Every example of natural selection you will ever be asked about follows the same five steps in the same order. Learn the sequence once and you can answer a question about moths, rabbits, bacteria or anything else the examiner invents.
📘 What you need to know
Evolution is the cumulative change in the heritable characteristics of a population or species.
Natural selection is the mechanism that drives evolutionary change.
Variation arises from differences in genes, ultimately from chance mutations.
Darwin’s theory became known as “survival of the fittest”.
Better-adapted organisms have a higher chance of survival, not a guarantee.
Speciation is the generation of new species, occurring when populations become isolated and can no longer interbreed to produce fertile offspring.
Evolution and natural selection
Biodiversity arises from evolutionary processes.
DefinitionsEvolution — the cumulative change, the overall change over time, in the heritable characteristics of a population or species. Natural selection — the mechanism that drives evolutionary change.
Natural selection occurs continuously and can take place over billions of years. The result of that process is the biodiversity of life on Earth we see today.
The five steps
In any environment, the individuals with the best adaptive features are the ones most likely to survive and reproduce.
Steps 3 and 4 are where the selection pressure acts. Steps 1 and 5 are what make the change heritable rather than temporary.
Individuals in a species show a range of variation caused by differences in genes.
When organisms reproduce, they produce more offspring than the environment can support.
This leads to competition for food and other resources — a “struggle for survival”.
Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce.
The genes for those characteristics are passed on at a higher rate, so the next generation contains more individuals with the better-adapted variation.
This theory was put forward by Charles Darwin and became known as “survival of the fittest”.
A worked example: rabbit fur colour
Imagine a population of rabbits showing variation in fur colour, preyed on by foxes. The foxes act as a selection pressure.
The illustrative figures show the shape of the change. The point is the direction, not the exact percentages.
Rabbits with white coats do not camouflage as well as brown rabbits, so predators are more likely to see them when hunting.
White rabbits are therefore less likely to survive; brown rabbits have a selection advantage.
Brown rabbits are more likely to survive to reproductive age and pass on their genes.
Over many generations, the frequency of the gene for brown fur increases and the frequency of the gene for white fur decreases.
⚠ Three things this example does not mean
Survival is not guaranteed. Better-suited organisms are more likely to survive, not certain to. Less-suited individuals can still survive and reproduce — their chances are simply lower.
Selection pressures change. An environment can change over time, and when it does a different characteristic may become the advantageous one.
It is not only animals. All organisms experience selection pressures from their environment.
The peppered moth is another standard example of natural selection, following exactly the same sequence.
Speciation
Speciation is the generation of new species through evolution. It occurs when populations of a species become isolated and adapt to their environments in different ways. Over time those populations become so different that they can no longer interbreed to produce fertile offspring — at which point they are considered separate species.
There are far more examples of natural selection than anyone could memorise, and you do not need to. Every one follows the same logic: within a species there is always variation from chance mutations; some individuals develop a characteristic giving them a survival advantage, so they live longer, breed more, and pass their genes on more often; repeated over generations, that characteristic becomes the norm. Learn the sequence, then slot in whatever organism the question gives you.
Worked examples
WE 1
Explaining natural selection
Explain how natural selection leads to a change in a population over time. (4 marks)
Step 1: variation
Individuals in a species show a range of variation caused by differences in genes, arising from chance mutations.
Step 2: competition
More offspring are produced than the environment can support, leading to competition for food and other resources.
Step 3: differential survival
Individuals with characteristics best suited to the environment have a higher chance of surviving to reproductive age and more chances to reproduce.
Step 4: inheritance
Their genes are passed on at a higher rate, so the next generation contains a greater number of individuals with the better-adapted characteristic. Repeated over generations, it becomes the norm.
Variation, competition, differential survival, inheritancesay “more likely to survive”, never “will survive”
WE 2
Applying it to an example
A population of rabbits contains both brown and white individuals and is hunted by foxes. Explain how the population will change. (4 marks)
Step 1: the variation and the pressure
The rabbits vary in fur colour because of genetic differences. Predation by foxes acts as a selection pressure.
Step 2: the disadvantage
White rabbits camouflage less well, so predators are more likely to see them when hunting.
Step 3: the advantage
Brown rabbits therefore have a selection advantage: they are more likely to survive to reproductive age and pass their genes to offspring.
Step 4: the change
Over many generations the frequency of the gene for brown fur increases while the frequency of the gene for white fur decreases.
Gene frequencies shift; individual rabbits do not changetalk about gene frequency, not about rabbits “adapting” during their lives
WE 3
Speciation
Outline how speciation occurs. (3 marks)
Step 1: isolation
Populations of a species become isolated from one another so that they no longer interbreed.
Step 2: divergence
Each population adapts to its own environment in different ways through natural selection, and accumulates different genetic changes.
Step 3: the threshold
Over time the populations become so different that they can no longer interbreed to produce fertile offspring, at which point they are considered separate species.
Isolation, divergence, then loss of the ability to interbreed“fertile offspring” is essential — offspring alone is not enough
💡 Exam tips
Learn the five-step sequence and use it as your template for any example.
Always start with variation caused by genetic differences, from chance mutations.
Use the phrase selection pressure and name what it is in the scenario.
Say more likely to survive, never “will survive”.
Talk about gene or allele frequency changing, not individuals changing.
For speciation, the test is inability to produce fertile offspring.
⚠ Common mistakes
Saying organisms adapt during their lifetime. Populations change across generations; individuals do not.
Claiming the best-adapted always survive. They have a higher chance, not a certainty.
Forgetting overproduction. Without more offspring than can be supported, there is no competition.
Treating selection pressures as fixed. Environments change, and so does what counts as advantageous.
Applying it only to animals. All organisms face selection pressures.
Defining species by offspring alone. They must be fertile offspring.
Up next: Measuring Biodiversity. You can describe biodiversity and explain where it comes from. Now you need to put a number on it — and there is a calculation you will be asked to perform.
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