The variations may be present in natural populations or they may have to be introduced or induced artificially. The following are the chief sources of variations among natural populations.
Spontaneous mutations bringing about variations in traits get stabilized due to recombinations. Natural hybridizations occurring frequently bring in some more variations. Most of these variations are heritable.
Another source of variation is the introgressive hybridization that occurs normally between varieties, or even species. This type of hybridization many a time will not breed true, but can be propagated by vegetative means.
This refers to increase in chromosome numbers from the normal diploid. Increased policy in most instances brings in change in the phenotypic traits, like increased growth, size, yield etc. This is one of the principle sources of variations. Polyploidy can even be induced artificially. Polyploids are quite common among flowering plants.
Selecting plants with favorable or suitable variations is the second step in hybridization. In fact, in earlier days, selection was the only method of breeding. During the pre Mendelian era, selection was primarily based on phenotypic variations.
But as we know today, phenotypic selections may not breed true, off they could be heterozygote. Selection practiced now a day is exact and based on the genetic mechanism for a particular trait. Among the various individuals of a species growing in a Field, few are picked based on their phenotype.
Subsequently they are interbred and several generations of progenies are raised in order to find out whether the plant breeds true for the chosen character. There are two types of selection – Single plant selection and mass selection.
In single plant or pure line selection, several individual plants with superior phenotype are selected.
Usually a large number of plants are selected and are sown in separate lines. From these progeny, again individual plants are selected with the desired character. This is called Progeny testing.
Seeds from these are sown separately again and selection continued in this way for several generations.
Finally a progeny is selected possessing the desired qualities. The entire process may take about 5-8 years. Wheat varieties pv-18 and Kalyan sona are developed by selection process described above.
In mass selection, many similar plants are selected for each desired trait. The seeds from these are sown together (not separately as in single plant selection) and harvested. The seeds are mixed again forming a composite bulk and used for rising the next generation.
The main difference between mass selection and single line selection lies in mixing the seeds and allowing interbreeding, so as to obtain a different genotype in the mass selection, which is not there in single line selection.
Plants selected by mass selection possess fewer genotypes than the original because of recombination but more variety in the genotype could be produced than in single line selection.
The two types of selection mentioned above are called artificial selection as against natural selection. The interference of human beings selectively allows perpetuation of some desirable traits and at the same time eliminates the undesirable ones.
This artificial selection may follow the natural selection or as it happens in most of the times it may work exactly opposite. In other words, traits which would have got selected under natural selection maybe eliminated or vice versa.
Plants that are selected on the basis of possession of favorable traits may be large; as a result the desired traits are distributed in many individuals.
The idea of the plant breeder is to put together as many good traits as possible in a single individual.
Hence the plants have to be hybridized to put together the traits in a single individual. In addition to this, hybridization provides further variety and improvement.
For instance a pure line plant with a good trait may have another undesirable trait in the recessive condition. But by hybridization, this recessive trait can be masked. Hybridization in plants is usually of two basic types Inbreeding and out breeding.
Mating between closely related individuals is called inbreeding. There may be different degrees of inbreeding, the extreme case being represented by self pollination in a bisexual flower.
Johanssen (1903) recognized that inbred plants possess identical characters, when grown under similar environmental conditions.
He called such populations as pure lines. In his experiments concerning garden beans, he selected weight as the main criterion and obtained 19 progenies. He found that the seeds have the some mean weight with some minor variations.
As a result of his experiments, Johanssen concluded that selection becomes non operative in a pure line as there is no change in the genotype.
Inbreeding experiments in maize, which is normally cross pollinated has revealed, that, there will be decline in quality, vigor and yield as a result of continued inbreeding. East and Jones have conducted intensive investigations in maize plants and have demonstrated Inbreeding depression’.
After about five generations of inbreeding, they found that, there was decrease in height and yield such degenerative changes increased with the increase of inbreeding ultimately leading to the formation of non viable seeds. To summaries, the following will be the genetic effects due to inbreeding –
1. Pure lines are produced, Homozygosis for many characters increases.
2. Recessive traits get a chance to appear phenotypically.
3. Rate of physiological activities generally comes down.
4. Vigor, yield, growth etc get reduced.
5. Lethality will increase.
Out breeding refers to crosses between plants which are not closely related. This increases the vigor, growth, yield etc., in the progeny. But while cross breeding one has to work within a set parameter.
If a cross is made between two diverse individuals, there will be genome non homogeneity resulting in sterility, even though such individuals may be vigorous. In such an instance the only method of propagation is vegetative.
Such individuals however can be made to reproduce sexually by doubling the chromosomes and reintroducing nomogenity (see polyploidy for details).
Hence it can be said that out breeding may produce hybrid sterility or hybrid vigor or Heterosis depending on to what extent the parents involved in crossing are related.
The phenomenon of out breeding and hybrid vigour has been noticed since a long time. Hybridization in out breeding is of three types.
Individuals belonging to the same species, but with different variations are selected as parents this may be called Intervarietal crossing.
Individuals belonging to two different species, but belonging to the same genus are chosen for crossing. This may be called Intrageneric crossing.
This is the widest parameter within which a cross is possible. Beyond this, the crosses will not succeed. The individuals belong to two different genera, but belong to the same family Raphanobrassica.