Natural Selection

The Cambridge-required sequence — Core

Natural selection is a population process, and Cambridge wants it explained as a sequence of steps rather than a single sentence definition.

1. Genetic variation exists

Individuals in a population already differ genetically, and some of those inherited differences happen to suit the current environment better than others.

2. Many offspring are produced

Populations typically produce more offspring than the environment can support all the way to adulthood.

3. There is a struggle for survival

Because resources are limited, individuals compete for them. The resource in question can be food, water, space, light in the case of plants, or breeding opportunities — whichever one the environment in a given question actually restricts.

4. Better-adapted individuals reproduce more successfully

Individuals whose inherited features happen to suit the environment are more likely to survive long enough to reproduce, and more likely to reproduce successfully once they do.

5. Their alleles are inherited

Those individuals pass the alleles responsible for their advantageous features on to their offspring.

6. The population changes over generations

Repeating this differential reproduction generation after generation can increase how common the advantageous alleles or phenotypes are in later generations. Cambridge’s Core list of steps stops at inheritance, but this final step is worth stating explicitly, because it is the part that actually explains whyselection changes a population rather than just describing one generation’s outcome.

Applying the sequence to an unfamiliar example

The mechanism is identical whether the question is about colour, disease resistance, feeding structures, temperature tolerance, or something else entirely. Work through five things in order: the heritable variation that exists, the selection pressure acting on the population, which phenotype has the advantage under that pressure, why that phenotype survives or reproduces more successfully, and how the advantageous allele is then inherited and becomes more common over generations. If a coloured form of an organism gives better camouflage, for instance, individuals with that colouring may be caught by predators less often, reproduce more as a result, and pass the relevant allele to more offspring than individuals without it — the organism changes in each version of the question, but the causal chain never does.

Language to avoid

Purpose-driven wording is the fastest way to lose marks here even when the biology sounds roughly right: “the animal changed because it needed to survive,” or “the environment gave the organism the useful gene,” both describe the wrong causal direction. The safer structure to write toward is: variation already exists, the environment selects among the variants that are already there, differential reproduction follows, inheritance carries the advantageous allele forward, and the population changes as a result. It is also worth knowing that “fittest” does not mean physically strongest in this context — it means most successful at surviving and reproducing in the current environment, which can mean almost anything depending on what that environment demands.

Adaptation as a process — Supplement

At Supplement level, keep adaptation distinct from an adaptive feature. An adaptive feature is the inherited trait itself. Adaptation is the process — occurring across many generations, resulting from natural selection, and affecting a whole population rather than one individual — by which that population becomes better suited to its environment. The mechanism is exactly the sequence above, run repeatedly: variation exists, the environment favours certain phenotypes, those individuals leave more offspring, their alleles become more common, and repetition across generations gradually changes the population. If the environment changes, a different phenotype can become the advantageous one, and the direction of selection can shift accordingly. It is also worth separating adaptation from acclimatisation, which is a change within one individual’s own lifetime and is not the same as evolutionary adaptation at all.

Antibiotic resistance as natural selection — Supplement

Antibiotic-resistant bacteria are Cambridge’s required worked example of the full natural-selection sequence, and the same causal-direction rule applies here as everywhere else in this topic: the antibiotic does not create resistance. It selects for bacteria that were already resistant before it was ever used.

1. Genetic variation already exists in the bacterial population — a random mutation can produce resistance to a particular antibiotic.
2. The antibiotic is applied, becoming a selection pressure on the population.
3. Susceptible bacteria are killed or have their growth stopped.
4. Resistant bacteria are far more likely to survive that same exposure.
5. The surviving resistant bacteria reproduce.
6. Resistance is inherited by their descendants.
7. Across repeated generations, resistant bacteria make up a growing proportion of the population.
8. A resistant strain can therefore develop.

The key causal distinction worth stating explicitly in an answer: mutation supplies the variation; the antibiotic selects among the variants that already exist.The antibiotic does not deliberately cause the correct resistance mutation, does not make every bacterium it touches transform into a resistant form, and does not create resistance because the bacteria “need” it. A surviving resistant bacterium is not resistant because it survived — it survived because it already carried the resistance before the antibiotic was ever applied.

This page covers the natural-selection mechanism behind resistance — why resistant strains become more common once they exist. What antibiotics actually do to bacteria, why essential-only use slows resistance down, and the named example of MRSA belong to a different part of the syllabus: see antibiotic resistance and MRSA for that side of the topic.

Cross-links

The heritable variation this whole page depends on is explained on the mutation and genetic variation page, and the inherited features that selection acts on are defined on the adaptive features page. When the selecting force is a human being rather than the environment, the process is called selective breeding, which shares the same underlying logic but a different source of the selection pressure.