Unit Overview
Selection and Evolution
CIE A-Level BiologyΒ· 5 min read π n/a
1. Unit at a Glance
This unit follows a logical sequence from the source of diversity within populations to the origin of entirely new species. We start by examining how variation arises and is maintained, then move to how selection acts on that variation to change allele frequencies over generations.
After covering both natural and human-driven artificial selection, we end by exploring how populations diverge to become separate species, and the barriers that maintain that separation. All concepts build sequentially, so mastering earlier topics will make later, more complex ideas much easier to follow.
This unit is divided into the following connected sub-topics:
Variation
Explore the types, causes, and evolutionary importance of genetic and phenotypic variation in populations.
β β β± 7 min
Natural selection
Learn how environmental pressures act on phenotypic variation to change allele frequencies over generations.
β β β β± 8 min
Evolution by natural selection
Connect natural selection to observable adaptations and real-world examples of evolutionary change.
β β β β± 8 min
Artificial selection
Compare human-driven selection for desired traits to natural selection, with examples of agriculture and breeding.
β β β± 6 min
Speciation
Understand how evolution leads to new species, comparing allopatric and sympatric speciation.
β β β β β± 9 min
Isolating mechanisms
Describe prezygotic and postzygotic barriers that prevent gene flow between distinct species.
β β β β β± 7 min
2. Common Pitfalls
Wrong move:
Confusing evolution with change within an individual
Why:
Evolution occurs at the population level over generations, not within a single individual during its lifetime
Correct move:
Remember: populations evolve (as advantageous alleles become more common over time), individual organisms do not evolve
Wrong move:
Mixing up natural selection and evolution
Why:
Natural selection is the mechanism that drives evolution, they are not interchangeable terms
Correct move:
Natural selection = differential survival/reproduction; evolution = change in allele frequency in a population over time
Wrong move:
Assuming all phenotypic variation is heritable
Why:
Many phenotypic differences are caused by environmental factors, not genetic differences
Correct move:
Only genetic variation can be acted on by selection to drive evolutionary change
3. Quick Reference Cheatsheet
Concept | Key Summary |
|---|---|
Continuous variation | Polygenic control, range of overlapping phenotypes (e.g. height in humans) |
Discontinuous variation | Single gene control, distinct non-overlapping phenotypes (e.g. ABO blood groups) |
Natural selection requirements | Heritable variation + differential survival/reproduction of individuals |
Evolution definition | Change in allele frequency of a population over generations |
Artificial selection | Human-driven selection for desired traits, faster evolution than natural selection |
Allopatric speciation | Speciation driven by geographic isolation of populations |
Sympatric speciation | Speciation without geographic isolation (e.g. polyploidy in plants) |
Isolating mechanisms | Prezygotic = prevent fertilization; Postzygotic = reduce hybrid fitness after fertilization |
What's Next
Start your study of this unit by exploring the foundation of all selection and evolution: variation. Mastering this topic first will help you build a solid understanding of how selection acts on heritable differences to drive evolutionary change. Once you complete all sub-topics in this unit, you can move on to explore classification and biodiversity in the next unit.