Variation

1. Overview

Variation is the engine of evolution and the reason why no two individuals (except identical twins) are exactly alike. Understanding variation allows us to categorize how characteristics are passed down and how environmental factors interact with our DNA to shape the physical traits (phenotypes) we see in a population.

Key Definitions

• Variation: The differences between individuals of the same species.
• Phenotype: The observable features of an organism.
• Genotype: The genetic makeup of an organism in terms of the alleles present.
• Mutation: A genetic change; the way in which new alleles are formed.
• Gene Mutation: A random change in the base sequence of DNA.
• Continuous Variation: Variation that results in a range of phenotypes between two extremes.
• Discontinuous Variation: Variation that results in a limited number of phenotypes with no intermediates.

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Core Content

Types of Variation

Variation can be classified into two main categories based on the distribution of phenotypes:

1. Continuous Variation

• Description: Features that show a complete range of measurements from one extreme to the other. There are no distinct groups.
• Causes: Influenced by both genes and the environment.
• Examples:
  • Body length/Height
  • Body mass (weight)
• Data Representation: Usually forms a "Bell Curve" (Normal Distribution) when plotted on a histogram.
• 📊A histogram showing a bell-shaped curve. The x-axis represents 'Height' and the y-axis represents 'Number of individuals'. The peak is in the middle, showing most people are of average height.

2. Discontinuous Variation

• Description: Features that fall into distinct categories. You either have the characteristic or you don't.
• Causes: Usually caused by genes only.
• Examples:
  • ABO Blood Groups (A, B, AB, or O)
  • Seed shape in peas (round or wrinkled)
  • Seed colour in peas (yellow or green)
• Data Representation: Plotted as a bar chart with gaps between the bars.
• 📊A bar chart with four separate bars representing Blood Groups A, B, AB, and O. The bars do not touch, indicating distinct categories.

Mutations

• What is it? A mutation is a change in the DNA of a cell.
• The Result: Mutations are the only way new alleles are created. This provides the raw material for natural selection.
• Increasing Mutation Rate: While mutations happen spontaneously, the rate can be increased by:
  1. Ionising radiation: e.g., Gamma rays, X-rays, and Ultraviolet (UV) rays.
  2. Chemicals: e.g., certain chemicals in tobacco smoke or industrial pollutants.

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Extended Content (Extended Curriculum Only)

Gene Mutation at the Molecular Level

A gene mutation is defined specifically as a random change in the base sequence of DNA. Because the sequence of bases (A, T, C, G) codes for specific amino acids, a change in this sequence can change the protein produced, leading to a different phenotype.

Sources of Genetic Variation

While mutation is the primary source of new alleles, several processes during reproduction "shuffle" the existing alleles to create variation in a population:

1. Mutation: Creates brand new alleles.
2. Meiosis: During the formation of gametes (sperm and egg), alleles are redistributed through independent assortment and crossing over.
3. Random Mating: Any male in a population can potentially mate with any female; this mixes different combinations of alleles.
4. Random Fertilisation: Any one of the millions of sperm cells can fertilise the egg, ensuring the resulting zygote has a unique combination of DNA.

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Key Equations

There are no specific mathematical formulas for topic 18.1. However, you should be able to:

• Calculate Percentages: (Number of individuals with a trait ÷ Total population) × 100.
• Calculate Means: For continuous variation data (Sum of all values ÷ Number of individuals).

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Common Mistakes to Avoid

• ❌ Wrong: Saying continuous variation is only caused by the environment.
• ✓ Right: Continuous variation is caused by both genes and the environment (e.g., you may have the genes to be tall, but poor nutrition stops you from reaching that height).
• ❌ Wrong: Using a line graph to show discontinuous variation.
• ✓ Right: Use a bar chart for discontinuous data and a histogram for continuous data.
• ❌ Wrong: Thinking mutations are always "bad."
• ✓ Right: Mutations are "random"—they can be harmful, neutral, or occasionally beneficial to an organism's survival.

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Exam Tips

• Command Word: "State": This is the most common command word for this topic. Give a short, factual answer (e.g., "State two causes of mutation" -> "UV radiation and tobacco smoke").
• Command Word: "Explain": If asked to explain why height is continuous variation, you must mention that it is controlled by multiple genes AND influenced by environmental factors like diet.
• Look for Chemicals: Exam questions often use "chemicals" or "radiation" as a context for how mutations occur. Be ready to link these to DNA damage.
• Data Analysis: Be prepared to identify the type of variation from a graph. If it's a smooth curve, it's continuous; if it's separate bars, it's discontinuous.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Define the term variation as it applies to a population of organisms. [2]

(b) State two sources of variation within a population. [2]

(c) Suggest one environmental factor that might contribute to continuous variation in the height of a plant species. [1]

Worked Solution:

(a)

1. Variation is the differences between individuals Differences between members of the same species. This is the core definition of variation.

How to earn full marks:

• Must mention "differences".
• Must specify "same species" or "population".

(b)

1. Meiosis produces genetically different gametes Meiosis. Meiosis results in different combinations of alleles in gametes.

2. Mutation introduces new alleles Mutation. Mutations create new genetic variations.

How to earn full marks:

• Accept any two of: mutation, meiosis, random mating, random fertilisation.
• 'Sexual reproduction' is not specific enough.

(c)

1. Light availability is the key environmental factor Availability of light. Plants need light for photosynthesis and growth.

How to earn full marks:

• Accept any reasonable environmental factor, such as water availability, nutrient levels in the soil, temperature.

Common Pitfall: Make sure you understand the difference between sources of variation (like meiosis and mutation) and examples of variation (like height or eye color). Don't confuse the two in your answers. Also, remember that "sexual reproduction" is too general; you need to be specific about the mechanisms involved.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

A scientist is studying the seed shape of pea plants. Some plants produce round seeds, while others produce wrinkled seeds.

(a) State whether seed shape in peas is an example of continuous or discontinuous variation. [1]

(b) Explain why seed shape in peas is an example of the type of variation you stated in (a). [3]

(c) Describe one other example of discontinuous variation in organisms. [2]

Worked Solution:

(a)

1. Seed shape in peas has distinct categories Discontinuous variation. This is because there are only two distinct phenotypes.

How to earn full marks:

• Must state "discontinuous".

(b)

1. Only two distinct phenotypes are present There are only two phenotypes: round or wrinkled. This is a key characteristic of discontinuous variation.

2. No intermediate phenotypes exist There are no intermediate phenotypes, such as slightly wrinkled or partly round. The phenotype is either one or the other.

3. Variation is controlled by a single gene This type of variation is typically controlled by a single gene. Single-gene control is typical of discontinuous traits.

How to earn full marks:

• Must mention the limited number of phenotypes (round or wrinkled).
• Must mention the absence of intermediate phenotypes.
• Must link this to genetic control (single gene).

(c)

1. Human blood groups are another example of discontinuous variation Human ABO blood groups. There are only four distinct blood groups.

2. No intermediate blood groups exist Individuals have blood type A, B, AB, or O; there are no intermediate blood types. This makes it a good example of discontinuous variation.

How to earn full marks:

• Must name a valid example of discontinuous variation, like human blood groups or the ability to roll your tongue.
• Must explain why it's an example of discontinuous variation, i.e., limited number of phenotypes.

Common Pitfall: Students often confuse continuous and discontinuous variation. Remember that discontinuous variation has distinct categories with no intermediates, while continuous variation has a range of values. Also, be sure to provide a reason why your example in part (c) is discontinuous.

Exam-Style Question 3 — Extended Response [9 marks]

Question:

A population of snails lives in a garden. The color of their shells varies from light brown to dark brown. A researcher measures the shell color of 100 snails using a color chart and records the frequency of each color.

(a) State whether shell color in these snails is an example of continuous or discontinuous variation. Explain your answer. [3]

(b) Suggest two environmental factors that could influence the shell color of these snails. Explain how each factor might affect shell color. [4]

(c) Describe how the researcher could present their data to show the distribution of shell colors in the snail population. [2]

Worked Solution:

(a)

1. Shell colour varies between extremes Continuous variation. This is the core concept of continuous variation.

2. A range of phenotypes exist Shell color varies across a range from light brown to dark brown. Continuous variation results in a spectrum of phenotypes.

3. Intermediate shell colours are observed There are intermediate shades of brown between the extremes. This is a key characteristic of continuous variation.

How to earn full marks:

• Must state "continuous variation".
• Must explain that there is a range of phenotypes.
• Must mention the existence of intermediate phenotypes.

(b)

1. Sunlight exposure affects pigment production Sunlight exposure. Snails in areas with more sunlight may produce darker shells due to increased melanin production. Sunlight stimulates the production of pigments.

2. Predation affects survival of different coloured snails Predation. If birds are more likely to spot light-colored snails on dark backgrounds, darker shells may become more common in the population. Selective pressure from predators.

How to earn full marks:

• Must identify a valid environmental factor.
• Must explain how that factor could influence shell color, linking it to pigment production or survival.

(c)

1. A histogram is ideal to display continuous data The researcher could present their data as a histogram. Histograms show the distribution of continuous data effectively.

2. The x-axis shows the range of shell colours The x-axis would represent the different shell colors (e.g., shades of brown), and the y-axis would represent the frequency of snails with each color. This allows for easy visualization of the distribution.

How to earn full marks:

• Must suggest a suitable graph (histogram or bar chart).
• Must explain how the axes would be labelled.

Common Pitfall: When describing environmental factors, be specific about how they influence the trait. Don't just say "temperature affects shell color"; explain the mechanism, such as its effect on enzyme activity or pigment production. Also, remember that a bar chart is also acceptable for continuous data if the data is grouped into discrete categories.

Exam-Style Question 4 — Extended Response [10 marks]

Question:

A scientist is studying a population of fruit flies in a laboratory. She exposes some of the fruit flies to low levels of ionizing radiation. She then observes changes in the offspring of the exposed flies.

(a) Define the term mutation. [2]

(b) State two sources of genetic variation in populations other than mutation. [2]

(c) Explain how exposure to ionizing radiation can increase the rate of mutation in fruit flies. [3]

(d) Suggest one possible effect of a mutation on the phenotype of a fruit fly. Explain your answer. [3]

Worked Solution:

(a)

1. Mutation is any alteration to the genetic material Mutation is a change in the genetic material (DNA). This is the fundamental definition of mutation.

2. May involve a change in the base sequence It can involve a change in the base sequence of DNA. Base-pair changes are the most common type of mutation.

How to earn full marks:

• Must mention a change in genetic material or DNA.
• Must mention a change in the base sequence or gene.

(b)

1. Meiosis creates genetic variation Meiosis. During meiosis, crossing over and independent assortment create new combinations of alleles.

2. Random fertilisation is also a source of variation Random fertilisation. Any sperm can fertilise any egg.

How to earn full marks:

• Accept any two of: meiosis, random mating, random fertilisation.
• "Sexual reproduction" is not specific enough.

(c)

1. Ionising radiation is high energy Ionizing radiation is high-energy radiation. This is what makes it dangerous.

2. Ionising radiation damages DNA directly It can directly damage DNA molecules. The radiation has enough energy to break chemical bonds in DNA.

3. Damaged DNA causes errors during replication This damage can lead to errors during DNA replication and transcription, resulting in mutations. The cell's repair mechanisms might not be perfect, leading to mutations.

How to earn full marks:

• Must mention the high energy of ionizing radiation.
• Must explain that it damages DNA molecules.
• Must explain how this damage leads to errors in replication/transcription.

(d)

1. A mutation can change enzyme structure Change in wing shape. A mutation could affect a gene involved in wing development, leading to deformed or smaller wings. Mutations in genes for enzymes can alter metabolic pathways.

2. Change in protein structure can affect function This change in the protein's structure can alter its function, leading to a visible phenotypic change. The new protein may not function as well, or at all.

3. Gene mutations are random Mutations are random and may affect any cell, and therefore any function. This is why the outcome is not predictable.

How to earn full marks:

• Must suggest a plausible phenotypic change in a fruit fly.
• Must explain how the mutation could lead to that change, linking it to gene expression or protein function.

Common Pitfall: When describing the effects of mutations, avoid vague answers like "it will cause a change." Be specific about the type of change and how it relates to gene expression or protein function. Also, remember that mutations are random and not always harmful; they can be neutral or even beneficial in some cases.