Understanding Variation and selection: IGCSE Biology Explained

Why no two organisms are identical

Line up thirty students in your year group and measure their heights. You will get thirty different numbers. Ask each of them for their blood type and you will get one of four letters: A, B, AB or O. Both sets of results show variation, but they behave in fundamentally different ways, and understanding that difference is one of the first things the IGCSE Biology examiner will test you on.

Variation is the existence of differences between individuals of the same species. It sounds simple, but the topic branches into genetics, environment, adaptation, natural selection and even antibiotic resistance. Getting the logic right here sets up nearly every genetics and ecology question you will face.

Two types of variation

The Cambridge IGCSE syllabus draws a hard line between two categories. Mixing them up is one of the most common mark-losing mistakes in the exam.

Why continuous variation involves the environment

Height is a useful example. Your genes set a potential range, but your actual height depends on nutrition, health during childhood, and other environmental factors. Two people with the same height-related genes can end up at different points on the curve if one had better nutrition. That is why continuous variation is described as being caused by both genes and the environment.

Discontinuous traits like blood type, on the other hand, are fixed by your genotype. No amount of diet or exercise will change your blood group from A to B. One gene (or a small number of genes) determines the outcome, and the environment has no influence.

Extended: Mutation as a source of variation

A mutation is a change in a gene or chromosome. Mutations are the ultimate source of new genetic variation because they create new alleles that did not previously exist in a population.

• Mutations are rare and random: they are not directed toward any particular outcome.
• Most mutations are neutral or harmful. Occasionally, one produces a beneficial change.
• The rate of mutation can be increased by exposure to ionising radiation (such as ultraviolet light, X-rays or gamma rays) or certain chemicals called mutagens.

Adaptive features

An adaptation is a feature of an organism that increases its chances of survival and reproduction in its environment. Adaptations arise because individuals with those features are more likely to survive, reproduce and pass on the genes responsible.

Worked example 1: The cactus (xerophyte)

A cactus lives in a hot, dry desert. Its adaptive features can be mapped directly to the problems that environment poses.

• Problem: water loss through transpiration. Adaptation: leaves reduced to spines (minimal surface area for evaporation), thick waxy cuticle on the stem, stomata sunken or few in number.
• Problem: infrequent rainfall. Adaptation: extensive shallow root system to absorb water quickly after rain, thick fleshy stem to store water.
• Problem: herbivory. Adaptation: spines deter animals from eating the plant.

Worked example 2: The polar bear (predator adaptation)

• White fur: camouflage against snow and ice, allowing the bear to approach prey without being seen.
• Thick layer of body fat: insulation against extreme cold and an energy reserve when food is scarce.
• Large body size: reduces the surface-area-to-volume ratio, minimising heat loss.
• Large paws: spread weight on thin ice and provide grip.

Natural selection: the mechanism step by step

Natural selection is the process by which organisms with features best suited to their environment are more likely to survive and reproduce. Over many generations, this shifts the characteristics of a population. The logic follows a chain of five steps.

1. Variation exists within a population due to genetic differences (ultimately from mutation).
2. Competition occurs for limited resources such as food, water, territory and mates. More offspring are produced than the environment can support.
3. Survival of the fittest: individuals with features better suited to the environment are more likely to survive. "Fittest" means best adapted, not physically strongest.
4. Reproduction: survivors are more likely to reproduce and pass on the alleles responsible for the advantageous features.
5. Inheritance: over many generations, the proportion of the advantageous allele increases in the population, and the population evolves.

Extended: Antibiotic resistance as a worked example

Antibiotic resistance in bacteria is one of the clearest real-world demonstrations of natural selection, and it appears frequently in IGCSE exams. Here is how to walk through it using the five steps above.

1. Variation: within a population of bacteria, a random mutation occurs in one individual that makes it resistant to an antibiotic.
2. Competition: when the antibiotic is applied, all bacteria compete to survive in the presence of the drug.
3. Survival: non-resistant bacteria are killed. The resistant individual survives because the antibiotic cannot affect it.
4. Reproduction: the resistant bacterium reproduces rapidly (bacteria can divide every 20 minutes), passing on the resistance gene to all its offspring.
5. Inheritance over generations: the entire new population is resistant. The antibiotic is now ineffective against this strain.

Selective breeding (artificial selection)

Selective breeding is the process by which humans choose organisms with desirable characteristics and breed them together. Over several generations, the desired trait becomes more common in the population.

The logic mirrors natural selection, but with one key difference: the selection pressure is human choice, not environmental fitness.

• Step 1: identify individuals with the desired trait (e.g. cows that produce the most milk).
• Step 2: breed those individuals together.
• Step 3: from the offspring, select those that show the trait most strongly and breed them again.
• Step 4: repeat over many generations until the trait is reliably present.

Examples: high-yield crop varieties, disease-resistant wheat, cows with increased milk production, dogs bred for specific temperaments or physical features.

Disadvantage: selective breeding reduces genetic variation within the population. If all individuals are genetically similar, a single new disease could wipe out the entire population because none have resistance. This is the same vulnerability that makes monocultures risky in agriculture.

Common exam mistakes

1. Saying "survival of the fittest" means the strongest. "Fittest" in biology means best adapted to the environment. A small, camouflaged moth is "fitter" than a large, visible one if predators spot the visible one first.
2. Stating that antibiotics cause mutations in bacteria. The mutation happens randomly before exposure. The antibiotic is a selection pressure, not a mutagen.
3. Confusing continuous and discontinuous variation. If the question gives you height data with a bell curve, it is continuous. If it gives you blood groups in a bar chart with gaps, it is discontinuous. Check both the data type and the graph.
4. Writing that the environment "causes" evolution directly. The environment provides the selection pressure. Variation already exists in the population from genetic differences. The environment does not create new alleles; it selects for alleles that are already present.
5. Forgetting to mention inheritance when describing natural selection. The chain must end with the advantageous allele being passed to offspring. Without inheritance, the trait dies with the individual and the population does not change.
6. Mixing up natural and artificial selection. In natural selection, the environment determines which organisms survive. In artificial selection, humans choose which organisms breed. The mechanism (variation, selection, inheritance) is the same; the selecting agent is different.

Self-check questions

1. Give one example of continuous variation and one example of discontinuous variation. State what causes each type.
2. A population of beetles lives on a dark-coloured tree trunk. Most beetles are brown, but a few are green. A bird that eats beetles hunts by sight. Explain, using the steps of natural selection, what would happen to the proportion of brown and green beetles over many generations.
3. Explain why selective breeding can make a population more vulnerable to disease.
4. A farmer wants to breed chickens that lay more eggs. Describe the steps the farmer would follow using selective breeding.
5. Extended: Explain how a population of bacteria can become resistant to an antibiotic. In your answer, state the role of mutation and the role of the antibiotic.