Evolution and Natural Selection

The method, step by step

1. 1

   Start with variation

   individuals in a population differ

   Variation exists before any selection happens, arising from mutation and from sexual reproduction. Without pre-existing variation there is nothing for selection to act on, which is why this step comes first.

2. 2

   Note the competition

   more offspring are produced than can survive

   Resources such as food, territory and mates are limited, so not every individual survives to reproduce. This competition is what makes the differences between individuals consequential.

3. 3

   Identify differential survival

   individuals with advantageous variations survive more often

   Those whose characteristics suit the environment are more likely to survive long enough to reproduce. This is 'survival of the fittest', where fittest means best suited rather than strongest.

4. 4

   Follow the inheritance

   survivors pass their alleles to offspring

   Because the advantageous characteristics are genetic, they are inherited. The next generation therefore contains a higher proportion of individuals with those alleles.

5. 5

   Repeat over many generations

   the population's characteristics gradually shift

   The change accumulates across generations. Given enough time and sufficient divergence, populations can become distinct species. No individual evolves — the population does.

Individuals do not evolve

The most common error is describing an organism as changing to suit its environment during its lifetime — giraffes stretching their necks, or animals growing thicker fur because it turned cold. Natural selection does not work that way and answers phrased like this lose the marks.

The correct description is that variation already existed, and those individuals whose characteristics happened to suit the conditions survived and reproduced more. The population changes across generations; no individual changes at all.

• Variation exists in a population
• More offspring are produced than survive
• Better-suited individuals survive and reproduce
• Their alleles are passed on
• Over generations, the population changes

The evidence for evolution

Fossils show sequences of change over time and organisms that no longer exist. Comparative anatomy reveals shared structures across species, such as the same bone arrangement in a human arm, a bat wing and a whale flipper. DNA comparisons show that more closely related species share more genetic sequence.

Antibiotic resistance in bacteria is natural selection observable within human timescales: resistant individuals survive treatment and multiply, so the population becomes resistant. It is the most immediately relevant example available.

Fittest does not mean strongest

Fitness in this context means suited to the environment, not physically powerful. Being smaller, better camouflaged, more efficient at digesting a food source, or simply more attractive to mates can all constitute higher fitness.

It is also entirely relative to conditions. A characteristic advantageous in one environment can be a disadvantage in another, which is why the same species can evolve differently in different places.

How we teach natural selection

We require answers to follow the four steps in order, because mark schemes are structured that way and because the sequence is what prevents the individual-adapts error creeping in.

We use antibiotic resistance as the worked example rather than a historical one. It happens on a human timescale, it matters, and students engage with it in a way that peppered moths no longer reliably produce.