Natural selection, Mutation, and Migration all of these directional forces causes change in gene frequency of variants. But apart from these directional forces there is one non directional force called genetic drift that controls and change the gene frequency in a population. Genetic drift eliminate the particular allele from a population or causes one allele to be present only in population. This happened in variable or random sampling of population
Genetic drift occur in small population. If the population is large the effect of genetic drift is not noticeable but in real the populations are smaller , the genetic drift is visible as it ill cause gene frequency changes because of sampliin error. If the number of prents is considerably large and no directional forces are acting on population there is a great chance of obtaining a good sample of genes of previous generation without any change in the frequency of alleles.
But in small population the genetic drift causes deviation of frequencies from original frequencies in previous population.
Mathematical expression for deviation.σ=√pq/N The extent of population deviation is measured by this equation here p is the frequency of one allele, q is the frequency of other allele N is the number of genes sampled For diploid parents each carrying two alleles σ=√pq/2N where N is the number of actual parents For a large population, the value of fluctuation ranges from 0.5 +/- 0.005 if we consider 5000 parents generation a population and p and q will be equal to 0.5 For a small population, if we consider 2 parents N=4 then the deviation of variants will be given as 0.50 +/- 0.25 In other words sampling accidents because of smaller population size can yield gene frequencies that depart considerably from the initial 0.5 values in a single generation. If the population is small and we began from the above mention extremes from 0.24 or 0.75 for a particular allele the frequency of that allele may fall to almost zero or increase almost to 1.
Rate of fixation in small population the chances of a particular allele increases to reach fixation( to eliminate or remain only in population). The proportion of such population to attain fixation is called the rate of fixation and is given as 1/2N. If N is large the fixation will be slow and vice versa.
The Ne is called the Effective population size. It is different from observed population size because not all members of population are necessarily parents.
Buri experiment on Drosophila Melanogaster