Nothing in nature decides to change. A beetle does not choose to be brown and a giraffe does not want a longer neck. What really happens is much simpler: living things are already slightly different from one another, some of those differences help them stay alive long enough to breed, and those are the ones that get copied into the next generation.
📚 What you need to know
Evolution is a change in the heritable characteristics of a population over generations.
Only heritable changes count. If a change is not written into the DNA, it cannot be passed on.
New alleles appear by random mutation. Mutation is random, but survival is not.
Natural selection: variation → selection pressure → the best-suited individuals survive and breed → their alleles become more common.
Individuals do not evolve. Populations do. One beetle never changes colour, but the mix of beetles can.
Lamarck said characteristics gained during a lifetime are inherited. That idea is rejected, because those changes never reach the gametes.
What “evolution” actually means
Species are not fixed. The animals and plants around you today are not the same as the ones that lived here a million years ago, and they will not be the same a million years from now. That slow change is evolution.
Definition — learn this wording
Evolution = a change in the heritable characteristics of a population over generations
Three words in that sentence are doing all the work, and examiners give marks for all three.
Heritable — the characteristic must be controlled by alleles, so it can be passed to offspring.
Population — a group of the same species living together. Evolution is a change in the group, not in one animal.
Generations — it happens across parents, offspring, grandchildren. Not within one lifetime.
If you only remember one sentence from this page, make it that one. A huge number of marks in this topic are just that definition, said carefully.
Heritable or not? The gamete test
Here is a quick way to check whether a change can be inherited. Ask yourself: did it change the DNA in the sperm or egg cells? If the answer is no, it stops with that individual.
✓ CAN BE PASSED ON
A mutation in a plant’s flower colour gene
A bacterium carrying a resistance allele
Fur thickness controlled by alleles
Beak shape set by inherited genes
✗ CANNOT BE PASSED ON
A plant that had its leaves eaten by a caterpillar
Muscles built up in the gym
A scar, a tan, a broken bone
A skill you learned, like reading
Watch out: this is where students lose easy marks. Writing “the giraffes stretched their necks and their babies were born with longer necks” describes Lamarck, not Darwin, and it will not be credited.
Where the variation comes from
Natural selection cannot invent anything. It can only work with differences that are already sitting in the population. So the first question is: where do those differences come from?
Random mutation. Every time DNA is copied there is a small chance of a mistake. Most mistakes do nothing, some are harmful, and just occasionally one is useful. Mutation is the only source of brand new alleles.
Sexual reproduction. Meiosis and fertilisation shuffle existing alleles into new combinations, so offspring are not identical to either parent.
🤔 Why “random” matters so much
Mutations do not appear because an organism needs them. A rabbit living somewhere cold does not start producing thick-fur mutations. The mutations happen anyway, all the time, in every population — and the cold environment simply decides which of them do well. Mutation is random; selection is not. Say that in an exam and you have shown you understand the whole mechanism.
Natural selection, step by step
Darwin’s explanation is really a chain of five short statements. Learn it as a chain, because exam questions usually want you to write it out in order.
1
VARIATIONIndividuals differ, because of random mutation
2
PRESSUREToo many young, not enough food, space or mates
3
SURVIVALBetter-suited individuals are more likely to live
4
BREEDINGSurvivors reproduce and pass on their alleles
5
SHIFTThe helpful allele gets more common each generation
Every beetle here keeps the colour it was born with for its whole life. The only thing that moves is how common each colour is — and that is exactly what “a change in heritable characteristics of a population” means.
Darwin and Lamarck: same question, different answers
Before Darwin, the French scientist Jean-Baptiste Lamarck had his own explanation for why species change. It sounds sensible until you ask how the information would actually get into the next generation.
The grey animal with the red cross is the one that does not get enough food and does not breed. Its alleles simply drop out. Nothing about it was “improved” during its life.
Question
Lamarck’s answer (rejected)
Darwin’s answer (accepted)
Where does the new feature start?
The individual develops it during its own life
It is already there as variation, caused by mutation
What causes it?
Using a body part a lot, or not using it
Random mutation, then selection by the environment
How does it reach the offspring?
The changed body part is somehow inherited
Through alleles in the gametes
Why is it rejected or kept?
Acquired characteristics do not change the DNA of gametes
Fits genetics, fossils, DNA evidence and observed change
A fair point for Lamarck: he was one of the first to argue that species change at all, which was a brave thing to say at the time. He got the mechanism wrong, not the big idea.
Why we still call it a “theory”
In everyday English a “theory” means a guess. In science it means almost the opposite: an explanation that has survived a huge amount of testing and pulls a lot of separate observations together.
Natural selection explains fossils, DNA similarities, antibiotic resistance, pesticide resistance and selective breeding — all with one mechanism.
No competing scientific explanation has held up against the evidence.
It has been updated rather than overturned. For example, we once assumed evolution is always slow, but resistant bacteria can appear in a matter of years.
We cannot rerun the last few hundred million years in a lab, so it cannot be “proved” the way a maths result is proved. That is normal for science, not a weakness.
Worked examples
WORKED EXAMPLE
Explain how a population of bacteria becomes resistant to an antibiotic. [4]
Step 1: start with variation, not with the drugRandom mutation means a few bacteria already carry a resistance allele before the antibiotic is used.Step 2: name the selection pressureThe antibiotic is the selection pressure. Non-resistant bacteria are killed.Step 3: survival and reproductionResistant bacteria survive and reproduce, passing the resistance allele to their offspring.Step 4: state the population-level resultOver generations, the frequency of the resistance allele rises until most of the population is resistantNever write “the bacteria became resistant because of the antibiotic” — that is Lamarck, and it scores zero.
WORKED EXAMPLE
A weightlifter trains for 20 years and builds huge muscles. Explain why her children are not born unusually muscular. [2]
Step 1: classify the changeMuscle built by training is an acquired characteristic. It happens to body cells only.Step 2: link it to inheritanceTraining does not alter the DNA in her gametes, so nothing about it can be passed to her children.Only heritable changes, carried by alleles in gametes, can be inheritedThis is exactly why Lamarck’s model was dropped.
WORKED EXAMPLE
A beetle population on pale sand has 400 individuals: 320 pale and 80 dark. After a fire blackens the ground, 10 generations later the same population of 400 has 60 pale and 340 dark. Calculate the percentage of dark beetles before and after, and explain the change. [4]
Step 1: percentage before the fire80 ÷ 400 × 100 = 20%Step 2: percentage after 10 generations340 ÷ 400 × 100 = 85%Dark beetles rose from 20% to 85%, an increase of 65 percentage pointsStep 3: explain it properlyBoth colours were already present, so the variation existed before the fire. On blackened ground, dark beetles are harder for predators to see, so more of them survive to breed and pass on the dark allele. Its frequency rises each generation.Notice the fire did not create dark beetles — it only changed which ones did well.
💡 Exam tip
Learn the definition word for word: a change in the heritable characteristics of a population over generations. It is free marks.
In any “explain how X evolved” question, always start with variation already exists due to random mutation. Most lost marks are here.
Name the selection pressure explicitly: the predator, the drug, the cold, the lack of food.
Finish with the population, not the individual: “the allele becomes more frequent in the population”.
Use the words allele and frequency. They signal to the examiner that you understand the genetic side.
If the question gives numbers, do the percentage or ratio first, then explain. Both are usually worth marks.
⚠ Common mix-up
“They evolved because they needed to.” Organisms never evolve on demand. The useful allele either happens to be there or it does not.
“The individual evolved.” An individual is stuck with the alleles it was born with. Only the population changes.
Confusing mutation with selection. Mutation creates the variation; selection decides what happens to it.
Saying the environment causes the mutation. The environment filters mutations, it does not order them.
Writing “survival of the fittest” and stopping there. “Fittest” means best suited to that environment right now, not strongest. Say what makes it better suited.
Treating “theory” as “guess”. In science a theory is a well-tested explanation, and you can say so in a NOS question.
Up next: Evidence for Evolution — how DNA sequences, selective breeding and the bones in your own arm all point to the same conclusion.
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