Every species alive today is the result of the same simple process running for billions of years: more offspring are born than can survive, they are not all identical, and the ones that fit their surroundings best leave the most young. That is it. Everything else is detail.
📚 What you need to know
Evolution is the cumulative change in the heritable characteristics of a population or species over time.
Natural selection is the mechanism that drives it, and it runs continuously.
It needs three things: variation, overproduction of offspring, and a selection pressure.
Variation comes from differences in genes, originally created by chance mutations.
Better-adapted individuals are more likely to survive and reproduce, so their genes become more common.
The population changes, not the individual. Individuals do not adapt during their lifetime.
Speciation happens when isolated populations diverge until they can no longer interbreed to produce fertile offspring.
What evolution is
DefinitionEvolution is the cumulative change in the heritable characteristics of a population or species over time.
Two words in there are doing heavy lifting. Cumulative means it is the overall change built up over many generations, not one dramatic event. Heritable means it only counts if it can be passed on — a scar or a suntan changes an individual but changes nothing about the next generation.
Biodiversity, all three levels of it, is what this process has produced. Natural selection has been running for billions of years, and it has not stopped.
Natural selection, step by step
🧩 The sequence that fits every example
Individuals in a species show variation, caused by differences in their genes.
Organisms produce more offspring than the environment can support.
This causes competition for food, space, mates and other resources — a struggle for survival.
A selection pressure acts. Individuals whose characteristics are best suited have a higher chance of surviving to reproduce.
Those individuals pass on their genes at a higher rate than the less well suited ones.
In the next generation, a greater proportion carry the advantageous characteristic. Repeat for many generations and it becomes the norm.
Learn this as a sequence rather than as a set of facts. Whatever organism a question gives you — a moth, a bacterium, a plant, a fish — the same six steps apply and you only have to change the nouns.
Some pale moths survive on the right-hand side. Selection shifts the odds; it does not guarantee anything for any single individual.
A worked example: moths on darkened bark
A moth population contains both pale and dark individuals. The difference is genetic, so it can be inherited.
The moths rest on tree trunks by day, where birds hunt them. Predation is the selection pressure.
Soot from industry darkens the bark. Pale moths now stand out against it; dark moths are camouflaged.
Pale moths are eaten more often, so fewer of them survive to breed. Dark moths have a selection advantage.
Dark moths pass on their genes at a higher rate, so the frequency of the dark form rises generation after generation.
Clean the air again and the pressure reverses, because the environment — and therefore what counts as well adapted — has changed.
That last bullet is the one people skip. There is no such thing as a permanently “better” characteristic. An advantage exists only in relation to a particular environment, and environments change.
A second example: antibiotic resistance
A bacterial population varies. By chance, a few individuals carry a mutation that lets them survive an antibiotic.
The antibiotic is the selection pressure. Almost all the non-resistant bacteria die.
The resistant few survive and reproduce, and bacteria reproduce very fast.
Within days the population can be almost entirely resistant.
This is natural selection you can watch in real time, and it is the clearest evidence that the process is not something that finished in the past.
Three things to be careful about. Better-adapted organisms are more likely to survive, not certain to. Less well-adapted organisms can still survive and breed, just at lower rates. And every organism experiences selection pressure, not only animals — plants, fungi and bacteria all do.
Where variation comes from
The ultimate source is random mutation: chance changes to genes that happen regardless of whether they are useful.
Sexual reproduction then shuffles gene forms into new combinations each generation.
Selection does not create variation. It filters variation that is already there.
🧠
Easy way to remember it
Mutation proposes, selection disposes. Mutations arrive by chance and are not aimed at anything. The environment then decides which of them get passed on.
Speciation
DefinitionSpeciation is the generation of new species through evolution, when populations become isolated and adapt to their environments in different ways until they can no longer interbreed to produce fertile offspring.
The barrier can be a mountain range, a sea, a river or a stretch of unsuitable habitat. What matters is that genes stop moving between the two groups.
The sequence
A population becomes isolated into two groups that no longer breed with each other.
The two environments impose different selection pressures.
Different characteristics are favoured in each group, so they diverge over many generations.
Eventually the differences are large enough that the two groups cannot interbreed to produce fertile offspring.
At that point they are counted as separate species.
The definition of a species that ESS uses is about fertile offspring, and that word matters. A horse and a donkey can produce a mule, but mules are sterile, so horses and donkeys stay two species.
Worked examples
WE 1
Explain natural selection in an example
A population of insects is sprayed with a pesticide each year. Explain how the population becomes resistant. (4 marks)
Step 1: variation
The population shows genetic variation, and by chance a few individuals carry a mutation giving resistance.
Step 2: selection pressure
The pesticide is the selection pressure. Non-resistant individuals are killed, resistant ones survive.
Step 3: reproduction
Survivors reproduce and pass the resistance gene on at a much higher rate than the rest.
Step 4: the outcome
Over generations the frequency of the resistance gene increases until most of the population is resistant.
Variation, pressure, differential survival, changed gene frequencysay the mutation existed before the spraying — the pesticide selects, it does not create resistance
WE 2
Explain speciation
Explain how a single species can give rise to two separate species. (4 marks)
Point 1: isolation
A population becomes split, for example by a mountain range or a sea, so the two groups no longer interbreed and gene flow stops.
Point 2: different pressures
The two environments impose different selection pressures, so different characteristics are favoured in each.
Point 3: divergence
Over many generations the gene frequencies in the two populations become increasingly different.
Point 4: the threshold
Eventually they can no longer interbreed to produce fertile offspring, so they are classed as separate species.
Isolate, diverge, and at some point the two can no longer breedthe word “fertile” is not optional; without it the definition is incomplete
WE 3
Correct a misconception
A student writes: “The moths turned darker so they could hide on the sooty bark.” Explain why this statement is wrong. (3 marks)
Point 1: individuals do not change
An individual moth cannot change its colour in response to its surroundings; its colour is determined by its genes.
Point 2: the variation was already there
Both pale and dark forms already existed in the population before the bark darkened.
Point 3: what actually happened
Darker moths survived predation more often and reproduced more, so the proportion of dark individuals in the population increased.
The population changed. No individual moth changed.avoid words like “wanted”, “tried” or “in order to” — they imply organisms choose to adapt
💡 Exam tips
Learn the six-step sequence and reuse it for every example. Only the nouns change.
Always name the selection pressure explicitly.
Say more likely to survive, never “will survive”.
Finish with gene frequency in the population — that is the actual definition of evolution happening.
Keep speciation to four beats: isolation, different pressures, divergence, no fertile offspring.
Remember variation comes from mutation and is random, not directed.
⚠ Common mistakes
Saying individuals adapt. Populations evolve; individuals do not change their genes.
Saying organisms mutate because they need to. Mutations happen by chance, regardless of need.
Guaranteeing survival. Being better adapted improves the odds; it does not promise anything.
Leaving out “fertile” in the species definition. The offspring must be able to breed too.
Forgetting that selection pressures change. An advantage today can be a disadvantage tomorrow.
Thinking only animals evolve. Plants, fungi and bacteria all face selection pressures.
Up next: Measuring Biodiversity — you can describe diversity in words all day, but conservationists need a number they can compare.
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