Natural selection is the force which ensures the differential contribution of alleles of gene to a gene pool, in other way we can say that natural selection determines the favourable individuals that can contribute more offsprings to the next generation into a gene pool to provide ultimate efficient adaptive individual in relationship with thier environment.
Concept of Fitness :
Fitness is the measure of the natural selection. The degree of fitness is a measure of reproductive efficiency of a character. Fitness also defined the favourable adaptive character will be produced in next generation in relation with environmental obstacles.
Selection Co-efficient :
Selection Co-efficient is the measure of the intensity of the selection force acting against unfavorable or inefficient genotypes in a heterozygous population.
In another sense selection co-efficient can be known as difference between relative fitness of different alleles or genotypes.
W = 1 – s
s = 1 – W
Here , W = Fitness and s = Selection co-efficient.
Derivation for one unit of dominant allele;
Suppose one offspring produced by dominant allele then 1-s offspring produced by recessive one ( homozygous stage ). This is called co-efficient of selection against the recessive and designated by letter ‘s’ . It is a positive number and ranging between ‘ 0 to 1 ‘.
Genetic Load :
The genetic variability is the main reason of advantages and disadvantages in a population, where natural selection play a key role to promote the survival of fit organism among other recessive ( homozygous ). This genetic variability create the genetic load, it means due to variability of fitness there is a loss of unfit genes and result in a difference between the fitness of an expected optimal genotype with the fitness of observed average genotype.
Simply we can understand that in some environmental conditions due to natural selection process there will be a factors which always put pressure or load upon an existing genes and finally a gene survive and another one get perish and it’s diffrence indicates the comparison between two variable genes based on fitness. Thus genetic load is the difference between the actual fitness and an observed fitness.
It was studied that the population with high genetic load will be in danger of going extinct as the organism are unable to survive further to produce viable offsprings.
Example : – If a gene is deleterious in homozygous state the frequency of homozygotes before and after selection will be as follows;
AA Aa aa
Frequency (fertilization) – P2 2pq q2
Fitness – 1 1 (1 – s)
Frequency (After selection)-p2 2pq q2- sq2
Here the uneven genetic load by individual with aa genotype is sq2 . If there were 100 individuals in the population before selection then 100sq2 are estimated due to genetic inviability.