Evolution and the History of Life on Earth
Evolution questions ask you to explain natural selection as a sequence of steps, to interpret evidence, and to place events such as the first life, the Cambrian explosion and the mass extinctions on a geological timescale.
The mechanism
- Individuals in a population vary, and the variation is inherited.
- More offspring are produced than can survive.
- Individuals whose features suit the environment leave more offspring.
- Those alleles become more frequent in the population over generations. The population evolves - not the individual. Adaptation is the result of this process, not something an organism chooses.
Evidence
Fossils show change over time and extinct forms; homologous structures (forelimb bones of bat, whale, human) indicate common ancestry; embryos of vertebrates resemble each other early on; DNA and protein sequences show degrees of relatedness; observed cases include antibiotic resistance in bacteria and industrial melanism in the peppered moth.
History of life
Life originated in water roughly 3.5–4 billion years ago; oxygen accumulated in the atmosphere through photosynthesis; the Cambrian explosion produced most animal body plans; plants and animals colonised land; dinosaurs dominated the Mesozoic and were lost at the end-Cretaceous extinction; mammals diversified afterwards; humans appear very recently on this scale.
Worked example
A patient takes an antibiotic and the infection returns. Reasoning: bacteria in the population already vary; a few carry a mutation giving resistance; the antibiotic kills non-resistant cells; resistant cells survive and reproduce; the proportion of resistant bacteria rises, so the next course fails. Note that the mutation existed before the antibiotic was used, so the antibiotic selected rather than created resistance - a required point.
Traps
Saying individual bacteria "adapt" or "learn"; describing evolution as progress toward complexity; confusing extinction with failure; claiming the environment creates the needed mutation; forgetting that humans did not evolve from modern apes but share an ancestor.