Sunday, March 02, 2008

Basic Concepts of Selection in Plant Breeding


Generally breeders in two ways can change variability in their working population. Firstly by allowing individuals to produce next generation within limits of breeders preference or by natural choice in which which some of these individuals may be allowed to produce many offsprings and some of which may be allowed to produce which few or no offsprings. This non-random reproduction in a breeding population is called selection. The two agencies involved in the selection, one is by the natural choice is called natural selection and the other that is done by the breeder is the artificial selection. Secondly, breeders can decide how the selected individuals will be mated to each other either by inbreeding and cross-breeding.

Following basic principles operates on selection:

1. Selection works only on heritable variation
Variation that is governed by genetic makeup of the individual alone can be recovered in the progenies, not the variation that is environmentally dependent.

2. Selection works only in existing variability ie., it cannot create new variation. 
Hence, selection can only shuffle the variation and also can combine variations of different individuals of the population, which is however, pre-exist within the population itself.

3. Selection works by providing preference to certain individuals to reproduce, therefore certain set of genes get preferential inheritance at the expense of the unselected genes. Asa consequence gene and genotypic frequency  get altered. 

4. Continuous selection leads to loss of variability, because unselected genes get eliminated in the course of generation advancement.

5. When a new variability is created in a population either by mutation or by movement of genes from outside (from another unrelated population), new selection pressure is generated on the introduced genes.

6. Purpose and consequence of selection varies from type of selection, individual genes, mating methods etc.

Plant breeding employs artificial selection to generate plant populations that are cultivated for the benefit of the mankind. Artificial selection is fundamentally aimed at the following facts:

1. To change the phenotype of the selected individual in the most desired way to the man
2. To bring down the variability in the selected population
3. To increase uniformity in the selected population
4. To increase number of populations with different level of variability between them
5. To increase the range of uniform populations

Various artificial selection methods used in plant breeding are, 

In self pollinated crops,
1. Mass selection
2. Pureline selection
3. Pedigree selection

In cross pollinated crops,
1. Mass selection
2. Progeny selection (ear to row methods, modified ear to row methods etc)
3. Family selection (half sib, full sib family selection and their modifications)
4. Recurrent selection

Clonally propagated crops,  
1. Clonal selection.