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Variation has two main causes: genetic and environmental. Variation is the differences in characteristics between individuals in a population. Free notes.
Causes of Variation is part of Variation and evolution in GCSE Edexcel Higher Biology.
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Variation has two main causes: genetic and environmental. Variation is the differences in characteristics between individuals in a population. Free notes.
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Use this lesson to stay inside Variation and evolution while connecting the detail back to the wider Topic 3: Genetics topic.
Read the core explanations from Causes of Variation before testing yourself from memory.
Look around your classroom. Everyone's different - height, eye colour, hair texture, even how they react to the same joke. This isn't random. There's a scientific explanation for why no two people are identical (except identical twins, and even they aren't completely the same).
These differences between individuals are called variation. Understanding variation is crucial because it explains everything from why some people are naturally athletic to why certain diseases run in families. It's also the raw material that makes evolution possible.
But here's where it gets interesting - variation has two main causes, and they work in completely different ways. Sometimes they even fight against each other.
Your genes are like a detailed instruction manual written in your DNA. They determine many of your characteristics before you're even born. Think of them as the 'factory settings' for your body.
Key examples of genetic variation:Blood type - You're born with A, B, AB, or O and it never changesEye colour - Determined by genes controlling melanin productionAbility to roll your tongue - Either you can or you can't, no amount of practice changes thisGenetic disorders - Like cystic fibrosis or sickle cell anaemia
Trap: Many students think all physical features are genetic. Actually, your final height depends on genes AND nutrition. Your genes set the potential, but environment determines if you reach it.
While genes provide the blueprint, your environment acts like the construction crew - it determines how that blueprint gets built. Environmental factors can dramatically change how you develop, sometimes overriding genetic tendencies entirely.
Major environmental factors:Nutrition - Poor diet stunts growth, even with 'tall' genesExercise - Builds muscle mass and cardiovascular fitnessClimate - Sun exposure affects skin pigmentationDisease - Infections can affect developmentLifestyle choices - Smoking damages lung capacity
Here's a powerful example: identical twins have identical genes, but if one twin exercises regularly and the other doesn't, they'll develop very different muscle mass and fitness levels. Same genes, different environment, different outcome.
Most characteristics aren't purely genetic or purely environmental - they're the result of both working together. This interaction is where biology gets really fascinating, and where exam questions get tricky.
Worked example: How Genes and Environment Interact - Height Example
Explain how both genetic and environmental factors determine a person's final height.
Answer: Height is determined by genetic potential (maximum possible) modified by environmental factors (nutrition, health) during critical growth periods.
More examples of gene-environment interactions:Skin colour - Genes determine potential, sun exposure affects actual pigmentationIntelligence - Genetic capacity influenced by education, nutrition, and stimulationBody weight - Genetic predisposition modified by diet and exerciseDisease susceptibility - Genetic risk factors triggered by environmental conditions
Here's something that surprises many students: there's usually more genetic variation within a single population than between different populations. Humans are remarkably genetically similar, yet we show extensive variation.
This extensive genetic variation exists because:Sexual reproduction - Mixes genes from two parents in new combinationsIndependent assortment - Chromosomes separate randomly during meiosisCrossing over - Chromosomes exchange segments during meiosisMutations - Rare changes in DNA create new alleles
This variation is crucial for species survival. If everyone was genetically identical, a single disease could wipe out the entire population. Genetic diversity means some individuals will always survive environmental challenges.
Variation questions are common in GCSE exams, but students often lose marks by being vague or confusing genetic and environmental factors. Here's how to get full marks.
Common exam mistakes to avoid:Saying 'DNA causes variation' - Be specific: different alleles cause variationConfusing correlation with causation - Just because tall parents have tall children doesn't prove it's only geneticForgetting environmental examples - Always give concrete examples like nutrition affecting heightMissing the interaction - Most characteristics involve BOTH genes and environment
Remember: variation is the foundation of evolution and the reason why life is so diverse. Understanding its causes helps explain everything from family resemblances to why populations can adapt to changing environments.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-23.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Phenotype is the observable characteristics of an organism, resulting from the interaction between its genotype (genetic makeup) and environmental influences during development. For example, height = genetic potential modified by environmental factors such as nutrition and health.
Variation is the differences in characteristics between individuals in a population. These differences can be physical, like height, behavioural, like personality, or physiological, like blood type, and they have genetic and environmental causes.
Examples of environmental variation include poor diet stunting growth, exercise building muscle mass, sun exposure affecting skin pigmentation, infections affecting development and smoking damaging lung capacity. These differences are caused by external factors and can change throughout life.
Tongue rolling is genetic: either a person can roll their tongue or they cannot, and no amount of practice changes this. It is an example of genetic variation caused by inherited alleles.
Examples of genetic variation include blood type, eye colour, the ability to roll the tongue and genetic disorders such as cystic fibrosis or sickle cell anaemia. These are caused by different alleles inherited from parents and are fixed at fertilisation.
Variation has two main causes: genetic and environmental. Genetic variation is caused by different alleles inherited from parents, while environmental variation is caused by external factors such as nutrition, exercise, climate, disease and lifestyle during development. Most characteristics result from both.
Crossing over and independent assortment are two causes of genetic variation during meiosis. In crossing over, chromosomes exchange segments, and in independent assortment, chromosomes separate randomly, creating new combinations of alleles.
Genetic variation is caused by different versions of genes (alleles) inherited from parents and is fixed at fertilisation, such as blood type. Environmental variation is caused by external factors during development and can change throughout life, such as muscle mass from exercise.
Height is both genetic and environmental. Genes set the potential maximum height, but nutrition, health and lifestyle during childhood and adolescence determine whether a person reaches it, which is why well-nourished populations are generally taller than previous generations.
Identical twins have identical genes but can differ because of their environment. If one twin exercises regularly and the other does not, they develop very different muscle mass and fitness levels: same genes, different environment, different outcome.
Nature versus nurture is the question of how much a characteristic is caused by genes (nature) or by environment (nurture). Most characteristics involve both: for height, genes set the potential and nutrition determines whether it is reached.
Genetic variation within a population comes from sexual reproduction mixing genes from two parents, independent assortment and crossing over during meiosis, and mutations creating new alleles. This variation is crucial for species survival, as some individuals will always survive environmental challenges.
Blood type is genetic: a person is born with blood type A, B, AB or O and it never changes. It is an example of genetic variation caused by alleles inherited from parents.
Skin colour is both genetic and environmental. Genes determine the potential, while sun exposure affects the actual pigmentation, so skin colour is an example of genes and environment interacting.
Body weight is both genetic and environmental: a genetic predisposition is modified by diet and exercise. Most characteristics are not purely genetic or purely environmental but result from both working together.