18.1 Variation

2026 Syllabus Objectives

  1. Describe variation as differences between individuals of the same species
  2. State that continuous variation results in a range of phenotypes between two extremes; examples include body length and body mass
  3. State that discontinuous variation results in a limited number of phenotypes with no intermediates; examples include ABO blood groups, seed shape in peas and seed colour in peas
  4. State that discontinuous variation is usually caused by genes only and continuous variation is caused by both genes and the environment
  5. Investigate and describe examples of continuous and discontinuous variation
  6. Describe mutation as genetic change
  7. State that mutation is the way in which new alleles are formed
  8. State that ionising radiation and some chemicals increase the rate of mutation
  9. Describe gene mutation as a random change in the base sequence of DNA
  10. State that mutation, meiosis, random mating and random fertilisation are sources of genetic variation in populations

What is Variation? 🔑

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Variation refers to the differences between individuals of the same species. When you look around any group of organisms - whether humans in a classroom or horses in a field - you will notice that no two individuals are exactly alike.

These differences can be observed in many characteristics:

  • Physical features (height, hair type, coat colour)
  • Physiological features (blood groups, metabolic rates)
  • Behavioural traits (sociability, athletic ability)

Key Definition: Variation is the differences between the individuals of the same species.

Example: Mimicry in Butterflies

The common mormon butterfly (Papilio polytes) demonstrates remarkable variation within a single species. Male butterflies always have the same appearance - black wings with white spots. However, female butterflies show two distinct forms:

  • Standard form: Looks identical to the male
  • Stichius form: Has elongated hindwings with white and red/pink markings

The stichius form is an example of mimicry - it resembles the poisonous common rose swallowtail butterfly. This mimicry provides protection from predators, who have learned to avoid the genuinely poisonous species. This survival advantage only works when the stichius form is found in the same locations as the common rose swallowtail.

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