Welcome to Variation and Natural Selection!
Have you ever looked around your classroom and noticed how different everyone looks? Some people are tall, some have curly hair, and some have blue eyes. Even though we are all the same species (Homo sapiens), no two humans are completely identical (except for identical twins!).
In this chapter, we will explore why living things are different from one another, how these differences help organisms survive in the wild, and how humans have learned to use this to breed animals and plants with features we want. Don't worry if biology sometimes feels full of tricky terms—we will break every idea down step-by-step with clear examples!
1. What is Variation?
Variation refers to the differences that exist between individuals of the same species.
There are two main types of variation that you need to know for your exam:
A. Continuous Variation
• Definition: Variation that has a continuous range of values between a minimum and a maximum, with no distinct categories.
• Key feature: It is quantitative (it can be measured on a scale with numbers).
• Examples: Height, body mass (weight), hand span, and shoe size.
• How it is displayed on a graph: A histogram that forms a smooth, bell-shaped curve (known as a normal distribution curve). Most people are around the average value in the middle, while very few are at the extreme ends (extremely tall or extremely short).
• Causes: It is usually caused by a combination of genes and the environment (for example, your genes give you the potential to be tall, but poor nutrition can limit your growth).
B. Discontinuous Variation
• Definition: Variation where individuals fall into distinct, clearly defined categories with no in-between values.
• Key feature: It is qualitative (you either have the feature or you do not).
• Examples: ABO blood group (you are group A, B, AB, or O), ability to roll your tongue, and attached vs. free earlobes.
• How it is displayed on a graph: A bar chart with distinct separate bars and gaps between them.
• Causes: It is caused almost entirely by genes alone (your environment cannot change your blood group!).
Causes of Variation: Nature vs. Nurture
Why do these differences exist? There are two main causes:
1. Genetic factors (inherited): Controlled entirely by the alleles passed on from your biological parents during sexual reproduction. Examples include natural eye colour, natural hair colour, and blood group.
2. Environmental factors: Differences caused by your surroundings, lifestyle, and choices. Examples include scars, tattoos, language spoken, and dyed hair colour.
3. Combined factors (Genes + Environment): Many characteristics are influenced by both. For example, your skin colour is genetically determined, but sunlight (environment) causes your skin to tan and become darker.
Common Mistake to Avoid:
Students often think weight or height is purely genetic. Remember: you inherit genes for body size, but diet and exercise (environmental factors) have a massive impact on your final weight and height!
Key Takeaway for Section 1:
Continuous variation has a continuous range with no separate groups (e.g. height, bell curve), caused by genes + environment. Discontinuous variation falls into distinct categories (e.g. blood group, bar chart), caused by genes alone.
2. Mutations: The Source of New Variation
Where does brand-new genetic variation come from in the first place? The answer is mutation.
• What is a mutation? A mutation is a random, spontaneous change in the structure of a gene or chromosome (a change in the DNA sequence).
• Mutations create new alleles (different versions of a gene).
Effects of Mutations
Mutations can have three possible effects on an organism:
• Harmful: Most mutations disrupt normal body functions (e.g., genetic conditions like cystic fibrosis or mutations leading to uncontrolled cell division called cancer).
• Neutral: Some mutations have no noticeable effect on an organism's survival (e.g., having detached earlobes).
• Beneficial: A few rare mutations give an organism an advantage in its environment (e.g., a bacterium mutating to become resistant to an antibiotic, or a prey animal developing camouflage colouring).
What Increases the Rate of Mutations?
Mutations happen naturally and randomly all the time. However, exposure to certain environmental factors called mutagens can increase the mutation rate:
• Ionising radiation: Ultraviolet (UV) rays from sunlight, X-rays, and gamma rays.
• Chemical mutagens: Chemicals found in tobacco smoke, tar, and certain industrial pollutants.
Key Takeaway for Section 2:
A mutation is a spontaneous, random change in DNA. While many are harmful or neutral, rare beneficial mutations provide the raw genetic material needed for evolution.
3. Natural Selection and Evolution
Evolution is the gradual change in a species over long periods of time (many generations). The mechanism that drives evolution was proposed by Charles Darwin and is known as natural selection (often nicknamed "survival of the fittest").
Step-by-Step: How Natural Selection Works
Whenever you are asked to explain natural selection in an exam, remember these 5 key steps:
1. Variation: There is natural genetic variation within a population of the same species due to random mutations and different alleles.
2. Overproduction & Competition: Organisms produce more offspring than the environment can support. This leads to a struggle for survival (competition for limited resources like food, water, light, mates, or shelter, as well as avoiding predators and disease).
3. Survival of the Fittest: Individuals possessing advantageous alleles/adaptations best suited to their environment are more likely to survive.
4. Reproduction: The surviving individuals reproduce and pass on their advantageous alleles to the next generation.
5. Evolution Over Time: Over many generations, the frequency of the advantageous allele increases in the population. The species becomes better adapted to its environment—this is evolution.
Memory Trick (V-C-S-R-E):
• Variation (random differences exist)
• Competition (struggle for resources)
• Survival (best-adapted survive)
• Reproduction (pass on beneficial alleles)
• Evolution (allele becomes more common over time)
Classic Real-World Example: Antibiotic Resistance in Bacteria
Natural selection is not just something that happened millions of years ago—it is happening right now in hospitals!
• Within a large population of bacteria, a random mutation produces a new allele that gives an individual bacterium resistance to an antibiotic (such as penicillin).
• When the patient takes the antibiotic, the non-resistant bacteria are killed, but the resistant bacterium survives.
• With all competition eliminated, the resistant bacterium rapidly reproduces by binary fission.
• All of its offspring inherit the resistant allele.
• Soon, the entire bacterial population is resistant to the antibiotic (for example, the superbug MRSA). The antibiotic is no longer effective!
Did You Know?
To prevent the spread of antibiotic resistance, doctors avoid over-prescribing antibiotics for minor viral infections (like colds or flu, which antibiotics cannot kill anyway), and patients are always advised to complete their full course of medication.
Key Takeaway for Section 3:
Natural selection occurs because individuals with advantageous characteristics are more likely to survive, reproduce, and pass on their beneficial alleles to their offspring.
4. Extinction
If the environment changes rapidly and a species lacks the variation needed to adapt, all members of that species may die out. This is called extinction.
Causes of Extinction
• Climate change: Rapid changes in temperature or weather patterns (e.g. ice ages or global warming).
• Habitat destruction: Deforestation, urbanization, and pollution destroying living spaces and food sources.
• New predators: Introduction of a predator that native prey species have no defence against.
• New diseases: An infectious pathogen spreading through a population with no natural immunity.
• Increased competition: A new, better-adapted competitor arriving and taking away food or nesting sites.
Key Takeaway for Section 4:
Extinction occurs when a species is unable to adapt quickly enough to environmental changes, causing every individual of that species to die.
5. Selective Breeding (Artificial Selection)
While natural selection is driven by the environment, selective breeding (also called artificial selection) is controlled entirely by humans.
• Definition: The process by which humans breed plants and animals for particular, desirable characteristics over many successive generations.
How Selective Breeding is Carried Out (Step-by-Step)
1. Humans choose individual parents from a population that show the desired characteristic.
2. These selected individuals are bred together.
3. From the resulting offspring, the individuals that display the best expression of the desired trait are selected.
4. These chosen offspring are bred together.
5. This process is repeated over many generations until all offspring reliably show the desired characteristic.
Examples of Selective Breeding
• Dairy cattle: Bred for high milk yield and gentle temperament.
• Beef cattle: Bred for fast muscle growth and high meat yield.
• Wheat and crop plants: Bred for high crop yield, large grain size, and resistance to fungal diseases and drought.
• Domestic dogs: Bred for specific tasks (e.g., greyhounds for speed, sheepdogs for herding) or appearance.
Disadvantages and Problems of Selective Breeding
Although selective breeding produces organisms that are useful to humans, it has serious biological drawbacks:
• Reduced Gene Pool (Decreased Genetic Variation): Breeding closely related individuals (inbreeding) reduces the number of different alleles in the population.
• Vulnerability to Disease: If all individuals in a crop or herd are genetically similar, a single new disease could potentially wipe out the entire population because none have natural resistance.
• Inherited Physical Defects: Inbreeding increases the chance of offspring inheriting two copies of harmful recessive alleles. For example, pedigree dogs often suffer from hip dysplasia (German Shepherds) or severe breathing difficulties (Pugs and Bulldogs with flattened faces).
Comparing Natural Selection and Selective Breeding
• Natural Selection: Driven by environmental pressures; features selected improve the organism's chances of survival and reproduction in the wild; happens slowly over very long periods.
• Selective Breeding: Driven by human choice; features selected are useful or attractive to humans (even if they harm the animal's survival in the wild); happens much more rapidly over fewer generations.
Key Takeaway for Section 5:
Selective breeding involves humans repeatedly mating individuals with desirable traits over generations. However, it leads to inbreeding, reduced genetic variation, and health problems in animals.
Chapter Quick Review
• Continuous variation: Range of values, no distinct groups (height); caused by genes and environment.
• Discontinuous variation: Distinct categories (blood group); caused by genes alone.
• Mutation: A spontaneous, random change in DNA that can produce new alleles.
• Natural selection: Variation \(\rightarrow\) Competition \(\rightarrow\) Survival of the fittest \(\rightarrow\) Reproduction \(\rightarrow\) Advantageous alleles passed on \(\rightarrow\) Evolution.
• Extinction: Complete loss of a species when it cannot adapt to environmental changes.
• Selective breeding: Humans choosing desirable traits over many generations; leads to reduced genetic diversity and health issues.