IGCSE Study Buddy: Variation and Selection (Chapter 18 Part 1)

Introduction

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Key Concepts of Variation

Definition of Variation

• Variation: Differences between individuals of the same species.
  • Can be genetic and/or phenotypic.

Types of Variation

1. Genetic Variation

   • Differences in genetics among individuals.
   • Examples: Blood group, eye color, gender, ability to roll the tongue.

2. Phenotypic Variation

   • Variability of observable features (phenotypes) within a population.
   • Can be caused by:
     • Genetic factors (controlled by genes).
     • Environmental factors (influenced by the environment).
       • Examples of environmental factors: Climate, diet, lifestyle.
       • Example: Difference in body type due to activity levels.

Causes of Genetic Variation

• Factors that cause genetic variation:
  • Mutation
  • Meiosis
  • Random mating
  • Random fertilization

Types of Variation: Continuous vs Discontinuous

Continuous Variation

• Traits can have many different values within a group.
  • Example: Height (spectrum of short to tall).
  • Characteristics: Measured on a scale (mass, height).
  • Graphs show smooth bell curves.
  • Caused by both genes and the environment.
  • Example: Siblings with genetic predisposition for height influenced by nutrition.

Discontinuous Variation

• Limited number of phenotypes with no intermediates.
  • Examples: ABO blood groups, seed shape and color in peas.
  • Characteristics: Specific, distinct options (e.g., blood types A, B, AB, O).
  • Graphs show step-like shape.
  • Generally caused by genes only.

Mutation

• Mutation: Random genetic change in DNA base sequence.
  • Causes:
    • Ionizing radiation (X-rays, gamma rays).
    • Exposure to chemicals (e.g., tobacco tar).
  • Mutations lead to new alleles.
  • Example: Mutation in rabbit fur color aiding in camouflage and survival.

Adaptive Features

• Adaptive Feature: Inherited feature aiding survival and reproduction.

Hydrophytes

• Plants adapted to grow in water (e.g., ponds, lakes).
  • Wide and flat leaves for sunlight absorption.
  • Small roots for direct water absorption.
  • Stomata on the upper side for gas exchange.

Xerophytes

• Plants adapted to survive in dry environments.
  • Thick waxy cuticle to prevent water loss.
  • Stomata in sunken pits to minimize water loss.
  • Small, needle-shaped leaves to reduce evaporation.
  • Extensive roots for deep water access.
  • Thick stems/leaves for water storage.

Conclusion

• Part 1 of Chapter 18 covers variation and selection.
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