Variation

This lesson covers the causes and types of variation — the differences that exist between individuals of the same species. Understanding variation is essential for understanding evolution and natural selection. For AQA GCSE Biology, you need to know the difference between genetic and environmental variation, and how they interact.

---

What Is Variation?

Variation refers to the differences between individuals within a species. No two individuals are exactly alike — even identical twins show some differences due to their environments.

Variation can be caused by:

1. Genetic factors (inherited from parents)
2. Environmental factors (the conditions in which an organism lives)
3. A combination of both (most characteristics)

---

Genetic Variation

Genetic variation is caused by differences in the genes (and alleles) that organisms inherit from their parents. There are several sources of genetic variation:

Source	How it creates variation
Sexual reproduction	Offspring receive a unique combination of alleles from two parents
Meiosis	During gamete formation, chromosomes are shuffled randomly (independent assortment) and sections of chromosomes are swapped (crossing over), creating genetically unique gametes
Random fertilisation	Any sperm can fuse with any egg, creating a unique combination of alleles
Mutations	Random changes to the DNA base sequence can create new alleles

Why sexual reproduction creates more variation than asexual reproduction:

Feature	Sexual reproduction	Asexual reproduction
Number of parents	Two	One
Gametes involved	Yes (sperm and egg)	No
Offspring genetically identical?	No — all genetically unique	Yes — clones of the parent
Sources of variation	Meiosis, random fertilisation, crossing over	Mutations only
Evolutionary advantage	Greater variation helps population adapt to changing environments	Rapid reproduction, no need to find a mate

Exam Tip: A very common exam question asks why organisms that reproduce sexually show more variation than those that reproduce asexually. The answer must include: mixing of alleles from two parents, independent assortment in meiosis, and random fertilisation.

---

Environmental Variation

Environmental variation is caused by the conditions in which an organism lives and develops. The environment can affect how genes are expressed, even if the genetic makeup is the same.

Examples of environmental variation:

Factor	Example of its effect
Diet and nutrition	A plant given more fertiliser grows taller; a person with a poor diet may not reach their full potential height
Sunlight	Plants grow towards light; skin colour can darken with sun exposure
Climate and temperature	Himalayan rabbits have darker fur on cooler extremities (ears, nose, feet)
Exercise	Regular exercise increases muscle mass regardless of genetic potential
Disease and injury	Scars, amputations, and illnesses can alter physical characteristics
Soil conditions	Identical plant seeds grown in different soils produce plants of different sizes

---

The Interaction of Genes and Environment

Most characteristics are influenced by both genetic and environmental factors. This is sometimes called nature vs nurture.

Characteristic	Genetic influence	Environmental influence
Height	Genes determine maximum potential height	Diet, exercise, and health affect actual height achieved
Weight	Genes influence metabolic rate and body shape	Diet, exercise, and lifestyle have a major effect
Skin colour	Genes determine baseline skin colour	Sun exposure causes tanning
Intelligence	Genes contribute to cognitive potential	Education, nutrition, and stimulation affect development
Athletic performance	Genes influence muscle fibre type, lung capacity	Training, diet, and practice determine actual performance

graph TD
    A[Variation in characteristics] --> B[Genetic causes]
    A --> C[Environmental causes]
    A --> D[Both genetic and environmental]
    B --> E[Examples: blood group, eye colour, inherited disorders]
    C --> F[Examples: scars, accent, tattoos]
    D --> G[Examples: height, weight, skin colour, intelligence]

Exam Tip: When asked whether a characteristic is caused by genes, environment, or both, think carefully. Very few characteristics are purely genetic (blood group is one). Very few are purely environmental (scars, language spoken). Most are influenced by both.

---

Types of Variation: Continuous vs Discontinuous

Variation can also be classified by how it is distributed in a population:

Continuous Variation

Continuous variation shows a range of values between two extremes, with no distinct categories. The data forms a normal distribution (bell curve) when plotted on a graph.

Features of continuous variation	Examples
Range of values with no clear categories	Height, weight, hand span, foot length
Usually controlled by many genes (polygenic)	Skin colour, intelligence
Strongly influenced by the environment
Displayed as a histogram or line graph
Forms a normal distribution curve

Discontinuous Variation

Discontinuous variation shows distinct, separate categories with no intermediate values.

Features of discontinuous variation	Examples
Distinct categories with no overlap	Blood group (A, B, AB, O)
Usually controlled by a single gene	Ability to roll tongue, attached vs free earlobes
Little or no environmental influence
Displayed as a bar chart
No normal distribution — separate bars for each category

Comparison Table

Feature	Continuous	Discontinuous
Values	Range (no clear groups)	Distinct categories
Number of genes	Many (polygenic)	Usually one
Environmental influence	Strong	Weak or none
Graph type	Histogram / normal distribution	Bar chart
Examples	Height, mass, skin colour	Blood group, eye colour (simplified), tongue rolling

Exam Tip: If an exam question shows a histogram with a smooth bell curve, the data shows continuous variation. If it shows a bar chart with separate categories, it shows discontinuous variation. Be ready to identify the type from a graph.

---

Measuring Variation