# Variation

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u18-variation/

Variation describes differences in traits between individuals of the same species. It is the foundational raw material for natural selection and evolution. This guide covers key types, causes, and the evolutionary role of variation for CIE 9700 exams.

**Prerequisites:** [Basic Mendelian inheritance and genetics](https://www.owlsprep.com/study/cie-9700-u17-inheritance/)

## Learning objectives

- Distinguish between continuous and discontinuous variation
- Explain genetic and environmental causes of variation
- Describe why variation is required for natural selection
- Classify different traits by variation type in exam questions

## Types of Variation

**Variation** — Variation refers to measurable differences in phenotype or genotype between individual organisms within the same species. It is grouped into two broad categories based on how traits are distributed.

*Example:* Differences in height among adult humans is a common example of variation

| Characteristic | Continuous Variation | Discontinuous Variation |
| --- | --- | --- |
| Genetic control | Polygenic (many genes) | One or few genes |
| Environmental effect | Strong influence | Little to no influence |
| Trait distribution | Continuous range, no distinct groups | Discrete, distinct groups |
| Example | Height, skin colour, crop yield | ABO blood group, tongue rolling, pea seed shape |

**Worked example:** Classify the following human traits as continuous or discontinuous variation: (a) eye colour, (b) body mass, (c) sickle cell anemia status, (d) finger length

1. First recall the core difference: continuous variation has a full range of intermediate phenotypes, while discontinuous variation falls into distinct groups.
2. (a) Eye colour ranges from very light blue to very dark brown with many intermediate shades, so this is **continuous variation**.
3. (b) Body mass shows a full range from very low to very high mass with no distinct cut-offs between groups, so this is **continuous variation**.
4. (c) Sickle cell anemia status is either unaffected, carrier, or affected — three distinct groups with no intermediates, so this is **discontinuous variation**.
5. (d) Finger length varies gradually between individuals with many intermediate lengths, so this is **continuous variation**.

> **Exam tip:** Always link variation type to number of genes when answering explanation questions; CIE examiners expect this connection.

## Causes of Variation

All variation comes from a combination of genetic and environmental factors. Only genetic variation is heritable (can be passed from parent to offspring) and contributes to long-term evolutionary change.

- **Genetic causes**: Random gene/chromosome mutations, crossing over in prophase I of meiosis, independent assortment of homologous chromosomes in metaphase I, random fertilization of gametes
- **Environmental causes**: Differences in diet, climate, light exposure, lifestyle, and injury that modify phenotype

**Worked example:** Identical twins share 100% of their DNA, but often have slightly different heights. Explain this observation.

1. Since identical twins have identical genotypes, any phenotypic difference must come from environmental factors, not genetic variation.
2. Height is a continuous, polygenic trait that is strongly influenced by environmental factors during development.
3. Differences in factors like childhood nutrition, access to healthcare, exposure to illness, or even uterine conditions during development can lead to measurable differences in final adult height, even with identical genetics.

> **warning**
>
> Variation caused purely by environmental factors (such as a scar from injury or acquired muscle mass from exercise) cannot be passed to offspring. Only changes to gamete DNA are heritable.

## Variation and Evolution

Variation is an absolute requirement for natural selection and evolution. If all individuals in a population are genetically identical, no trait can be selected for or against, so the population cannot adapt to environmental change.

Advantageous variants (traits that improve survival and reproduction) will become more common over generations, while harmful variants will decrease in frequency. Over long periods, this process leads to adaptation and eventually speciation.

**Worked example:** How does variation allow a population of bacteria to evolve resistance to a new antibiotic?

1. Random genetic mutations create variation in the bacterial population: a small number of individuals carry a mutation that gives them antibiotic resistance.
2. When the population is exposed to the antibiotic, all non-resistant individuals die, while resistant individuals survive due to their advantageous trait.
3. Surviving resistant bacteria reproduce asexually, passing the resistance allele to all their offspring. Over multiple generations, the entire population becomes resistant, as all non-resistant individuals are eliminated.

## Common pitfalls

- **Wrong:** Claiming all continuous variation is purely genetic
  - Why it fails: Nearly all continuous polygenic traits are strongly influenced by environmental factors
  - Correct: Always state that continuous variation is caused by a combination of genetic and environmental factors
- **Wrong:** Claiming all variation is heritable
  - Why it fails: Only genetic variation can be passed to offspring; purely environmental variation is not heritable
  - Correct: Distinguish between heritable genetic variation and non-heritable environmental variation when answering questions about evolution
- **Wrong:** Defining variation as differences between different species
  - Why it fails: In the context of evolution, variation refers to differences between individuals of the same species
  - Correct: Remember that intra-species variation is the variation that drives natural selection
- **Wrong:** Claiming discontinuous variation is never affected by the environment
  - Why it fails: While discontinuous variation is primarily genetic, many traits can have their expression modified by environment
  - Correct: State that discontinuous variation is largely genetic, but acknowledge environmental modifiers may affect phenotype severity

## Cheatsheet

| Feature | Continuous Variation | Discontinuous Variation |
| --- | --- | --- |
| Number of genes | Many (polygenic) | 1-2 genes |

## What's next

Variation is the starting point for all evolutionary change, so your understanding here forms the base for all subsequent topics in selection and evolution. Next, you will explore how natural selection acts on variation to change allele frequencies in populations over time. This topic also connects directly to speciation, where divergent selection on variation in separated populations leads to the formation of new species. Mastering variation is critical for essay questions in Paper 4, which often require you to link variation to natural selection, adaptation, and evolution. Getting this foundation right will make more complex topics much easier to learn.

- [Natural Selection](https://www.owlsprep.com/study/cie-9700-u18-natural-selection/)
- [Evolution by natural selection](https://www.owlsprep.com/study/cie-9700-u18-evolution-by-natural-selection/)
- [Artificial selection](https://www.owlsprep.com/study/cie-9700-u18-artificial-selection/)

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