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Evolution - Darwinian Theory and Modern Synthetic Theory

Grade 12CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Darwinian Theory of Evolution is founded on two core concepts: Branching Descent (the pattern of evolution) and Natural Selection (the mechanism of evolution).

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Natural Selection occurs because individuals with inherited traits that are better suited to the environment survive and reproduce at higher rates (FitnessFitness).

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Modern Synthetic Theory (Neo-Darwinism) explains evolution as a change in allele frequency within a population over time. It considers five factors: Gene Mutation, Genetic Recombination, Gene Flow, Genetic Drift, and Natural Selection.

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Hugo de Vries proposed the Mutation Theory, stating that evolution is caused by sudden, large, and random mutations rather than the small, directional variations described by Darwin. He termed a single-step large mutation as 'Saltation'.

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Adaptive Radiation: The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats), exemplified by Darwin's Finches and Australian Marsupials.

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Hardy-Weinberg Principle: Allele frequencies in a population are stable and is constant from generation to generation (Genetic Equilibrium) if factors like mutation, gene flow, and natural selection are absent.

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Genetic Drift: A change in allele frequencies due to chance events, particularly in small populations. Extreme cases include the Founder Effect and the Bottleneck Effect.

📐Formulae

p+q=1p + q = 1

p2+2pq+q2=1p^2 + 2pq + q^2 = 1

Frequency of homozygous dominant (AA)=p2\text{Frequency of homozygous dominant (AA)} = p^2

Frequency of heterozygous (Aa)=2pq\text{Frequency of heterozygous (Aa)} = 2pq

Frequency of homozygous recessive (aa)=q2\text{Frequency of homozygous recessive (aa)} = q^2

💡Examples

Problem 1:

In a stable population of 10001000 individuals, 160160 individuals show the recessive phenotype (genotype aaaa). Calculate the frequency of the dominant allele AA and the frequency of the heterozygous genotype AaAa.

Solution:

  1. Frequency of homozygous recessive genotype q2=1601000=0.16q^2 = \frac{160}{1000} = 0.16.
  2. Frequency of recessive allele q=0.16=0.4q = \sqrt{0.16} = 0.4.
  3. Using p+q=1p + q = 1, the frequency of dominant allele p=1−0.4=0.6p = 1 - 0.4 = 0.6.
  4. Frequency of heterozygous genotype 2pq=2×0.6×0.4=0.482pq = 2 \times 0.6 \times 0.4 = 0.48.

Explanation:

Applying the Hardy-Weinberg equilibrium equations where pp represents the frequency of the dominant allele and qq represents the frequency of the recessive allele.

Problem 2:

Contrast Darwinian variations and de Vriesian mutations.

Solution:

Darwinian variations are small, continuous, and directional (gradualismgradualism), leading to slow evolution. De Vriesian mutations are large, random, and directionless (saltationsaltation), leading to sudden speciation.

Explanation:

This highlights the difference between the classical Darwinian view and the Modern Synthetic/Mutation theory regarding the speed and nature of evolutionary change.