asexually by budding and grafting. Triploid varieties of sugarbeet have higher sugar content than diploids Triploid watermelons, which are produced from a cross between tetraploid and diploid, are -seedless, early maturing and resistant to diseases. In USA, triploid watermelons cover about 10% of the area under this crop.
The commercial potato is regarded as an autotetraploid though interspecific hybridization may also be involved. Triploid oranges (which are seedless) are produced by pollinating diploid plants with pollen of tetraploid plants. Triploid tulips are propagated asexually. Autopolyploidy in Petunia, an ornamental plant, leads to brighter flower colour than diploid forms.
b. Allopolyploidy:
This is also known as hybrid polyploidy or bispecies or multispecies polyploidy depending upon the species involved. Such polyploidy is obtained by doubling of chromosome number by colchicine treatment. Allopolyploidy has been more instrumental in evolution of crop plants, because 50% of the crop plants are alloployploids.
Hybrid polyploidy has played significant role in the evolution of crops like wheat, tobacco, cotton, Brassica, oat, etc. Examples of artificially produced allopolyploids include triticale, strawberry and loganberry.
Triticale is a man-made new cereal (between wheat and rye), which combines high yield of wheat and disease and drought resistance of rye. Strawberry is a polyploid between North and South American species, and loganberry is a polyploid between raspberry and blackberry.
2. Introgression:
Incorporation of gene of one species into the genetic background of another species by means of interspecific hybridization and backcrossing is known as introgression (Anderson, 1949). The interspecific hybrid backcrosses in nature with one of the parental species.
As a result of introgression, genes from two divergent species are combined. A true breeding recombinant form favoured by natural selection may give rise to a new species. Introgressive hybridization between primitive maize and wild grass Tripsacum is considered to be responsible for the evolution of modern forms of maize.
3. Mutation:
Mutations are important sources of creating variability in a genetic population. Mutations can occur in nature as well as can be induced by the use of physical or chemical mutagenic agents. In hexaploid wheat, a natural mutation is responsible for homologous pairing.
Spontaneous mutations have played significant role in the evolution of crop plants. Spontaneous mutations can be used either as a cultivar or as a parent in the hybridization programme. Induced mutations have played key role in improving yield, quality, earliness, adaptation, and disease and insect resistance in various crop plants.
Role of Mutation:
The bread wheat is hexaploid, combining diploid chromosome complements from three different species. In nature, hexaploid bread wheat behaves as diploid [n = 21 and 2n = 42]. It has been found that hexaploid wheat has acquired this property of diploid pairing from a mutation on chromosome number 5B, which inhibits pairing between homologous chromosomes (chromosomes of different genomes).
This mutant gene is present in the long arm of chromosome number 5 in B genome. The mutant gene permits only homologous pairing (pairing between the chromosomes of the same species). Thus, this mutant gene enforces hexaploid wheat to have homologous pairing and behave like a diploid species.
This system acts as a barrier to pairing between the genomes of different species. This phenomenon is called diploidization. The 5B system in wheat was discovered by Riley in 1954.
Evolution of Some Crop Plants:
The modern crop plants have developed in nature from the primitive forms as a result of evolution.
The genetic evolution of some important field crops such as wheat, cotton, tobacco, Brassica, potato, maize and rice is briefly described below:
1. Evolution of Bread Wheat:
Wheat is a cereal crop of global importance. It belongs to the genus Triticum of the family Poaceae (old Gramineae). There are three types of species in the genus Triticum, viz., diploid, tetraploid and hexaploid. The somatic chromosome number of these species is 14, 28 and 42, respectively (Table 39.1).
Bread wheat (Triticumaestivum) is the predominantly cultivated species, which belongs to the hexaploid group. Other cultivated species are T. monococcum in diploid group and T. turgidum in tetraploid group.
Triticumdichasians (Aegilopscaudata), Triticumtauschii (Aegilopssquarrosa), Triticumturgidum (T. dicoccum, T. durum, T. polonicum, T. turgidum), and Triticumaestivum (T. vulgare, T. compactum, T. spelta). Parentheses denote old names.
The tetraploid species developed as an amphidiplod between two diploid species, and hexaploid species originated from a cross between tetraploid and diploid species. It is believed that tetraploid species Triticumturgidum evolved as an amphidiploid between Triticummonococcum (AA) and an unknown species (now probably extinct) with BB genome.
The hexaploid bread wheat originated as an amphidiploid between Triticumturgidum (AABB) and T. tauschii (DD). The overall process can be represented as follows (Fig. 39.1).
Thus, A genome of bread wheat has derived from T. monococcum, B genome from an unknown species which is now probably extinct, and D genome from T. tauschii. The F1 was sterile at both the stages, which became fertile through chromosome doubling in nature. Thus, interspecific hybridization and polyploidy have played key role in the evolution of bread wheat.
2. Evolution of Upland Cotton:
Cotton is one of the major fibre crops of global importance. It is grown in more than sixty countries in the world. Cotton belongs to the genus Gossypium of the family Malvaceae. There are about 50 species in the genus Gossypium. Some of them are diploid [2n = 26] and some tetraploid [2n = 52], Out of 50 species, only four species are cultivated, viz., G. arboreum, G. herbaceum, G. hirsutum and G. barbadense (Table 39.2).
This is generally believed by the evolutionists that Gossypium africanum (native of South Africa) is the ancestor or progenitor of all cultivated species of cotton. It reached India possibly by sea and over a long time branched into two species, viz., G. arboreum and G. herbaceum. These species have 13 large chromosomes in their haploid complements.
The American diploids have 13 small chromosomes in their diploid complements. Skovsted (1937) proposed that tetraploid cottons have developed from natural crossing between diploid species with small and large chromosomes and natural chromosome doubling in the long past resulted in the evolution of present day tetraploid species.
There are two opinions about the origin of upland cotton (Gossypium hirsutum). According to Beasley (1940) a possible origin of G. hirsutum is from the cross between Asiatic cultivated cotton, G. arboreum and American wild species, G. thurberi followed by chromosome doubling of F1 in nature (Fig. 39.2a).
According to more recent scheme G. hirsutum has originated from the cross between G. africanum and G. Raimondi followed by chromosome doubling of F] in nature (Fig. 39.2b). Now this theory is widely accepted.
3. Evolution of Tobacco:
Tobacco is a narcotic plant which belongs to the genus Nicotiana in the family Solanaceae. It is a native of America, but now it is grown in all the countries of South and South-East Asia. There are two cultivated species of tobacco, viz., Nicotiana tabacum and N. rustica. Both these species are tetraploid (2n = 48). The wild species are diploids (Table 39.3).
It is believed that Nicotiana tabacum has originated as an amphidiploid between two diploid wild species, viz., N. sylvestris and N. tomentosa (Fig. 39.3a). Similarly, N. rustica is believed to be an amphidiploid between wild diploid species N. paniculata and N. undulata (Fig. 39.3b).
The crossing of diploid species, viz., N. sylvestris and N. tomentosa with N. tabacum and N. paniculata and N. undulata with N. rustica leads to bivalent formation in F1 during meiosis (metaphase I), which confirms involvement of respective diploid species in the evolution of respective tetraploid species.
4. Evolution of Tetraploid Brassica:
The genus Brassica belongs to the family Crucifereae and has several oil bearing species. There are three basic species of Brassica from which three different tetraploid species have originated (Table 39.4). The tetraploid species have originated through interspecific hybridization between diploid species and chromosome doubling of the F1 in nature (Fig. 39.4).
Brassica Juncea is natural amphidiploid between B. campestris and B. nigra, B. napus is amphidiploid between B. oleracea and B. campestris, and B. carinata is amphidiploid between B. nigra and B. oleracea. The origin of different tetraploid species from three diploid basic species can be represented by triangle of U (Fig. 39.4).
Evolution of Potato:
Potato is an important vegetable crop of global importance. It belongs to the genus Solanum in the family Solanaceae. The commercially cultivated potato (Solanum tuberosum) is native of Central and South America from where it has spread to other parts of the world.
Andes region of South Peru and Bolivia is believed to be the center of origin of cultivated potato. S. tuberosum is a tetraploid species [2n = 48], which is believed to have originated from the diploid species S. stenotomum which is also found growing in the Andes region of South Peru and Bolivia (Table 39.5).
First it was believed that S. tuberosum is an autotetraploid from the species S. stenotomum. But according to more recent views, it is probably a segmental allopolyploid derived from crosses between cultivated diploid species S. stenotomum and wild diploid species S. sparsipilum.
5. Evolution of Maize:
Maize is a cereal crop of global importance. It belongs to the genus Zea of the family Poaceae (old gramineae). It is grown both for food and fooder purposes. Maize is native of America from where it has spread to other parts of the world. Maize (Zea mays) is the only species in the genus Zea. It has two close relatives, viz., Gamagrass (Tripsacum) and Teosinte (Euchlaena).
Teosinte is considered to be the closest relative of maize because maize crosses readily with Teosinte. Moreover, chromosome number is also similar in maize and annual forms of Teosinte (2n = 20).
The present day commercially cultivated forms of maize are believed to have originated through introgression of genes from its wild relative Teosinte, and Teosinte is considered to have originated from a cross between primitive corn and Tripsacum. This is the most widely accepted theory about the genetic origin of maize crop.
6. Evolution of Rice:
Rice is a staple food of global importance. It belongs to the genus Oryza in the family Poaceae (old Gramineae). It is grown in India and China from time immemorial. South and South-East tropical Asia is considered to be the native place of predominantly cultivated species of rice, Oryza sativa, because vast diversity of this species is found in this region.
The other cultivated species is Oryza glaberrima, which is native of West tropical Africa and also grown there.
It is generally believed that the Asian rice (O. sativa) has originated in Asia from the perennial Asian wild species Oryza rufipogon. The African cultivated species (O. glaberrima) is believed to have originated in West Africa from wild diploid species O. breviligulata.
Still there is controversy about the progenitor of Asian rice. Some researchers believe that Asian rice has originated from perennial wild species, while others consider annual form as the aneastor of cultivated rice.