Variation

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Explain, with examples, that phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and environmental factors
  2. Explain what is meant by discontinuous variation and continuous variation
  3. Explain the genetic basis of discontinuous variation and continuous variation
  4. Use the t-test to compare the means of two different samples

1. What is Variation?

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Variation refers to the differences between individuals of the same species. When you look around your classroom, you'll notice that no two students are exactly the same - some are taller, some have different eye colors, some have different blood types. These differences are called variation.

More specifically, phenotypic variation means differences in the observable characteristics (features you can see or measure) of organisms. Your phenotype is everything about you that can be observed - your height, weight, eye color, blood group, and so on.

The Causes of Phenotypic Variation

Phenotypic variation can be caused by three things:

1. Genetic Factors Only

These are differences caused purely by the genes you inherit from your parents. The environment has no effect on these characteristics.

Example: ABO Blood Groups

  • Your blood group (A, B, AB, or O) is determined entirely by your genes
  • You inherit two alleles (versions of the gene) for blood group - one from each parent
  • There are three possible alleles: I^A, I^B, and I^O
  • No matter what you eat, where you live, or what you do, your blood group will never change
  • This is a genetic factor because it's controlled only by your DNA

Example: Eye Color

  • The color of your eyes is determined by genes inherited from your parents
  • Environmental factors like light exposure do not change your eye color

2. Environmental Factors Only

These are differences caused purely by the surroundings and conditions an organism experiences. Genes play no role in these variations.

Example: Scars or Tattoos

  • If you get a scar from an injury or a tattoo, these are changes to your body caused by the environment (an accident or a choice you made)
  • These are not in your DNA and cannot be passed to your children
  • Two people with identical DNA (like identical twins) can have different scars

Example: Language Spoken

  • The language you speak is learned from your environment, not inherited genetically
  • A child born to Japanese parents but raised in England will speak English, not Japanese

3. Combination of Genetic AND Environmental Factors

Most characteristics are actually influenced by both your genes and your environment working together. This is the most common situation.

Example: Height

  • Your genes set a potential range for how tall you can grow
  • But whether you reach your full potential height depends on environmental factors like:
    • Nutrition: If you don't get enough protein, vitamins, and minerals during childhood, you won't grow as tall as your genes would allow
    • Disease: Serious illnesses during childhood can stunt growth
    • Exercise: Regular physical activity can help you reach your genetic potential
  • Two people with genes for being tall might end up different heights if one had poor nutrition

Example: Skin Color in Response to Sun Exposure

  • Your genetic makeup determines your base skin color
  • However, sun exposure (environmental factor) can darken your skin through tanning
  • Someone with genes for pale skin will still be paler than someone with genes for dark skin, even if they both tan

Example: Sickle Cell Trait and Malaria

  • People who are heterozygous for the sickle cell allele (Hb^A Hb^S) have some sickle-shaped red blood cells
  • In areas where malaria is common (environmental factor), this genotype provides an advantage
  • These individuals are less likely to die from malaria complications
  • This is why the sickle cell allele remains common in malaria-endemic regions - the environment affects which genotype survives better

The Variation Formula

We can summarize this relationship with a simple formula:

Phenotype = Genotype + Environment

Or written another way:

V_P = V_G + V_E

Where:

  • V_P = Phenotypic variation (the differences we can observe)
  • V_G = Genetic variation (differences due to genes)
  • V_E = Environmental variation (differences due to environment)

This formula tells us that the characteristics we observe in an organism result from both its genetic makeup and the environmental conditions it experiences.

More Examples of Environmental Effects

Hydrangea Flower Color

  • The same hydrangea plant with identical genes can produce different colored flowers depending on soil pH
  • In acidic soil: flowers turn blue
  • In alkaline soil: flowers turn pink or red
  • This happens because soil pH affects the availability of aluminum ions, which affects pigment production

Himalayan Rabbit Fur Color

  • These rabbits have a genotype that makes darker pigment at lower temperatures
  • Their ears, nose, paws, and tail are darker because these areas lose more heat (they have a high surface area to volume ratio)
  • The rest of their body stays warmer and remains lighter in color
  • If you apply an ice pack to a light area of their fur, that spot will grow dark fur - proving it's temperature (environment), not just genes

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