What you'll learn
Variation and Selection examines how organisms within a species differ from one another and how environmental pressures lead to evolutionary change. This topic connects genetics, ecology, and evolution, appearing regularly in Paper 2 (Core) and Paper 4 (Extended) through data analysis questions, adaptation explanations, and selective breeding scenarios. Understanding the mechanisms of natural selection and artificial selection is essential for explaining biodiversity patterns and agricultural practices.
Key terms and definitions
Variation — differences between individuals of the same species, caused by genetic factors, environmental factors, or both.
Continuous variation — variation where individuals in a population show a range of phenotypes between two extremes with no distinct categories (e.g. height, mass, leaf surface area).
Discontinuous variation — variation where individuals fall into distinct categories with no intermediates (e.g. ABO blood groups, ability to roll tongue).
Mutation — a random change in the DNA base sequence that can be inherited if it occurs in gametes.
Natural selection — the process by which organisms better adapted to their environment survive, reproduce, and pass on their advantageous alleles to offspring.
Artificial selection (selective breeding) — the process by which humans deliberately breed organisms with desirable characteristics to produce offspring with those characteristics.
Adaptation — an inherited characteristic that makes an organism well suited to survival in its environment.
Evolution — the change in the inherited characteristics of a population over time through the process of natural selection.
Core concepts
Types of variation
Variation exists in all populations and arises from two main sources.
Genetic variation results from:
- Different alleles inherited from parents
- Mutations creating new alleles
- Random assortment of chromosomes during meiosis
- Random fertilisation of gametes
Examples of genetically controlled characteristics include:
- ABO blood group
- Eye colour in humans
- Flower colour in pea plants
- Presence or absence of horns in cattle
Environmental variation results from:
- Diet and nutrition
- Climate and temperature
- Light intensity and exposure
- Physical accidents or disease
Examples of environmentally influenced characteristics include:
- Muscle development through exercise
- Sun tan in humans
- Language spoken
- Scars and body modifications
Most characteristics show both genetic and environmental influences. Height in humans, for instance, has a genetic component (tall parents typically have tall children) but nutrition during childhood significantly affects final height. Similarly, body mass has a genetic component but diet and exercise have major environmental effects.
Continuous and discontinuous variation
Continuous variation characteristics:
- Controlled by multiple genes (polygenic inheritance)
- Usually influenced by environmental factors
- Show a normal distribution curve when plotted
- Measured quantitatively
- Examples: height, mass, hand span, leaf length, intelligence
When continuous data is plotted as a histogram, the result typically shows a bell-shaped curve with most individuals near the mean and fewer at the extremes.
Discontinuous variation characteristics:
- Controlled by a single gene or small number of genes
- Not influenced by environmental factors
- Show distinct categories
- Counted rather than measured
- Examples: ABO blood groups, attached/free earlobes, tongue rolling ability, seed shape (round or wrinkled)
When discontinuous data is plotted as a bar chart, clear separate bars appear with no intermediates between categories.
Mutation as a source of variation
Mutations are random changes in DNA that occur spontaneously during DNA replication. Most mutations have no effect or are harmful, but occasionally a mutation produces an advantageous characteristic.
Mutation rate increases with exposure to:
- Ionising radiation (X-rays, gamma rays, UV light)
- Chemical mutagens (tar in cigarette smoke, certain pesticides)
Mutations in body cells affect only that individual and cannot be inherited. Mutations in gametes (sex cells) can be passed to offspring and become part of the population's gene pool. This introduces new alleles into populations, providing the genetic variation necessary for natural selection to occur.
Examples of mutations:
- Sickle cell anaemia results from a mutation in the haemoglobin gene
- Antibiotic resistance in bacteria arises through random mutations
- Some insect populations have evolved pesticide resistance through mutation
Natural selection and adaptation
Natural selection operates when organisms compete for limited resources. The theory, developed by Charles Darwin, explains how species change over time.
The process of natural selection follows these steps:
- Variation exists — individuals in a population show variation in their phenotypes due to different alleles
- Environmental pressure — there is competition for limited resources (food, water, space, mates) or predation, disease, or climate challenges
- Differential survival — individuals with characteristics better adapted to the environment are more likely to survive
- Differential reproduction — survivors reproduce and pass advantageous alleles to offspring
- Allele frequency changes — over many generations, the advantageous alleles become more common in the population
Adaptations develop through natural selection and can be:
- Structural (physical features): thick fur in Arctic mammals, streamlined bodies in aquatic animals, long necks in giraffes
- Behavioural (actions): migration, hibernation, territorial marking, nocturnal activity
- Physiological (internal processes): venom production, antibiotic resistance, antifreeze proteins in Antarctic fish
Example of natural selection: peppered moths
The peppered moth (Biston betularia) provides documented evidence of natural selection:
- Original population: mostly light-coloured moths camouflaged against pale lichen-covered trees; dark moths were rare
- Environmental change: Industrial Revolution caused air pollution that killed lichen and darkened tree trunks with soot
- Selection pressure: birds predated more visible moths
- Outcome: dark moths became camouflaged; light moths became conspicuous. Dark moth frequency increased dramatically in polluted areas
- Modern change: Clean Air Acts reduced pollution; lichen returned; light moths again became more common
This demonstrates how environmental change drives changes in allele frequency through natural selection.
Example of natural selection: antibiotic resistance
Bacterial antibiotic resistance demonstrates rapid natural selection:
- Random mutations in bacterial DNA occasionally produce resistance to specific antibiotics
- When antibiotics are used, susceptible bacteria die but resistant bacteria survive
- Resistant bacteria reproduce rapidly (every 20 minutes in optimal conditions)
- Resistant population increases, making the antibiotic ineffective
- New antibiotics required, but bacteria continue to evolve resistance
This explains why:
- Antibiotics should only be used when necessary
- Full antibiotic courses must be completed (to kill all bacteria, including those slightly more resistant)
- Agricultural use of antibiotics should be limited
Artificial selection (selective breeding)
Artificial selection involves humans choosing which organisms reproduce based on desirable characteristics. Unlike natural selection where the environment determines survival, humans decide which traits to select.
The selective breeding process:
- Identify organisms with desired characteristics from existing variation
- Select those organisms as parents
- Breed them together
- Select offspring showing the desired characteristics most strongly
- Repeat over many generations
Examples in agriculture:
Crop plants:
- Wheat bred for high yield, short stems (resist wind damage), disease resistance
- Rice bred for flood tolerance, vitamin A content (Golden Rice), yield
- Tomatoes bred for size, flavour, shelf-life, uniform ripening
Domesticated animals:
- Cattle bred for high milk yield (dairy breeds) or high meat production (beef breeds)
- Chickens bred for high egg production or rapid growth for meat
- Dogs bred for specific behaviour, size, and physical traits
Advantages of selective breeding:
- Increased yield and productivity
- Improved disease resistance
- Better quality products
- Organisms suited to specific environments or purposes
Disadvantages of selective breeding:
- Reduced genetic diversity — breeding from limited stock reduces variation
- Inbreeding problems — increased chance of harmful recessive alleles being expressed
- Vulnerability — if environment changes or new disease emerges, lack of variation means few individuals may survive
- Loss of other characteristics — selecting for one trait may reduce others (e.g. high-yield crops may be less drought-resistant)
Evolution
Evolution is the gradual change in inherited characteristics of a population over time through natural selection. Over many generations, these changes can result in new species formation.
Evidence for evolution includes:
- Fossil records showing how organisms have changed over millions of years
- Comparative anatomy revealing similar bone structures in different species (e.g. pentadactyl limb)
- DNA evidence showing genetic similarities between related species
- Observable changes in populations like antibiotic resistance and pesticide resistance
Darwin's theory of evolution by natural selection explains:
- Why organisms are adapted to their environments
- Why there is such diversity of life
- Why some species become extinct when unable to adapt to change
- How new species arise when populations become isolated and face different selection pressures
Worked examples
Example 1: Distinguishing variation types
Question: The table shows characteristics in humans. Complete the table by placing a tick (✓) in the correct column to show whether each characteristic shows continuous or discontinuous variation. [3 marks]
| Characteristic | Continuous | Discontinuous |
|---|---|---|
| Hand span | ||
| ABO blood group | ||
| Tongue rolling ability |
Answer:
| Characteristic | Continuous | Discontinuous |
|---|---|---|
| Hand span | ✓ | |
| ABO blood group | ✓ | |
| Tongue rolling ability | ✓ |
Explanation: Hand span is measured and shows a range with no distinct categories (continuous). Blood group and tongue rolling fall into clear categories with no intermediates (discontinuous). [1 mark per correct row]
Example 2: Natural selection explanation
Question: Giraffes have very long necks. Explain how natural selection has resulted in giraffes having long necks. [4 marks]
Answer:
- Variation existed in ancestral giraffe population; some had longer necks than others / mutation produced longer neck allele [1]
- Giraffes compete for food / leaves high in trees [1]
- Giraffes with longer necks could reach more food so were more likely to survive [1]
- These giraffes reproduced and passed on alleles for long necks to offspring / over many generations long neck alleles became more common [1]
Mark scheme notes: Must mention variation first. Need both survival advantage AND reproduction/inheritance. Generic statements without giraffe context score fewer marks.
Example 3: Selective breeding
Question: A farmer wants to breed sheep that produce high quality wool. Describe how the farmer could do this using selective breeding. [4 marks]
Answer:
- Select/identify sheep with the best quality wool from the flock [1]
- Breed these sheep together [1]
- Select offspring with the best quality wool [1]
- Repeat over many generations [1]
Alternative acceptable answers: Allow "cross-breed" for breed; allow "mate" for breed; must mention selection in parents AND offspring; must mention repetition/many generations.
Common mistakes and how to avoid them
Mistake: Stating that characteristics are either genetic OR environmental, not both. Correction: Most characteristics (like height, mass, intelligence) are influenced by both genetic and environmental factors. Only a few characteristics are purely genetic (blood group) or purely environmental (language spoken, scars).
Mistake: Confusing natural selection with evolution. Correction: Natural selection is the mechanism by which evolution occurs. Evolution is the outcome — the change in inherited characteristics over time. Natural selection is the process that drives evolutionary change.
Mistake: Writing that organisms "adapt during their lifetime" or "try to evolve". Correction: Individual organisms do not evolve. Populations evolve over many generations. Adaptations arise through random mutation and natural selection, not through organisms consciously changing or needing to change.
Mistake: Stating that mutations are always harmful. Correction: Most mutations are neutral (no effect). Some are harmful. Occasionally mutations are advantageous in certain environments. Advantageous mutations provide variation for natural selection.
Mistake: In selective breeding, only mentioning selecting parents or only mentioning breeding. Correction: Must describe both selection of parents and breeding them together, then selecting offspring and repeating. The process requires multiple generations.
Mistake: Confusing continuous and discontinuous variation based on whether it's important rather than the pattern. Correction: Continuous variation shows a range of phenotypes measured quantitatively (height, mass); discontinuous shows distinct categories counted qualitatively (blood group, eye colour). This is about the pattern of data, not the characteristic's importance.
Exam technique for Variation and Selection
"Explain" questions on natural selection: Follow a logical sequence — variation exists, competition/selection pressure, differential survival, reproduction and inheritance, change in allele frequency. Examiners expect all these elements. A 4-mark question typically needs all four steps. Use the specific organism/context from the question.
"Describe" selective breeding questions: Must include selection of parents, breeding, selection of offspring, and repetition over generations. Questions worth 3-4 marks expect all these stages. Simply saying "breed the best ones" scores only 1 mark.
Data analysis questions: Variation questions often include graphs or tables. For continuous variation, expect to calculate means, ranges, or describe distribution patterns. For natural selection scenarios, be ready to explain changes in population numbers over time using selection pressure logic.
Command word "Suggest": Applied to unfamiliar contexts (e.g. selection in a new organism). Use your knowledge of natural selection or selective breeding principles but apply to the novel situation. Mark schemes accept logical answers that show understanding of the process.
Quick revision summary
Variation arises from genetic factors (alleles, mutation) and environmental factors. Continuous variation shows a range (height, mass); discontinuous shows distinct categories (blood groups). Natural selection occurs when organisms with advantageous adaptations survive, reproduce, and pass on beneficial alleles, causing populations to evolve over generations. Examples include antibiotic resistance and peppered moth colour changes. Selective breeding involves humans choosing organisms with desired traits and breeding them over many generations, producing improved crops and livestock but reducing genetic variation.