Natural selection — the four-step chain
Variation — individuals in a population (same species, one area, one time) differ because they carry different alleles. Competition — they compete for limited resources and face predation and disease. Survival of the best-adapted — those suited to the environment are likelier to survive and breed. Inheritance — survivors pass on their advantageous alleles, which become more common each generation. Adaptation is the outcome, not a separate mechanism.
Evolution (Extended) — the antibiotic-resistance worked example
Evolution (Extended) is change in a population's characteristics over successive generations, caused by natural selection. Worked example: random mutation gives a few bacteria a resistance allele before any antibiotic is used. The antibiotic kills the rest; the resistant ones survive, reproduce rapidly and pass the allele on. Over many generations the resistant proportion increases until the population is mostly or entirely resistant.
Selective breeding — humans choose the parents
Selective (artificial) breeding is human-directed: identify the individuals showing the desired characteristic most strongly (e.g. milk yield), breed only those together, select again from the offspring, and repeat over many generations. The contrast with natural selection is who selects and for what — humans choose, for traits useful to people; the environment selects, for traits that aid survival. Breeding is deliberate; natural selection has no intention.
Drawn from real examiner reports.
Antibiotic resistance ≠ artificial selection
Because a human action appears in the scenario, candidates label antibiotic resistance "artificial selection". Prescribing the antibiotic only creates the selection pressure — nobody chooses which bacteria breed together, and the resistance allele already existed by random mutation. This is natural selection, and at Extended level, evolution.
Flagged w22 P22 Q34
"Struggle for survival" ≠ breeding
Asked to describe selective breeding, candidates write "the animals compete and only the strongest survive to breed" — that is natural-selection language and answers a different question. A full answer names three things: a human chooses parents with the desired characteristic, breeds them together, and repeats this over many generations.
Flagged s22 P13 Q35
Population ≠ community
"All the organisms in an area" describes a community — the different species living together. A population is all the organisms of the same species, in a particular area, at the same time. It matters here: natural selection acts on variation within one species, so survival is compared between individuals of the same species.
Flagged s22 P11/12/13 Q38 · w22 P11 Q38
The antibiotic doesn't cause the mutation
Writing "the bacteria mutate to become resistant when the antibiotic is applied" inverts the sequence. Mutations are random and occur before exposure, so a few bacteria already carry the allele. The antibiotic never triggers the mutation — it only kills the bacteria that lack it, leaving those that already had it.
Flagged w22 P41 Q4b i
Adaptation ≠ adaptive feature
Adaptation is the population-level outcome of natural selection over generations; an adaptive feature is the individual inherited trait itself (topic 18.2). The related slip is describing one organism "adapting" during its own lifetime. Individuals do not adapt — populations do, because only inherited variation is passed to the next generation.
Selective breeding is not evolution
Changing livestock by selective breeding is not evolution in this syllabus. Evolution is reserved for change brought about by natural selection. Antibiotic resistance in bacteria is evolution; a high-yield dairy herd is directed artificial change, however many generations it took. Change alone is not evolution — the agent of selection decides.
Flagged w22 P22 Q34
Use the four-step scaffold
For any "explain how natural selection has led to..." question, write the same four steps applied to the organism in the stem: the variation present, the selection pressure faced, which individuals survive, and survivors passing the allele to their offspring.
Ask: who is choosing?
Before naming the process, ask who decides which individuals reproduce. The environment — predators, disease, limited resources, an antibiotic — means natural selection. A human choosing parents with a desired trait means selective breeding.
Name the specific pressure
Generic answers such as "the fittest survive" earn little. Name the actual variation and pressure from the stem — darkened bark and birds hunting by sight, or the antibiotic. Then finish the chain: survivors reproduce and pass on the allele, the step where most marks are lost.
Two differences means two distinct points
When asked for two ways selective breeding differs from natural selection, take them from three genuinely separate contrasts: who selects, what is selected for, and whether the process is deliberate. Rewording a single point twice earns one mark, not two.
Population: all the organisms of the same species, living in a particular area, at the same time. (Not "all organisms in an area" - that is a community.)
Natural selection: the process by which individuals with characteristics better suited to their environment are more likely to survive and reproduce, passing their advantageous alleles to the next generation, so the population becomes progressively better adapted over many generations.
Adaptation: the outcome of natural selection - a population becoming better suited to its environment over successive generations. (Contrast with an adaptive feature, the individual inherited trait itself - see topic 18.2.)
(Extended) Evolution: the change in the characteristics of a population over many successive generations, as a result of natural selection.
Selective (artificial) breeding: the process by which humans choose organisms with a desired characteristic and breed them together, repeated over many generations, so the desired characteristic becomes more common/pronounced in the population.
Define a population.
(a) Define a population.
(b) State the four steps of the natural selection chain, in order.
