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Evolution - Mechanism of Evolution (Variation and Mutation)

Grade 12CBSEBiology

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

🔑Concepts

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According to Charles Darwin, evolution occurs through small, continuous variations that are directional and lead to gradual changes over generations.

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Hugo de Vries proposed the Mutation Theory based on his work on the evening primrose (OenotheraOenothera lamarckianalamarckiana), suggesting that mutations are the primary cause of evolution.

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Mutations are defined as sudden, random, and directionless changes in the genetic material (DNA).

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De Vries coined the term 'Saltation' to describe a single-step large mutation that can lead to the formation of a new species.

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In modern synthetic theory, variation is caused by multiple factors: gene mutations, chromosomal aberrations, genetic recombination during meiosis (crossingcrossing overover), and gene flow.

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While Darwinian variations are small and directional (leading to gradualism), mutations are large and random (leading to saltation).

📐Formulae

p+q=1p + q = 1

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

Mutation Rate=Number of mutationsTotal number of gene copies per generation\text{Mutation Rate} = \frac{\text{Number of mutations}}{\text{Total number of gene copies per generation}}

💡Examples

Problem 1:

Explain the molecular basis of the mutation that causes Sickle Cell Anemia as a mechanism of variation.

Solution:

A point mutation occurs in the β\beta-globin chain of hemoglobin. The triplet codon GAGGAG (coding for Glutamic acid) is mutated to GUGGUG (coding for Valine) at the 6th6^{th} position.

Explanation:

This single base substitution (A→UA \rightarrow U) results in a structural change in the HbHb molecule under low oxygen tension, leading to the sickling of RBCsRBCs. This represents a hereditary variation caused by mutation.

Problem 2:

Contrast Darwinian variation with Hugo de Vries' mutation concept in a population of size NN.

Solution:

Darwinian variation: Small, directional, and leads to gradualism. Mutation: Large, random, directionless, and leads to saltation.

Explanation:

Darwin believed that evolution was a slow, continuous process, whereas De Vries believed that a single large mutation could cause speciation (SaltationSaltation) instantly without intermediates.