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Home/Anthropology/Mains PYQ/2018/Paper I/Human Genetics & Human Biology
UPSC MainsAnthropology Optional Paper I 2018 20 Marks Model Answer Available

Mechanisms of Human Variation in Gene Frequencies — 2018 Paper I

Question · 2018 · Paper I · 20 Marks

Explain the mechanisms of human variation in gene frequencies.

Model Answer

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Approach

  • Demand of Question: Explain the evolutionary mechanisms through which allele frequencies differ and change among human populations.
  • Structuring the Response: Begin with Hardy-Weinberg equilibrium as the baseline, then discuss mutation, selection, drift, gene flow and mating-population structure.
  • Key Dimensions to Cover: Mutation, natural selection, genetic drift, founder effect, bottleneck, gene flow, endogamy, HbS-malaria and population history.

Model Answer

Introduction: Human populations differ in the frequencies of genetic variants because allele frequencies are continuously shaped by mutation, natural selection, genetic drift, gene flow and demographic structure. The Hardy-Weinberg principle provides the null model against which these evolutionary changes can be understood.

Under ideal conditions:

p² + 2pq + q² = 1

where p and q represent frequencies of two alleles.

Departures from these conditions generate evolutionary change.

  1. Mutation

Mutation is the ultimate source of new alleles.

It may involve:

  • nucleotide substitutions;
  • insertions and deletions;
  • gene duplication;
  • chromosomal changes.

Mutation rates are generally low, so mutation alone produces slow changes in allele frequency.

However:

Mutation → New variation → Raw material for evolution

The fate of the new allele is subsequently determined by selection, drift and migration.

  1. Natural selection

Natural selection produces differential survival and reproduction among genotypes.

Positive selection: Favours an advantageous allele, increasing its frequency.

Purifying selection: Removes deleterious variants.

Balancing selection: Maintains more than one allele in a population.

A classic human example is the HbS allele in malaria-endemic regions.

  • HbAA → greater susceptibility to severe falciparum malaria
  • HbSS → sickle-cell disease
  • HbAS → relative protection from severe malaria

Thus:

Malarial environment → Heterozygote advantage → Maintenance of HbA and HbS

This demonstrates interaction between gene frequency and environment.

  1. Genetic drift

Genetic drift refers to random changes in allele frequencies, particularly important in small populations.

Its effects include:

  • loss of alleles;
  • fixation of alleles;
  • increased differentiation between isolated populations.

Two important forms are:

Founder effect: A new population established by a small number of individuals carries only a sample of the original gene pool.

Population bottleneck: Sudden demographic reduction alters allele frequencies because surviving individuals constitute a non-random sample of earlier variation.

Thus:

Small population size → Sampling error → Rapid allele-frequency change

Drift has been particularly important in small, isolated or highly endogamous human populations.

  1. Gene flow

Gene flow occurs when migrants reproduce in another population and introduce their alleles into its gene pool.

It generally:

  • increases variation within populations;
  • reduces genetic differences between populations.

Thus:

Migration + Interbreeding → Transfer of alleles → Genetic convergence

Human migrations throughout prehistory and history have produced extensive admixture, making human biological boundaries highly permeable.

  1. Population isolation and endogamy

Geographical, linguistic, religious or caste-based barriers may restrict gene flow.

Endogamy → Restricted mating network → Greater population differentiation

Endogamy itself primarily alters mating patterns, but when combined with small population size, founder effects and drift, it can strongly influence allele frequencies.

This is particularly relevant to many historically endogamous caste and tribal populations of India.

  1. Non-random mating

Assortative mating and inbreeding primarily change genotype frequencies, especially homozygosity.

For example:

Consanguinity → Increased homozygosity → Greater expression of recessive alleles

Its long-term interaction with selection and population subdivision can contribute indirectly to population differentiation.

  1. Demographic history

Population:

  • expansions;
  • contractions;
  • migrations;
  • admixture;
  • isolation

also leave strong signatures in present-day gene frequencies.

Hence contemporary variation cannot be explained by selection alone.

Integrated mechanism

Mutation → Creates variation
Selection → Sorts variation adaptively
Drift → Changes frequencies randomly
Gene flow → Redistributes variation

Conclusion

Human variation in gene frequencies results from the interaction of evolutionary forces with population size, migration, mating patterns and environmental pressures. Consequently, human biological diversity is dynamic, clinal and population-based rather than divisible into fixed biological races.

2026 Genetic polymorphism and natural selection. 2026 Numerical chromosomal aberrations can be due to abnormalities in autosomes as well as sex chromosomes. Elaborate in terms of the effects these have on human body, giving suitable examples. 2026 Distinguishing features of nuclear and mitochondrial DNA. 2026 How would you find out that a particular disease/disorder is inherited and, if so, in which manner? Elaborate with suitable examples.

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