VariationCambridge IGCSE Biology: Revision notes
Section 1
What is variation and why does it matter?
Variation refers to differences between individuals of the same species. These differences can be in physical features (morphological), physiological traits, or behavioural characteristics. Variation is a fundamental feature of populations and provides the raw material for natural selection to act upon.
Within any population, you will find a range of phenotypes (observable characteristics). Understanding variation is crucial because it helps explain:
- How organisms adapt to their environments
- Why individuals within a species are not identical
- The basis of evolution and natural selection
Examiners want you to state that variation exists both between species AND within species. Focus on within-species variation as this is what matters for selection and evolution.
Section 2
What is continuous variation and how does it appear in populations?
Continuous variation produces a range of phenotypes between two extremes with many intermediate forms. This means individuals show a spectrum of traits rather than distinct categories.
Examples of continuous variation:
- Body length and body mass
- Height in humans
- Skin colour
- Intelligence
Key features:
- No clear-cut categories
- Results in a smooth distribution (often bell-shaped/normal distribution) when plotted on a graph
- Cannot easily classify individuals into groups
- Caused by both genetic factors AND environmental factors
Why is it continuous? Multiple genes control the trait (polygenic inheritance), and the environment influences expression. For example, human height is controlled by many genes, but nutrition and health also play major roles.
Think of continuous variation like a spectrum from light to dark: there are infinite shades in between the extremes, not just 'light' or 'dark' categories.
When describing continuous variation, always mention that it shows no clear-cut categories and that individuals cannot be placed into distinct groups.
Section 3
What is discontinuous variation and how does it differ from continuous?
Discontinuous variation results in a limited number of distinct phenotypes with no intermediates. Individuals fall into clear, separate categories with no 'in-between' forms.
Examples of discontinuous variation:
- ABO blood groups (types A, B, AB, or O only)
- Seed shape in peas (round or wrinkled, not intermediate shapes)
- Presence or absence of horns
- Flower colour in certain plants
Key features:
- Distinct, separate categories
- No intermediate forms possible
- Caused by genes only (not influenced by environment)
- Usually controlled by one gene or a few alleles
- Results in a bar chart distribution rather than a bell curve
Comparison table:
| Feature | Continuous | Discontinuous |
|---|---|---|
| Number of phenotypes | Many (range between extremes) | Few (distinct categories) |
| Intermediates | Yes, many | No |
| Genetic control | Many genes (polygenic) | One or few genes |
| Environmental influence | Yes, significant | No |
| Examples | Height, mass, skin colour | Blood groups, seed shape |
| Graph type | Bell curve | Bar chart |
Why is discontinuous variation not affected by environment? Once your blood group is determined by your alleles, no environmental factor can change it.
Students often say 'discontinuous variation is caused by environment only'. This is backwards — discontinuous variation is caused by genes ONLY; continuous variation is affected by both genes AND environment.
Blood type (discontinuous): Two people could be the same height (continuous) but have different blood types. Blood type cannot be changed by diet or climate, but height can be influenced by nutrition. This shows why blood type is discontinuous and height is continuous.
Section 4
What is mutation and how does it create new variation?
Mutation is a genetic change — specifically, a random alteration in the DNA sequence of a gene. Mutations are the primary source of entirely new alleles within populations.
Gene mutation at the molecular level:
- A gene mutation is a random change in the base sequence of DNA
- Bases are the chemical units (A, T, G, C) that make up the DNA code
- Even a change of a single base can result in a different allele
- This new allele may code for a different protein or amino acid sequence
Key points about mutations:
- Mutations are random — they occur unpredictably and without direction
- Most mutations are neutral or harmful, but some may be beneficial
- Mutations can be spontaneous (occurring naturally) or induced (caused by external factors)
- Mutations increase genetic variation in a population
Factors that increase mutation rate:
- Ionising radiation (X-rays, gamma rays, ultraviolet light) — damages DNA and causes breaks or alterations
- Some chemicals (certain pesticides, carcinogens, alkylating agents) — can bind to DNA and cause changes in the base sequence
Examples of chemical and radiation-induced mutations:
- Exposure to radioactive materials increases cancer risk
- Certain pesticides used in agriculture increase mutation rates in organisms
- UV radiation can cause skin mutations leading to melanoma
Examiners want to see that you understand mutations are random changes and that they increase the mutation rate when exposed to radiation/chemicals. State clearly: 'Ionising radiation and some chemicals increase the rate of mutation'.
A mutation is like a typo in a instruction manual: a single letter change might not matter (neutral), could make instructions useless (harmful), or occasionally improve the process (beneficial).
Section 5
What are the sources of genetic variation in populations?
Genetic variation within a population comes from four main sources:
1. Mutation
- Creates entirely new alleles
- Random changes in DNA base sequences
- Increases the pool of alleles available in a population
- Essential for long-term evolution
2. Meiosis
- The process that produces gametes (sex cells)
- Introduces variation through crossing over (exchange of genetic material between homologous chromosomes) and independent assortment (random distribution of chromosome pairs)
- Means each gamete is genetically unique
- Produces variation in sexually reproducing organisms
3. Random mating
- Individuals do not choose partners based on specific traits
- Different combinations of alleles come together unpredictably
- Results in offspring with different genetic combinations than parents
- Increases the variety of genotypes in the next generation
4. Random fertilisation
- The random fusion of sperm and egg cells
- Any sperm can fertilise any egg
- Produces genetically unique offspring
- Combined with random mating, creates tremendous genetic diversity
Summary: Mutation provides new alleles; meiosis, random mating and random fertilisation shuffle and recombine existing alleles into new combinations.
| Source | Effect on Variation |
|---|---|
| Mutation | Creates new alleles |
| Meiosis | Recombines alleles through crossing over and independent assortment |
| Random mating | Brings different alleles together unpredictably |
| Random fertilisation | Creates unique combinations of alleles |
Why does this matter? Without these sources of variation, populations would be genetically uniform and unable to adapt to environmental changes.
Examiners test whether you can name all four sources and explain briefly what each does. Focus on: mutation creates new alleles; meiosis recombines them; random mating and fertilisation shuffle combinations.
In a population: Mutation creates a new 'blue feather' allele (rare). During meiosis, this allele is shuffled into different gametes. Random mating ensures the blue-feathered bird doesn't only mate with feather-colour mates. Random fertilisation means the blue allele could end up in any offspring combination—increasing variation in the next generation.
Must Know
- Variation is differences between individuals of the same species; all populations show variation
- Continuous variation shows a range of phenotypes between two extremes (height, mass, skin colour) with many intermediates; caused by genes AND environment; plotted as a bell curve
- Discontinuous variation shows limited distinct phenotypes with no intermediates (blood groups, seed shape); caused by genes only; plotted as a bar chart
- Mutation is a random change in DNA base sequence that creates new alleles; ionising radiation and certain chemicals increase mutation rate
- Four sources of genetic variation in populations: mutation (creates new alleles), meiosis (crossing over and independent assortment), random mating, and random fertilisation (all shuffle and combine alleles)
- Gene mutations are random, spontaneous or induced, and provide the only source of genuinely new genetic material in populations
That's the notes covered.
Carry on to the next subtopic.