Welcome to Unit 1 Biology: Variation and Adaptation
Have you ever looked around your classroom and noticed how different everyone is? Some people are tall, some have curly hair, some have blue eyes, and some can roll their tongue while others cannot. These differences are what scientists call variation.
In this chapter, we will explore why living things of the same species look and behave differently, how these differences help animals and plants survive in their environments, and how Charles Darwin's Theory of Natural Selection explains the gradual change of species over time. Don't worry if biology sometimes feels overwhelming—we will break down every idea into clear, step-by-step stages!
---1. What is Variation?
Variation is defined as the differences in characteristics between individuals of the same species.
Even though all humans belong to the same species (Homo sapiens), no two people look or act completely identical (even identical twins have slight differences like fingerprints or scars!).
Types of Variation
In your CCEA GCSE exam, you need to know the two main types of variation: continuous and discontinuous.
A. Continuous Variation
Characteristics that vary across a continuous range with no distinct categories. These are measurable features that can take any numerical value within a range.
• Examples: Height, body mass (weight), foot length, hand span.
• How it is plotted: Continuous data is shown using a histogram or a smooth line curve known as a normal distribution curve (a bell-shaped curve where most individuals are in the middle average range).
B. Discontinuous Variation
Characteristics where individuals fall into distinct, separate categories with no in-between values. You either have the trait or you do not!
• Examples: Blood groups (A, B, AB, or O), earlobe attachment (free earlobes vs attached earlobes), and the ability to roll your tongue (roller vs non-roller).
• How it is plotted: Discontinuous data is always plotted on a bar chart with distinct, separate bars (leaving spaces between the bars).
Quick Review Box: Continuous vs Discontinuous
• Continuous: Measured in numbers along a range (e.g., Height). Plotted on a histogram or bell curve.
• Discontinuous: Sorted into set categories (e.g., Blood group A, B, AB, O). Plotted on a bar chart with spaces between bars.
2. Causes of Variation
Why do we have these differences? Variation can be caused by our genes, our environment, or a combination of both.
1. Genetic Variation
These are characteristics determined entirely by the genes and alleles inherited from our biological parents at fertilisation. External surroundings cannot change them.
• Examples: Eye colour, blood group, natural hair colour, inherited genetic conditions.
2. Environmental Variation
These are differences caused entirely by your surroundings, lifestyle, climate, or life experiences.
• Examples: Scars, tattoos, spoken language, regional accent.
3. Combined Genetic and Environmental Variation
Many of our most common traits are a mixture of both! You inherit a genetic potential from your parents, but your environment influences how that potential develops.
• Example 1 (Human height/weight): You might inherit genes from your parents to grow tall, but if you do not get a healthy, balanced diet with enough nutrients, you may not reach your full potential height.
• Example 2 (Plant growth): A plant may have the genes to grow large and produce bright green leaves, but without sunlight and water, it will remain small and yellow.
Key Takeaway: If a feature is coded purely in your DNA (like blood group), it is genetic. If it is caused purely by what happens to you (like a scar), it is environmental. If it is influenced by both your DNA and your lifestyle/diet (like height or weight), it is combined.
---3. Charles Darwin and Natural Selection
How do living organisms become so well suited to their habitats? The British naturalist Charles Darwin proposed the mechanism of Natural Selection, which explains how evolution happens over time.
The 6 Steps of Natural Selection (A Step-by-Step Guide)
In extended 6-mark exam questions, CCEA examiners look for these specific steps in order:
1. Overproduction: Organisms produce more offspring than the environment's resources can support.
2. Competition: Because resources are limited, individuals must compete with one another for food, water, shelter, territory, light (for plants), and mates.
3. Variation: Natural phenotypic differences exist among individuals in a population (caused by random mutations and genetic differences).
4. Survival of the Fittest: Individuals with characteristics or adaptations best suited to their environment have a selective advantage and are more likely to survive.
5. Reproduction and Inheritance: The surviving individuals reproduce and pass on their advantageous genes/alleles to their offspring.
6. Evolution over Time: Over many generations, the frequency of the advantageous characteristic increases in the population.
Easy Memory Aid: "Oh, Cold Vipers Sneak Right Everywhere!"
• Overproduction
• Competition
• Variation
• Survival of the fittest
• Reproduction & Inheritance
• Evolution over time
4. Key Case Studies
Case Study 1: The Peppered Moth (Biston betularia)
The peppered moth is a famous real-world demonstration of natural selection in action during the Industrial Revolution in Britain.
1. Before the Industrial Revolution:
• Tree trunks were light-coloured and covered in pale lichens.
• Pale/speckled moths were well camouflaged against the tree bark, protecting them from predatory birds.
• Dark (melanic) moths stood out easily against the light bark, so birds spotted and ate them.
• Result: Pale moths were common; dark moths were rare.
2. After Industrialisation and Pollution:
• Heavy coal smoke and soot covered the trees, killing the pale lichens and blackening the tree trunks.
• Dark moths were now perfectly camouflaged on the soot-covered trees.
• Pale moths were easily seen by birds and heavily predated.
• Result: Dark moths were more likely to survive, reproduce, and pass on the gene for dark colour. Over generations, dark moths became the dominant form in industrial areas.
Case Study 2: Antibiotic Resistance in Bacteria
Bacteria reproduce very rapidly, allowing us to observe natural selection over very short time periods.
• A random genetic mutation occurs in a single bacterium, giving it resistance to a specific antibiotic medicine.
• When a person takes the antibiotic, the non-resistant bacteria are killed.
• The resistant bacterium survives because the antibiotic cannot harm it.
• The surviving bacterium multiplies rapidly without competition, passing the resistance gene to millions of offspring.
• Eventually, the entire population becomes resistant to that antibiotic.
5. Extinction
What happens if the environment changes and a species does not have the adaptations needed to survive?
Extinction occurs when an entire species completely dies out and no living individuals remain.
Causes of Extinction Include:
• Rapid climate change or environmental shifts.
• Introduction of a new predator that hunts the species faster than it can reproduce.
• Introduction of a new disease to which the population has no natural immunity.
• Habitat destruction or severe human activity (such as poaching or deforestation).
• Inability to compete successfully with a new invasive species for food and resources.
6. Examiner Tips & Common Pitfalls to Avoid
CCEA examiners regularly point out common student mistakes in their yearly reports. Keep these tips in mind to maximise your marks:
Mistake 1: Lamarckian Thinking ("Animals change on purpose")
Incorrect: "Giraffes stretched their necks because they needed to reach high leaves."
Correct: Random variation and mutations already existed in the population. Giraffes born with slightly longer necks had a survival advantage, allowing them to feed better, survive, and reproduce.
Mistake 2: Forgetting the Reproduction Step
In 6-mark questions, many students write that the fittest animals "survive", but stop there! You must always state that they survive AND reproduce, passing on the advantageous alleles/genes to their offspring.
Mistake 3: Confusing Graphs for Variation
• Use a bar chart (with separate bars) for discontinuous data like blood groups.
• Use a histogram or continuous distribution curve for continuous data like height.
Mistake 4: Saying Humans Become Resistant to Antibiotics
Humans do not become resistant to antibiotics. It is the bacteria that mutate and develop resistance!
Chapter Summary Checklist
Can you answer these quick revision checks?
• Can I state the definition of variation?
• Can I identify continuous vs discontinuous characteristics and their correct graph types?
• Can I give examples of genetic, environmental, and combined variation?
• Can I write down the 6 key steps of natural selection in order?
• Can I explain the peppered moth story before and after the Industrial Revolution?
• Can I explain how bacteria become resistant to antibiotics?
• Can I list at least three factors that lead to extinction?