• Selection is basic to any crop improvement.
• Isolation of desirable plant types from the population is known as selection.
• It is one of the two fundamental steps of any breeding programme viz.,
  • Creation of variation
  • Selection
• There are two agencies involved in carrying out selection: One is Nature itself (Natural selection) and the other is artificial selection.
• Though both may complement each other in some cases, they are mostly opposite in direction since their aims are different under the two conditions (nature and domestication). The effectiveness of selection primarily depends upon the degree to which phenotype reflects the genotype.
• Before domestication, crop species were subjected to natural selection. The basic for natural selection was adaptation to the prevailing environment. After domestication man has knowingly or unknowingly practiced some selection. Thus crop species under domestication were exposed to both natural and artificial selection i.e. selection by man. For a long period, natural selection played an important role than selection by man. But in modern plant breeding methods natural selection is of little importance and artificial selection plays an important role.

Basic Principles of Selection

👉🏻 Selection has two basic characterics viz

1. Selection is effective for heritable differences only.
2. Selection does not create any new variation.

👉🏻 It only utilizes the variation already present in a population. The two basic requirements for selection to operate are:

1. Variation must be present in the population.
2. The variation should be heritable.

Selection intensity

• Parcentage of plants selected, to be advanced to next generation, from a population. Selection intensity (I).
• It is the amount of selection applied expressed as the proportion of the population favoured (selected). The selection intensity is inversely proportional to the percentage proportion selected (PS), as reflected in Table.

  

• Thus, larger the size of I, more stringent is the selection pressure (hence low fraction is selected) and vice versa. Then, no selection means all the members of a population are allowed to reproduce (I = 0, PS = 100 %), and zero selection means the whole population is rejected (PS = 0). However, in real selection experiments, as the desired alleles become preponderant after each cycle of selection, I is also changed.
• The I is of the greatest consequence is bringing about changes in the gene frequency under selection. However, since the latter does not mean undue loss of desirable alleles, or undue load of population size, the choice of an arbitrary value of I may be hazardous in a plant breeding programme.
• The small size of I (i.e. low selection pressure) may cause a large population size to be handled in the next generation, which will unnecessarily be taxing on time and resources.
• On the other hand, a large size of I (high selection pressure) might cause allelic erosion due to genetic drift (i.e. changes in gene frequencies due to sampling error, or small sample size under selection in a finite population not due to genetic causes). Therefore, an appropriate level of I should be chosen based upon the range of variability present in the population subjected to selection.
• The I = 2.06 to 1.76 (i.e. around 5-10 per cent of individuals selected) has generally been found appropriate in plant populations.
• However, the limit of selection intensity is set by two factors: (i) population size, and (ii) inbreeding. Under natural selection, selection intensity is expressed as the relative number of offsprings produced by different genotypes, and is termed as selection coefficient.

Selection differential

• Difference between the mean of the population and mean of the selected individuals.
• Expressed in terms of standard deviation and is designated as ‘S’ Selection differential (S) S is the average superiority of selected individuals over the mean of population of their origin. It is considered in the same parental generation before selection is made. An arbitrary culling level, k (i.e. I) is fixed for a trait and individuals beyond that level are selected. The average of all such selected individuals can be designated by X. Then the mean of selected individuals (X) exceeds the parental population (µ) mean by the measure of S.

S = X - µ

• Therefore, wider the phenotypic variability (i.e. Phenotypic Variance, and Phenotypic Standard Deviation, that measures Variability), greater is the possibility of S being large.

Heritability

• In crop improvement, only the genetic component of variation is important since only this component is transmitted to the next generation.
• The ratio of genetic variance to the total variance i.e. phenotypic variance is known as heritability.

H = Vg / Vp

Vp = Vg + Ve

• Where,
  • Vp = phenotypic variance
  • Vg = genotypic variance
  • Ve = environmental variance
• Heritability estimated from the above formula is known as the broad sense heritability. This is valid when homozygous lines are studied. But when segregating generations are studied genotypic variance consists of (a) additive variance (b) dominance variance (c) and variance due to epistasis.
• Dominance variance is important when we are dealing with hybrids i.e. F1 generations. In self-pollinated crops we release varieties only after making them homozygous lines. Hence additive variance is more important in such cases. The proportion of additive genetic variance to the total variance is known as narrow sence heritability.
• If heritability is very high for any character it can be improved. Improvement of characters with low heritability is very difficult.

Genetic Advance

• Genetic advance is the difference between the mean of the selected plants in the original population and the mean of the progeny raised from the selected plants in the next generation. It can be predicted by the following formula.
• Genetic advance (GA) = s P * H * K
  • K = selection intensity 2.06 when 5% of the population is selected
  • P = phenotypic standard deviation of the character in the population
  • H = heritability in broadsense

Mass Selection

• It is the earliest method of selection. Man has always practiced mass selection consciously or unconsciously from the time of domestication. In its most basic form mass selection consists of selecting individuals on the basis of phenotypic superiority and mixing the seeds for using as planting material for next season.

👉🏻 Procedure for evolving variety by mass selection

• First year: Large number of phenotypically similar plants having desirable characters are selected. The number may vary from few hundred to few thousand. The seeds from the selected plants are composited to raise the next generation.
• Second year: composited seed planted in a preliminary field trial along with standard checks. The variety from which the selection was made should also be included as check. Phenotypic characterlistics of the variety are critically examined and evaluated.
• Third to Sixth year: The variety is evaluated in coordinated yield trials at several locations. It is evaluated in an initial evaluation (IET) trial for one year. If found superior it is promoted to main yield trials for 2 or 3 years.
• Seventh year: if the variety is proved superior in main yield trials it is multiplied and released after giving a suitable name.

Modification of mass selection

• Mass selection is used for improving a local variety. Large number of plants are selected (I year) and individual plant progenies are raised (II year). Inferior, segregating progenies are reflected.
• Uniform, superior rows are selected and the seed is bulked.
• Preliminary yield trials are conducted in third year.
• Fourth to seventh year multilocation tests are conducted and seed is multiplied in eight year and distributed in ninth year.
• Many other modifications also are followed depending on the availability of time and purpose for which it is used.

✅ Merits of Mass selection

• Can be practiced both in self and cross pollinated crops.
• The varieties developed through mass selection are more widely adopted than pure lines.
• It retains considerable variability and hence further improvement is possible in future by selection.
• Helps in preservation of land races
• Useful for purification of pureline varieties
• Improvement of characters governed by few genes with high heritability is possible.
• Less time consuming and less expensive.

❌ Demerits of mass selection

• Varieties are not uniform
• Since no progeny test is done, the genotype of the selected plant is not known
• Since selection is based on phenotype and no control over pollination the improvement brought about is not permanent. Hence, the process of mass selection has to be repeated not and then.
• Characters which are governed by large number of genes with low heritability cannot be improved.
• It cannot create any new genotype but utilizes existing genetic variability.

🏆 Achievements

• Mass selection must have been used by pre historic man to develop present day cultivated cross from their wild parents.
• It was also used extensively before pureline selection came into existence.
• Cotton: Dharwad American Cotto
• Groundnut: TMV-1 & TMV-2
• Pearl Millet: Pusa moti, Baja puri, Jamnagar gaint, AF3
• Sorghum: R.S. 1
• Rice: SLO 13, MTU-15
• Potato: K122

• Selection is basic to any crop improvement.
• Isolation of desirable plant types from the population is known as selection.
• It is one of the two fundamental steps of any breeding programme viz.,
  • Creation of variation
  • Selection
• There are two agencies involved in carrying out selection: One is Nature itself (Natural selection) and the other is artificial selection.
• Though both may complement each other in some cases, they are mostly opposite in direction since their aims are different under the two conditions (nature and domestication). The effectiveness of selection primarily depends upon the degree to which phenotype reflects the genotype.
• Before domestication, crop species were subjected to natural selection. The basic for natural selection was adaptation to the prevailing …

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