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Home/Anthropology/Mains PYQ/2020/Paper I/Human Evolution & Primatology
UPSC MainsAnthropology Optional Paper I 2020 15 Marks Model Answer Available

Darwin, Post-Darwin and the Synthetic Theory — 2020 Paper I

Question · 2020 · Paper I · 15 Marks

Elucidate how Darwin and Post-Darwin theories of evolution resulted in the development of Synthetic theory of evolution.

Model Answer

VAID ICS

Approach

  • Demand of Question: Trace how Darwinian natural selection and later developments in genetics and population biology converged into the Synthetic Theory.
  • Structuring the Response: Explain Darwin’s contribution, its limitations, post-Darwinian advances, and their integration in the Modern Evolutionary Synthesis.
  • Key Dimensions to Cover: Darwin, Mendel, mutation, Weismann, Hardy-Weinberg, Fisher, Haldane, Wright, Dobzhansky, Mayr and natural selection.

Model Answer

Introduction

Darwin’s theory of evolution by natural selection provided the basic mechanism of adaptive evolutionary change, but lacked a satisfactory theory of heredity. The rediscovery of Mendelian genetics and subsequent developments in population genetics eventually united heredity and natural selection into the Synthetic Theory or Modern Evolutionary Synthesis.

Darwinian foundation

In On the Origin of Species (1859), Charles Darwin proposed:

  1. organisms show variation;
  2. more offspring are produced than can survive;
  3. individuals compete for limited resources;
  4. favourable heritable variations enhance reproductive success;
  5. natural selection gradually alters populations.

Thus:

Variation → Struggle → Differential reproduction → Natural selection → Evolution

Limitations of Darwinism

Darwin could not satisfactorily explain:

  • the mechanism of heredity;
  • origin of variation;
  • maintenance of variation;
  • quantitative changes in gene frequencies.

His concept of pangenesis failed to provide a valid genetic mechanism.

Post-Darwinian developments

  1. Weismann: August Weismann’s germ-plasm theory distinguished germ cells from somatic cells and challenged inheritance of acquired characters.
  2. Rediscovery of Mendel: Around 1900, Mendel’s principles established particulate inheritance through discrete hereditary units.
  3. Mutation theory: Hugo de Vries emphasized mutations as a source of discontinuous variation, initially presenting mutation as an alternative to gradual Darwinian selection.

Population genetics

The crucial synthesis occurred when Mendelian inheritance was expressed mathematically at the population level.

Hardy-Weinberg principle established the baseline against which evolutionary changes in allele frequencies could be measured.

R.A. Fisher: Demonstrated compatibility between Mendelian inheritance and continuous variation and emphasized natural selection.

J.B.S. Haldane: Mathematically analysed changes in gene frequencies under selection.

Sewall Wright: Emphasized genetic drift, population structure and interaction between selection and chance.

Thus:

Mendelian genetics + Natural selection + Population genetics = Evolution as change in gene frequency

Emergence of the Synthetic Theory

During the 1930s–1940s, several disciplines were integrated.

  • Theodosius Dobzhansky: Linked genetics with natural populations in Genetics and the Origin of Species (1937).
  • Ernst Mayr: Integrated systematics, geographical variation and speciation.
  • George Gaylord Simpson: Incorporated palaeontology.
  • G.L. Stebbins: Integrated plant evolution.

Major propositions

The Synthetic Theory explains evolution through changes in allele frequencies produced by:

  1. mutation — source of new variation;
  2. recombination — creates new genetic combinations;
  3. natural selection — differential reproductive success;
  4. genetic drift — random frequency changes;
  5. gene flow — movement of alleles between populations;
  6. reproductive isolation — facilitates speciation.

Synthesis

Darwinian selection explained which variations are favoured, while Mendelian genetics explained how variations are inherited.

Together:

Variation → Genetic inheritance → Population processes → Selection/isolation → Speciation

Conclusion

The Synthetic Theory emerged by integrating Darwinian natural selection with Mendelian genetics, mutation theory and population genetics, transforming evolution from a mainly descriptive theory of adaptation into a genetic theory of changing populations.

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