GENETIC DRIFT

Published in Zoology
Sunday, 11 June 2017 10:49
Genetic drift as one of the evolutionary force
 
The genetic architecture of small population changes irrespective of elective advantage or disadvantage. Analogously genes attain Hardy-Weinberg’s equilibrium in large populations only. The random changes in gene frequencies occurring by chance and not under the control of natural selection are called genetic drift. A series of steps and at each step the movement made is random, in directed which Is known as stochastic process.

The theory of genetic drift was developed by a geneticist SE WALL WRIGHT in 1930. It is also known as Sewall Wright effect or ‘scattering of variability’. It denotes that the random fluctuations in the gene frequencies In a small population from generation to generation.

In demes of limited sizes, random genetic drifts arise by chance. These cannot arise in large population. For example if we compare two populations of two extreme sizes - Population ‘A’ consisting of 5,000 breeding individuals and population ‘B’ of only 50, the gene pool of each contains equal number of Land I. If their gene frequencies are represented by p & q
 
pL= qI -0.5

In the next generation, the gene frequency is expected to deviate from the original 0. 5 by an amount of equal to the ‘standard error’
 
geneticdrift1
 
The standard error is determined as the sequence root of the product of original frequencies (p & q) divided by the number of genes available. This number of genes will be double the number of breeding organisms.
 
In small population B with organisms, the standard error would be J(o. 5 x 0.5 + 1oo) = 0.05. So in the next generation the gene frequencies will change to 0.45 and 0.55 either way. This amounts to 10% change in the gene frequency. Thus the standard error in a large population’s) + 10,000 = 0.0005) is negligible and it significantly high in a small population.

Effects of Genetic Drift on Gene frequency
 
In small populations or demes, the genetic drifts have the following, effects on the gene-frequency.

i) Homozygocity: In small populations, due to genetic drift gene frequencies continue to fluctuate until one of the allele lost and other fixed. This leads homozygosity in small populations. It means the genetic drift reduces genetic variability by eliminating one of the two alleles ‘either new or old one.
 
ii) Fixation of new mutations: Since genetic drift tend to eliminate one allele and fix the other one, irrespective of its dominance or recessiveness or advantageous or non advantageous nature. So a new mutation has 50% chances of either being lost or be fixed in small population.

iii) Genetic divergence: The demes become progressively genetically different. In each sub population, the genes fixed and lost will be different. Thus, In due course of time, (each deme gradually diversifies from the other sister demes) lead to the establishment of new species.
 
geneticdrift21
 
Genetic Drift and Evolution:
 
The role played by the genetic drifts actually in the evolution of organisms in nature is doubtful. A widely ranging broad base population is isolated into small sub groups - ‘DEMES’. The causes for isolation may be either on account of ecological or geographical discontinuities, home instinct. The size of these small demes is such that they appear to be affected by chance of events underlying genetic drift. The limited size of small breeding populations, the gene pool of their new generations may not be the same of the parental gene pools due to the action of genetic drift. The changed gene pools gradually lead to the formation of new species.

Founder Effect: Whenever a few organisms from large population encroaches a new or isolated geographical region, these form the “founders or ‘founder members’. The founders carry only a limited portion of the parental gene pool. The descendants of the founder i.e. the founder population or marginal isolates in a new area will tend to have ratios similar to the founders. The resemblance of the descendants of the founders is called founders effect’ or ‘founder principle’ (Maw).

The diffusion of genes into populations through migrations and interbreeding is known as Gene flow. The gene flow links all the demes of a population. It tends to counteract the loss of variability due to genetic drift in small population.
 
Rh gene (r) was introduced into the Chinese population by American immigrants. This Rh factor is associated with erythroblastosis fetalis or hemolytic disease in new born.

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