Covers Lessons 10–14: sources of variation, allele frequency, types of selection, adaptations, speciation and the key evidence for human evolution.
Lesson Summaries
Evolution acts on variation that already exists in populations. Mutation creates new alleles, recombination reshuffles them, and gene flow moves them between populations. Allele frequency measures how common an allele is in the gene pool, and natural selection changes those frequencies over generations. Genetic drift can also change allele frequencies, but it does so randomly rather than through fitness advantage.
Directional selection favours one extreme phenotype and shifts the distribution. Stabilising selection favours the intermediate phenotype and narrows variation around the mean. Disruptive selection favours both extremes and can split the distribution into two peaks. Reading bell-curve patterns is central to recognising these selection types in unfamiliar scenarios.
An adaptation is an inherited characteristic that increases fitness in a particular environment and results from natural selection over generations. Structural adaptations involve body form, physiological adaptations involve internal processes, and behavioural adaptations involve actions or response patterns. Convergent adaptation explains why unrelated organisms can evolve similar features under similar selection pressures.
Speciation is the divergence of one ancestral species into reproductively isolated populations. In allopatric speciation, a geographic barrier reduces gene flow, allowing populations to diverge through mutation and selection until reproductive isolation is complete. Reproductive barriers can be pre-zygotic or post-zygotic, and sympatric speciation can also occur without geographic separation, especially through polyploidy in plants.
Humans are one of the great apes and share a common ancestor with other apes. The hominid record shows major changes in bipedalism, brain size, jaw structure and tool use across species such as Australopithecus afarensis, Homo habilis, Homo erectus, Homo naledi and Homo sapiens. Fossils, tools and ancient DNA together support a branching evolutionary tree rather than a simple linear chain.
1. What is the ultimate source of new alleles in a population?
2. Which statement best distinguishes genetic drift from natural selection?
3. Which graph pattern best indicates stabilising selection?
4. Why can disruptive selection be important in speciation?
5. Which example is a physiological adaptation?
6. Which statement best explains convergent adaptation?
7. What is the defining feature of the biological species concept?
8. Which example is a post-zygotic barrier?
9. Which statement about humans and chimpanzees is scientifically accurate?
10. Which anatomical feature gives strong evidence for bipedalism?
11. Why is Homo naledi especially important in this topic?
12. What is the best overall model of human evolution?
13. Explain how mutation, heritability and selection pressure can lead to a change in allele frequency over generations. 4 MARKS
1 mark: mutation/variation | 1 mark: heritability | 1 mark: selection pressure/differential reproduction | 1 mark: allele frequency change
14. Distinguish between structural, physiological and behavioural adaptations using one example for each. 4 MARKS
1 mark each for three correct definitions/examples, plus 1 mark for clear distinction
15. Assess the statement: “Human evolution is best understood as a branching tree rather than a straight line.” In your answer, refer to both fossil evidence and one named hominid species. 4 MARKS
1 mark: judgement | 1 mark: fossil evidence idea | 1 mark: named species example | 1 mark: evaluative conclusion
13. Mutation creates new alleles and therefore provides variation within the population. If that variation is heritable, offspring can inherit the alleles from their parents. A selection pressure then causes some variants to survive or reproduce more successfully than others. Over many generations, the favoured allele becomes more common in the gene pool, so allele frequency changes and the population evolves.
14. A structural adaptation is a physical feature of anatomy, such as thorny devil skin grooves or acacia phyllodes. A physiological adaptation is an internal functional process, such as a desert mammal producing concentrated urine or red kangaroo embryonic diapause. A behavioural adaptation is an action or response pattern, such as crepuscular activity or migration. The distinction is whether the adaptive feature is body form, internal function or behaviour.
15. The statement is accurate because the hominid fossil record shows overlapping species and mixed trait combinations rather than one straight ladder from primitive to advanced forms. Fossils provide direct evidence of anatomy, including changes in bipedalism, skull shape and brain size, but they also show that different traits did not all evolve in one simple sequence. Homo naledi is a strong example because it combines primitive and more modern features, supporting a branching model of human evolution. The best conclusion is that human evolution is a branching tree reconstructed from multiple lines of evidence, not a simple line of replacement.