Natural selection -- the five required elements
Describe it in this order: (1) genetic variation already exists within the population; (2) many offspring are produced, more than the environment can support; (3) a struggle for survival, including competition for resources such as food, space, water and mates; (4) individuals better adapted to the environment have a greater chance of surviving and reproducing; (5) they pass their alleles on, so the allele spreads. All five are needed for full marks.
Selective breeding -- a three-step cycle
Humans (1) select individuals with a desirable feature, (2) cross them to produce the next generation, and (3) select the offspring showing that feature. Repeating the cycle over many generations raises the proportion carrying it: higher yield, disease resistance or drought tolerance in crop plants; higher milk yield, leaner meat or faster growth in domesticated animals. Apply the cycle to the context named in the question.
(Extended) Antibiotic-resistant bacteria
A few bacteria already carry a resistance allele, arisen earlier by chance mutation, before the antibiotic is used. Bacteria reproduce rapidly by binary fission, so many offspring are produced. The antibiotic kills non-resistant cells; resistant cells survive, reproduce and pass the allele on, so the proportion of resistant bacteria rises each generation until the whole strain is resistant. Overusing antibiotics, or not finishing a course, leaves more survivors.
Drawn from real examiner reports.
Mutation is not natural selection
Mutation creates a new allele. It is the source of variation (B17.1) and happens by chance, whether or not the allele turns out to be useful. Natural selection is the separate process that then acts on variation already present, making an advantageous allele more common over generations. Candidates asked to name natural selection often write mutation instead.
Flagged Jun 2023 P32 Q7ciii
Natural selection vs selective breeding
In natural selection the environment selects: competition for limited resources decides which individuals survive to reproduce, with no human involved. In selective breeding a human chooses which individuals with desirable features are crossed, and which offspring are kept to breed on. The mechanism is the same; the selector is not. Name the right one.
Evolution is a population change
Evolution is a change in the characteristics (allele frequencies) of a species or population over many generations. One bacterium surviving one dose of antibiotic is not evolution: its alleles were fixed when the cell formed and have not changed. Only the proportion of resistant individuals in the population can change, and only across generations.
Flagged Nov 2022 P42 Q4b
Selective breeding takes MANY generations
This is the least-known point on the topic. Candidates give the three steps -- select, cross, select the offspring -- and then stop. One round of selection changes a herd or a crop very little; the large gains seen in real dairy cattle or disease-resistant wheat appear only after the cycle is repeated over many generations. Write those words explicitly.
Flagged Jun 2022 P43 Q10b
Crossing is sexual, not asexual
Selective breeding works by crossing two selected parents so their alleles combine in the offspring, which requires sexual reproduction. Chickens -- and almost every organism used in 0654 contexts, including cattle and wheat -- reproduce sexually, so describing the farmer as cloning the best bird is wrong.
Flagged Jun 2022 P43 Q10b
Bacteria do not acquire resistance
Writing that surviving bacteria became, adapted or developed resistance because they met the antibiotic is the classic error. The resistance allele was already there, by chance, in a few cells before treatment. The antibiotic only kills the non-resistant cells, so resistant ones survive to reproduce; the population changes across generations, not within one cell.
Flagged Jun 2023 P41 Q7a
Resistance is not immunity
Immunity is one organism's immune-system response -- making antibodies against a pathogen -- and it develops within that individual's own lifetime. Antibiotic resistance is a population-level genetic change produced by natural selection over many generations. Bacteria mount no immune response, so never call a resistant strain immune.
Flagged Nov 2022 P42 Q4aii
Run the five-step chain in order
For any "explain how natural selection resulted in..." question, use this chain whatever the organism: variation already present -> many offspring -> struggle for survival -> better-adapted individuals survive and reproduce -> allele passed on. Marks come from causal order.
Finish at the population
Do not end with "so the organism survives". End with "the proportion of the population carrying the allele increases over many generations". Examiners treat that population-level statement as what evolution means, and it is where the final mark usually sits.
Name the pressure; say "greater chance"
Say exactly what is selecting -- this antibiotic, this pesticide, this predator, drought, cold -- rather than "the environment changed". And write that better-adapted individuals have a greater chance of surviving; "they definitely survive and the rest die" is marked wrong.
Tie selective breeding to the context
Application marks need the desirable feature named in the question -- milk yield in dairy cows, grain yield or disease resistance in wheat -- carried through every step. "Select the best ones and breed them" is generic and scores no application marks.
Cambridge 0654 spec reference: Section B17 "Variation and selection", sub-topic B17.2 "Selection". Core content covers natural selection (5 elements) and selective breeding (3 steps, applied over many generations); Extended (Supplement) content adds antibiotic-resistant bacteria as a worked example of natural selection.
Out of scope for B17.2 (covered by other leaves, not here): continuous/discontinuous variation and the definition of mutation itself (B17.1); genetic diagrams, Punnett squares and monohybrid cross ratios (B16 Inheritance); the mechanism of immunity (B10 Diseases and immunity -- referenced only as a contrast below).
Natural selection -- describe with reference to all FIVE elements:
| Step | Idea |
|---|---|
| 1 | Genetic variation exists within a population (individuals already differ in their alleles) |
| 2 | Production of many offspring -- more than the environment can support |
| 3 | Struggle for survival, including competition for resources |
| 4 | Individuals better adapted to the environment have a greater chance of survival and reproduction |
| 5 | These individuals pass on their alleles to the next generation |
Selective breeding (artificial selection) -- describe with reference to THREE steps, then repeated over many generations:
Evolution = a change in the characteristics (allele frequencies) of a species/population over many generations -- it happens to a population over time, not to one individual during its own lifetime.
(Extended) Antibiotic resistance = development of a resistant bacterial strain by natural selection: a resistance allele already exists by chance in a few bacteria before treatment; the antibiotic kills non-resistant bacteria; resistant bacteria survive, reproduce and pass on the allele, so the proportion of resistant bacteria in the population rises over successive generations. Resistance is not the same as immunity (immunity is an individual organism's immune-system response within its own lifetime; resistance is a population-level genetic change over generations).
Define natural selection.
State, in the correct order, the three steps involved in selective breeding, and state how often this cycle must be repeated to produce a substantial improvement in a crop or livestock population.
(4 marks)
