GENETIC DRIFT

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Department of Zoology Bhavya VashishtKurukshetra University M.Sc. Zoology (P)Kurukshetra Roll No. 4 Semester - II

CONTENTS Introduction to Genetic Drift Theory of Genetic Drift Genetic Basis of Random Genetic Drift Fixation of Allele in Small Population Effective Population Size Effect on Gene Drift on Gene Frequency Genetic Drift and Evolution Role of Genetic Drift in Small Population Experimental Evidence to Demonstrate Role of Genetic

Drift Bottleneck Effect Founder Effect Consanguinity and Genetic Drift Conclusion

INTRODUCTION TO GENETIC DRIFT

Genetic Drift is random changes in gene frequencies occurring by chance and not under the control of natural selection.

These deviations are non directional, i.e., they are not under the effect of natural selection.

One of the reasons for these deviations in the gene frequency in small population is supposed to be sampling error.

THEORY OF GENETIC DRIFTGiven by Sewall Wright in 1930.Salient features of this theory are: Genetic drift is an evolutionary force

operating in small populations. Gene frequency in small populations

changes by chance because of sampling errors.

In small populations some genes may be lost or reduced and others may increase by sheer chance irrespective of their selective advantage or disadvantage.

By genetic drift a new mutation arising in small populations may either get fixed or lost irrespective of its adaptive value.

In small population heterozygosity tends to change to homozygosity by chance.

Genetic drifts may fix some non adaptive trait in small populations.

Genetic drifts tend to preserve or eliminate genes without distinction.

Isolated small populations of a large population come to posses some unusual characteristics not shown by large parental population.

GENETIC BASIS OF GENETIC DRIFT

In demes of limited size, random genetic drifts arise by chance deviations, which cannot arise in a large population

Standard deviation (σ)= √ (p*q /2N) Standard error in small population is

large as compared to large population. This means when population size is

small, there is a very large drift in gene frequencies.

FIXATION OF AN ALLELE IN SMALL POPULATION

Repetition of similar standard deviation may result in an insignificant change

It gradually alter the genotype such that one gene is fixed and other is lost.

Now the population becomes homozygous from the previous heterozygous condition.

A large population is divided into small demes. So a particular allele may get fixed in one population and it may be absent in another.

If p=q=0.550% of lines fixes one allele and rest 50% fixes second allele

If p=0.7 and q=0.370 % lines fixes one allele and 30 % lines fixes second allele

TIME REQUIRED FOR FIXATION OF AN ALLELE

Time (t) = (1/p*N) or (1/q*N)t= time p or q = gene frequenciesN= deme sizeExample:In demes with ten breeding individuals having

p=q=0.5t = (1/0.5*10)t = 20It would take about 20 generations to fix half of them.

EFFECTIVE POPULATION SIZE It represents number of function parents

in the population.

The sexually immature, sterile and old individuals are excluded

Example: If out of 1000 individuals there are 300 mating parents, the effective population size will be 600.

Effective population size is also limited by distribution of sexes in the population.

Example: If there are 300 females and only 3 males, the effective size of population will be less than 303 but more than 6.

This relationship has been expressed by Wright as follows:

Ne = 4NfNm / (Nf+Nm)

Where Nf is the number of female parents and Nm is the number of female parents.

EFFECT OF GENE DRIFT ON GENE FREQUENCY

Homozygosity and fixation of one and elimination of other gene

Fixation of new mutation Genetic divergence

GENETIC DRIFT AND EVOLUTIONThe role of genetic drift in evolution is explained because of following reasons-

In nature – Most breeding populations of animals are usually small. Even a widely ranging population is isolated into small

subgroups called demes. Seasonal, annual or cyclical fluctuations are observed

in population size in many species.

ROLE OF GENETIC DRIFT IN SMALL POPULATION

Thus genetic drift in small isolated populations leads to:

Variations among population or demes by fixing or eliminating certain alleles.

Fixation or elimination of gene mutations. Fixation of unfavorable, neutral or favorable

characteristics in populations. Establishing reproductive isolation between

different demes of a large population and origin of new species.

EXPERIMENTAL EVIDENCE TO DEMONSTRATE ROLE OF GENETIC DRIFT

Lamotte studied frequency of banded and bandless snails.

He grouped colonies according to size , small – 500 – 1000, intermediate – 1,000 – 3000, large – over 3000 individuals

Gene B – bandless gene b – banded In large population – frequency of gene B is 1-3% In small population – frequency of gene B is 1-100 %

This indicates that in some population one allele is fixed and its alternative allele is fixed in another.

BOTTLENECK EFFECTYearly or seasonal phenomenon of cyclic fluctuation in population density causing periodic squeezing of some of the genes in a gene pool in random fashion is called bottle neck phenomenon.

FOUNDER EFFECT

When a few individuals or a small group of individuals from some large population invades a new or isolated geographical region, these become the founders or founder members. These founders carry only a limited portion of parental gene pool. Their gene pool may contain certain alleles in a very low frequency or may lack a few alleles.

CONSANGUINITY AND GENETIC DRIFT

Small population provides opportunity for genetic drift as well as for consanguinity. Similarity between consanguinity and genetic drift lies in the end result that both lead to homozygosity. This may lead to appearance of rare recessive trait.

CONCLUSION

Genetic drift is applicable in small population and as a result of genetic drift one allele is fixed in one population and the another allele is fixed in another population. This leads to speciation. Also the heterozygous condition present in ancestral forms finally gets converted into homozygous form leading to the loss of an allele.

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