Unit Overview
Variation and Selection
CIE IGCSE Biology· 5 min read 📊 8-10% of Paper 1 (MCQ) and 10-12% of Paper 2/3/4 (theory)
1. Unit at a glance
You will begin by exploring the difference between genetic and environmental sources of phenotypic variation, including the role of random and induced mutations as the original source of all new alleles in a population. You will then connect this variation to selection processes, exploring how trait frequencies change over time in both natural ecosystems and human-managed agricultural settings.
Work through the following sub-topics in order to master this unit:
Variation and Mutation
Covers types of variation, causes of mutation, and the effects of mutation on phenotype and population genetic diversity.
★★⏱ 8 min
Adaptive Features, Natural Selection and Selective Breeding
Explains adaptive features, the process of natural selection, evolution, and the methods and real-world applications of selective breeding.
★★★⏱ 10 min
2. Common Pitfalls
Wrong move:
Confusing genetic and environmental variation, assuming all phenotypic differences are heritable
Why:
Only variation caused by differences in DNA sequence can be passed to offspring and acted on by selection processes
Correct move:
Test for heritability by comparing genetically identical organisms raised in different environments, or tracking trait passage across generations
Wrong move:
Stating that mutations are always harmful or always beneficial
Why:
Most mutations are neutral, some are harmful, and a small number confer a selective advantage depending on specific environmental conditions
Correct move:
Evaluate the effect of a mutation relative to the specific habitat and selection pressures the organism experiences
Wrong move:
Claiming that natural selection gives organisms the traits they 'need' to survive
Why:
Natural selection acts on existing random variation, it does not produce new traits in response to an organism’s survival needs
Correct move:
Frame selection as a filtering process that increases the frequency of pre-existing advantageous traits in a population over generations
3. Quick Reference Cheatsheet
Term/Concept | Definition | Related Sub-topic |
|---|---|---|
Phenotypic variation | Differences in observable traits between individuals of the same species, caused by genes, environment, or a combination of both | Variation and Mutation |
Mutation | A random, spontaneous change in the DNA sequence of a gene or chromosome, the source of all new alleles in a population | Variation and Mutation |
Adaptive feature | An inherited trait that increases an organism’s chance of survival and reproduction in a specific environment | Adaptive Features, Natural Selection and Selective Breeding |
Natural selection | Process by which individuals with advantageous traits are more likely to survive, reproduce, and pass their alleles to offspring | Adaptive Features, Natural Selection and Selective Breeding |
Selective breeding | Human-led process of breeding organisms with desired heritable traits to produce offspring that express those traits consistently | Adaptive Features, Natural Selection and Selective Breeding |
What's Next
Start your study of this unit with the first sub-topic on Variation and Mutation to build foundational knowledge of how variation arises in populations. Mastering the difference between heritable and non-heritable variation is critical to understanding selection processes covered in the second sub-topic. Once you complete both sub-topics in this unit, you will move on to the next unit on Organisms and their Environment, which builds on adaptation concepts to explore ecosystem dynamics.