Inheritance and ecology · GCSE Biology
Natural selection
Write natural selection as a sequence for GCSE Biology: variation, competition, survival, reproduction and alleles becoming more common, with antibiotic-resistant bacteria as the example.
Variation first, then struggle, then survivors reproduce, then the advantageous allele becomes more common. Organisms do not try to adapt. Antibiotic resistance is the cleanest exam story.
The important bits
What you need to know
- 1
Evolution is a change in the inherited characteristics of a population over time through natural selection, which may result in the formation of a new species (Higher/Triple may add speciation).
- 2
Darwin’s sequence: variation (mutation and sexual reproduction) → competition for limited resources → individuals with advantageous alleles are more likely to survive and reproduce → offspring inherit those alleles → the alleles become more common.
- 3
An adaptation is a feature that increases survival chance: structural (thick fur), behavioural (migration), functional (venom, concentrating urine). Do not call a scar an adaptation in the evolutionary sense.
- 4
Antibiotic resistance: mutation gives resistance → antibiotic kills non-resistant bacteria → resistant bacteria survive and reproduce → resistance becomes more common. MRSA is the named strain.
- 5
Peppered moths (industrial melanism) appear on some papers: dark moths survived better on sooty trees. Use it only as evidence of selection, not as moths choosing to change colour.
- 6
Do not write that organisms develop a feature because they need it (Lamarck). The variation has to exist first. Giraffes did not stretch their necks and pass on stretched necks as an acquired characteristic.
- 7
Fossils, antibiotic-resistant bacteria, and classification (three-domain system: archaea, bacteria, eukaryotes) are evidence streams. Extinction happens when the environment changes faster than selection can track, or when humans destroy habitat.
- 8
Speciation (Higher): isolation (often geographic) plus selection in different directions plus so much genetic change that the groups can no longer interbreed to produce fertile offspring.
Quotations worth analysing
Short evidence. Real method.
“Individuals with advantageous alleles are more likely to survive and reproduce, so those alleles become more common.”
This sentence is the spine. “Survival of the fittest” without alleles is a slogan, not an answer.
“Mutation produces variation; the antibiotic is the selection pressure.”
Do not say the antibiotic caused the mutation. Random mutation first, then selection.
“Organisms do not change because they need to; existing variation is selected.”
Need is not a mechanism. If the allele is not there, the population may go extinct.
Go deeper
How do I write a six-mark natural-selection answer that always scores?
Name the species and the environmental pressure. State that variation already exists (mutation / sexual reproduction). Describe competition (food, mates, space, oxygen, antibiotic). Link a named feature to survival (camouflage, long roots, resistance gene). Survivors reproduce. Offspring inherit the allele. Over generations the proportion of the allele / the phenotype rises. That is evolution by natural selection. Add a closing line: if the environment changes again, a different allele may be favoured. Avoid “the strong kill the weak” and “they all try harder”. If the example is bacteria, include binary fission and possibly plasmids. If it is a mammal, include meiosis and fertilisation as the inheritance route. The logic is the same; the reproduction word changes.
Go deeper
How is antibiotic resistance the model answer?
A random mutation or a plasmid gene lets one bacterium survive penicillin. The antibiotic is a selection pressure: it kills competitors. The resistant cell reproduces by binary fission, producing a resistant clone. Hospital use and unfinished courses increase the pressure. MRSA is difficult to treat because many antibiotics fail. Reducing resistance: finish prescriptions, do not use antibiotics for viral illness, restrict farm use, hygiene to stop spread. This story also sits on the infection page; here the emphasis is alleles becoming more common, not how penicillin works. If a graph shows percentage resistant over years, describe the rise, then give the sequence. Correlation with antibiotic use supports selection, not Lamarckian “practice”.
Go deeper
What extra do Higher papers want for speciation?
A population is split (river, mountain, island). No gene flow. Different selection pressures (food, climate, predators) favour different alleles. Mutations accumulate separately. Eventually individuals cannot produce fertile offspring if they meet — two species. Isolation must come first in the story. Do not write that they decide to become two species. Fossils cannot always show this process directly; we infer from living examples (Darwin’s finches as a teaching story if your board uses them). Three-domain classification (Woese) uses RNA evidence to split prokaryotes into archaea and bacteria, with eukaryotes as the third domain — evidence that classification follows evolutionary relationships, not just looks.
See the idea in action
A population of bacteria includes a few cells with a mutation for antibiotic resistance. When penicillin is used, non-resistant cells die. Resistant cells survive, reproduce, and the resistance allele becomes more common. The bacteria did not “learn” penicillin. Variation existed first; the antibiotic selected. In a similar logic, if a climate dries, plants with deeper roots (existing allele) survive and seed, so deep-root alleles increase. Plants that “needed” deep roots but had no such allele would die out.
Exam technique
Turn knowledge into marks
Always use the five-step sequence and the word allele. For resistance, mutation then selection, not “bacteria get used to it”. Ban “need” and “try” as causes. Finish with alleles becoming more common.
Common mistakes
Do not give these marks away
- 01
Writing that organisms adapt because they need to, or that the environment causes a directed mutation.
- 02
Omitting inheritance and reproduction, so the story stops at survival.
- 03
Saying the antibiotic created the resistance gene, rather than selected it.
Which sequence correctly describes natural selection?
ANeed → mutation directed by need → everyone changes in one lifetime
BVariation → competition → survival of better-adapted individuals → reproduction → alleles become more common
CAll organisms try to be fittest, so they become a new species overnight
DAntibiotics cause bacteria to mutate on purpose
Show the answer
Variation → competition → survival of better-adapted individuals → reproduction → alleles become more common. Variation must exist first. Selection then changes allele frequencies over generations. Need does not direct mutation, and antibiotics do not create resistance genes on purpose.
Quick questions
If this is the bit you searched
What is natural selection GCSE?
Individuals with advantageous alleles are more likely to survive and reproduce, so those alleles become more common in the population over generations.
How does antibiotic resistance evolve?
A random mutation (or plasmid) makes a bacterium resistant. Antibiotics kill non-resistant bacteria. Resistant bacteria survive, reproduce, and resistance becomes more common.
What is the difference between Darwin and Lamarck at GCSE?
Darwin: existing variation is selected. Lamarck (rejected): organisms change in life because they need to and pass on acquired characteristics. Use Darwin’s sequence.
What is speciation?
Higher: a population is isolated, selection differs, genomes diverge until the groups cannot interbreed to make fertile offspring — two species.