Unit 4: How Does Life Change Over Time?
Evolution as the unifying idea
Unit 4 examines how populations change over time. You must explain evolution as a change in allele frequencies in a population, and identify its causes: mutation, gene flow, genetic drift, and natural selection. Distinguish the different types of selection (directional, stabilising, disruptive) and understand the evidence for evolution, including fossils, comparative anatomy, embryology, biogeography and molecular comparisons such as DNA and protein sequences. Know that similarities may reflect common ancestry (homologous structures) or convergent evolution (analogous structures).
Populations, species and humans
Understand the biological species concept and its limitations, reproductive isolating mechanisms, and speciation (allopatric and sympatric). Be able to apply Hardy–Weinberg reasoning conceptually, recognising that allele frequencies remain constant only under stated conditions. Describe the evidence for hominin evolution, including bipedalism, brain size changes and tool use, and interpret phylogenetic trees. Explain how human activities affect evolutionary processes.
Traps examiners set
- Saying individuals evolve; populations evolve, and individuals are selected.
- Confusing homologous (shared ancestry) with analogous (similar function, different origin) structures.
- Treating natural selection as goal-directed or as producing "perfect" organisms.
- Claiming a mutation arises because it is needed; mutations occur randomly with respect to fitness.
- Misreading phylogenetic trees: branch tips are contemporary, not ancestors of one another.
Worked example and rule of thumb
In a moth population, a dark variant survives better on soot-darkened bark and leaves more offspring, so the dark allele's frequency rises. Rule of thumb: selection acts on phenotypes, but evolution is measured as change in allele frequencies across generations. Always state the selection pressure, the heritable variation and the reproductive consequence.
Practical skills
Interpret data on allele frequencies and phylogenies, and evaluate whether conclusions are supported by evidence.