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Cambridge IGCSE Biology · 0610

Chapter 18: Variation and selection (Part 1)

Variation

Definition
Differences between individuals of the same species.
Phenotypic variation
Can be caused by both genetic and environmental factors (e.g., an organism’s potential height is genetic, but nutrients from the environment determine if it is reached).
Genetic variation
Sources of genetic variation in populations include mutation, meiosis, random mating, and random fertilisation.

Exam Traps

  • Do not say all variation is genetic — environment also affects phenotype.

Continuous and discontinuous variation

Continuous variation
Results in a range of phenotypes between two extremes (e.g., body length and body mass). It is caused by both genes and the environment.
Discontinuous variation
Results in a limited number of phenotypes with no intermediates (e.g., ABO blood groups, seed shape/colour in peas). It is usually caused by genes only.

Exam Traps

  • Do not say body mass is discontinuous — it is continuous.
  • Avoid claiming discontinuous variation is always affected by the environment.

Mutation

Definition
A genetic change.
New alleles
Mutation is the way in which new alleles are formed.
Gene mutation
A random change in the base sequence of DNA.
Increased rate
The rate of mutation is increased by exposure to ionising radiation and some chemicals.

Exam Traps

  • Do not say all mutations are harmful or always beneficial — effects vary.
  • Avoid confusing mutation (source of new alleles) with natural selection (acts on existing variation).

Adaptive features

Definition
An inherited feature that helps an organism to survive and reproduce in its environment.
Fitness
The ability of an organism to survive and reproduce in its environment.

Exam Traps

  • Avoid using “fitness” in the everyday sense of exercise — in biology it means reproductive success.

Adaptations to extreme environments

Xerophytes
Plants adapted to dry climates (e.g., cacti).
  • Features: Thick waxy cuticle (waterproof barrier), sunken stomata (trap moist air), fewer stomata, and small, rolled leaves or spines to reduce surface area for water loss.
Hydrophytes
Plants adapted to very wet conditions (e.g., water lilies).
  • Features: Large, flat leaves (promote water loss), stomata on the upper surface, and small root systems as water is readily available.
Bell curve for continuous variation and bar chart for discontinuous variation
Diagram 1: Continuous vs. discontinuous variation. A bell-shaped normal distribution curve for continuous variation (e.g., height) and a bar chart with distinct, non-touching columns for discontinuous variation (e.g., blood groups).

Exam Traps

  • Do not give hydrophyte features (stomata on upper leaf surface) for xerophytes.
  • Avoid saying sunken stomata increase transpiration — they trap moist air and reduce water loss.

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