Breeding of Vegetable, Spice and Tuber crops (2+1)

Factors affecting the equilibrium in populations

Migration

• Migration is the movement of individuals into a population from a different population. Migration may have introduced new alleles into the population or may change the frequencies of existing alleles. In plant breeding programmes, migration is represented by intervarietal crosses, polycrosses etc., wherein the breeder brings together into a single population two or more separate populations.

Mutation

• Mutation is a sudden and heritable change in an organism and is generally due to a structural change in a gene. Mutation may produce a new allele not present in the population or may change the frequencies of existing alleles.

Random Drift

• Random drift or genetic drift is a random change in gene frequency due to sampling error. Random drift occurs in small population because sampling error is greater in a smaller population than in a larger one.
  
• Ultimately, the frequency of one of the alleles becomes zero and that of the other allele becomes one.
• The allele with the frequency of one is said to be fixed in the population because there would be no further change in its frequency.
• It may be expected that in a small population all the genes would become homozygous, or would be fixed in due course of time. Breeding populations are generally small, hence a certain amount of genetic drift is found to occur in them.
  
• The breeder cannot do anything to prevent this genetic drift, except to use very large populations, which is often not practicable. Alternatively, he may resort to phenotypic disassortative mating, which would again require time, labour and money.

Inbreeding

• Mating between individuals sharing a common parent in their ancestry is known as inbreeding.
  
• In small populations, a certain amount of inbreeding is found to occur.
  
• Inbreeding reduces the proportion of heterozygotes or heterozygosity and increases the frequency of homozygotes or homozygosity.

Selection

• Differential reproduction rates of various genetopes are known as selection.
  
• In crop improvement, selection is very important because it allows the selected genotypes to reproduce, while the undesirable genotypes are eliminated.
  
• In a random mating population, if plants with AA or aa genotypes are selected, the frequency of A allele in the selected of population would be 1 or 0, respectively.
• Thus selection in a random mating population is highly effective in increasing or decreasing the frequency of alleles, but it is unable to either fix or eliminate them.
• However, in combination with a system of inbreeding, selection is highly efficient in the fixation and elimination of alleles.