07/09/2026
*Grade 12 Life Science: Evolution notes
1. *What is Evolution?*
*Evolution* is the change in the genetic composition of a population over generations.
It does not happen to individuals – it happens to populations over time.
*Two main ideas:*
1. *Lamarckism*: "Use and disuse theory". Organisms acquire traits during life and pass them to offspring. _Incorrect_ – acquired traits aren’t inherited.
2. *Darwin’s Theory of Natural Selection*: Organisms best suited to their environment survive and reproduce more.
2. *Evidence for Evolution*
*a. Fossil Evidence*
- Fossils are remains of organisms that lived long ago.
- Older fossils are found in lower rock layers, younger ones on top.
- *Transitional fossils* like _Archaeopteryx_ show changes from reptiles to birds.
*b. Comparative Anatomy*
- *Homologous structures*: Same basic structure, different function. E.g. human arm, bat wing, whale flipper. Shows common ancestry.
- *Analogous structures*: Same function, different structure. E.g. insect wing vs bird wing. Shows convergent evolution.
- *Vestigial organs*: Organs with no function now, e.g. human appendix.
*c. Comparative Embryology*
Embryos of different vertebrates look very similar in early stages, suggesting common ancestry.
*d. Molecular Evidence*
Organisms with closer evolutionary relationships have more similar DNA and proteins. E.g. humans and chimps share ∼98% DNA.
*e. Biogeography*
Distribution of species matches continental drift. E.g. most marsupials are found in Australia.
3. *Mechanisms of Evolution*
*a. Natural Selection* – 5 steps:
1. *Overproduction*: More offspring are produced than can survive.
2. *Variation*: Individuals differ due to mutations and genetic recombination.
3. *Competition*: Limited resources cause struggle for survival.
4. *Survival of the fittest*: Individuals with favorable traits survive and reproduce.
5. *Reproduction*: Favorable traits are passed on, changing the population.
*Types of Natural Selection:*
- *Directional*: One extreme trait is favored, e.g. longer giraffe neck.
- *Stabilising*: Intermediate trait is favored, e.g. human birth weight.
- *Disruptive*: Both extremes are favored, middle is eliminated.
*b. Genetic Drift*
Random change in allele frequency, especially in small populations. Important in bottleneck and founder effects.
*c. Gene Flow*
Movement of alleles into and out of a population through migration.
4. *Speciation – Formation of New Species*
*Speciation* is the formation of a new species that cannot interbreed with the original.
*Allopatric Speciation* – most common in Grade 12:
1. A population is split by a geographical barrier like a river or mountain.
2. The two groups live in different environments.
3. Mutation and natural selection cause them to diverge over time.
4. If they meet again, they can no longer interbreed – a new species has formed.
5. *Human Evolution*
*Key hominids:*
- *Australopithecus afarensis*: "Lucy", bipedal, small brain.
- *Homo habilis*: First tool maker.
- *Homo erectus*: First to leave Africa, used fire.
- *Homo neanderthalensis*: Lived in Europe, large brain.
- *Homo sapiens*: Modern humans. Originated in Africa ∼200,000 years ago.
*Evidence humans evolved in Africa:*
- Oldest _Homo sapiens_ fossils found in Africa.
- Greatest genetic diversity is in African populations.
6. *Key Points to Remember*
1. *Evolution is not goal-directed* – there’s no plan for “improvement”. It’s driven by chance and selection.
2. *Mutations provide raw material* for evolution. No mutation = no new variation.
3. *Populations evolve*, not individuals.
4. *Microevolution* = small changes within a species. *Macroevolution* = formation of new species.
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*Exam tip*: Most questions come from natural selection, evidence for evolution, and human evolution. Practice drawing and interpreting graphs for directional, stabilising, and disruptive selection.
