Cambridge IGCSE Biology Notes 18: Variation and Selection

Topic 18 of Cambridge IGCSE Biology 0610 and 0970 follows differences within a species from their genetic and environmental sources to inherited adaptations and changing populations. Official sections 18.1 to 18.3 cover variation patterns, mutation, hydrophytes and xerophytes, natural selection, antibiotic resistance and selective breeding.

Variation within a species

Variation means differences between individuals of the same species. Variation can be observed in measurements, categories, physiology, behaviour or other phenotypes.

The pattern matters because it affects graph choice, explanation and how selection can act.

Continuous variation

Continuous variation produces a range of phenotypes between two extremes. Body length and body mass are named examples. Measurements can take many intermediate values and usually form a distribution rather than separate categories.

Continuous variation is caused by both genes and the environment. Many genes may contribute, while nutrition, health, activity and other environmental conditions influence the observed value.

A histogram is suitable when continuous measurements are grouped into class intervals. Adjacent bars touch because the scale is continuous. Sample size, measurement precision and representative sampling affect the distribution.

Discontinuous variation

Discontinuous variation produces a limited number of phenotypes with no intermediates. Cambridge names ABO blood groups, pea seed shape and pea seed colour.

It is usually caused by genes only in this level of model. Individuals fall into categories, so a bar chart with separated bars is appropriate.

Discontinuous does not mean that only two categories exist. ABO has four phenotypes, but no continuous intermediate between them.

Investigating variation

Define the population and sample before measuring. Use the same method and instrument for every individual, select participants or organisms without favouring particular outcomes and record units consistently.

For continuous data, measure rather than round individuals into vague labels. For discontinuous data, define each category before counting.

Differences within a sample do not automatically reveal cause. A phenotype may reflect genes, environment or both, and an observational investigation often cannot separate them completely.

Practical sampling, consent, measurement, data presentation and evaluation remain in the practical hub.

Mutation and genetic variation

A mutation is a genetic change. A gene mutation is a random change in the base sequence of DNA. Mutation can create a new allele.