Nothing in evolution is trying to happen. No rabbit decides to be browner, no giraffe stretches its neck for its children. What actually happens is much simpler and much stranger: some individuals leave more offspring than others, and because their traits are inherited, the whole population drifts. Get that idea straight and this topic becomes easy.
📘 What you need to know
Evolution is change in the heritable characteristics of a population over generations.
Natural selection is the mechanism that drives it: better adapted individuals are more likely to survive, reproduce and pass on their alleles.
Natural selection needs variation. Without differences between individuals, nothing can be selected.
Variation comes from mutation (the original source of new alleles), meiosis (crossing over and random orientation) and random fertilisation.
Species overproduce offspring, so there is competition for limited resources. Intraspecific competition matters most for evolution.
Only heritable characteristics can be passed on. Traits acquired during a lifetime cannot.
Darwin’s On the Origin of Species (1859) replaced Lamarckism and caused a paradigm shift.
Evolution and natural selection are not the same word
Students lose marks by using these two terms interchangeably. They are not the same thing.
Two definitions worth learning word for wordEvolution = change in the heritable characteristics of a population over generations Natural selection = the process that causes it
Evolution is the outcome. Natural selection is the mechanism. If a question asks “what is evolution?”, talking about survival of the fittest is not an answer — that is describing the process, not the result.
Notice something else in that definition: a population. Individuals do not evolve. An individual is stuck with the alleles it was born with for its whole life. What changes across generations is the mix of alleles in the population as a whole.
If your exam sentence starts “the rabbit evolved brown fur so it could hide better”, rewrite it. Nothing evolves in order to do anything. The correct shape is: brown rabbits survived more often, so the brown allele became more common.
The logic of natural selection
Darwin’s argument is a chain of five statements, and each one follows from the last. Learn it as a chain and you can rebuild the whole answer under pressure.
Step 5 feeds back into step 1: the next generation still has variation, just a different mix of it. That is why a small advantage can build up into a large change.
🧩 The five sentences to write in an exam
There is variation in the population, caused by mutation and sexual reproduction.
More offspring are produced than the environment can support, so there is competition.
Individuals with the advantageous characteristic are more likely to survive and reproduce.
They pass on the alleles for that characteristic to their offspring.
Over many generations, the frequency of that allele increases in the population.
A worked example: fur colour in rabbits
Suppose a rabbit population lives on dark soil. There is variation in fur colour: one allele gives dark fur, another gives pale fur. Foxes hunt the rabbits, which makes predation a selection pressure.
Dark rabbits are better camouflaged, so they are more likely to escape a fox, survive to adulthood and breed. Each time they breed they pass the dark allele on. Pale rabbits are eaten more often, so they breed less often and pass their allele on less often. Over generations the dark allele becomes common and the pale allele becomes rare.
The two lines are mirror images because there are only two alleles here, so their frequencies must add up to 1. Notice the curve is S-shaped, not straight.
Careful with your wording. Say individuals are more likely to survive, never that they are guaranteed to. A poorly camouflaged rabbit can still get lucky and breed — its chances are simply lower. Examiners specifically look for this.
Where the variation comes from
Natural selection can only work if individuals differ. If every organism in a population were identical, none would be favoured over another, no characteristic would spread, and the population could not adapt to any change in its environment. So variation is not a detail — it is the raw material.
Variation comes from small differences in DNA base sequences, and there are three sources.
1. Mutation — the only source of brand new alleles
A mutation is a change in the DNA base sequence, usually from a copying error during DNA replication. This is the original source of genetic variation: every allele that exists today started as a mutation at some point. A new allele may be advantageous, disadvantageous, or have no apparent effect.
Two details that get tested:
Only mutations in the dividing cells of the sex organs change the alleles in gametes, so only those are passed to the next generation. A mutation in a somatic (body) cell dies with the individual and has no effect on natural selection.
In asexually reproducing species, mutation is the only source of variation — there is no meiosis and no fertilisation to shuffle anything.
2. Meiosis — shuffling what is already there
Meiosis does not create new alleles. It creates new combinations of existing alleles, in two ways.
Crossing over. During meiosis I, homologous chromosomes pair up. Non-sister chromatids become entangled at points called chiasmata, and a section of one chromatid breaks and rejoins with the chromatid from the other chromosome. Alleles are swapped, so the chromatids end up carrying combinations that neither parent chromosome had.
Two chromatids come out unchanged and two come out recombined. Which combinations appear depends on exactly where along the chromosome the chiasma formed.
Random orientation. At metaphase I the homologous pairs line up along the equator, and each pair can face either way round — completely at random, and independently of every other pair. This is also called independent assortment. When the pairs are pulled apart, the combination of chromosomes ending up in each daughter cell depends on how they happened to line up.
Possible chromosome combinations
number of combinations = 2n where n = number of chromosomes in a haploid cell
For humans n = 23, so 223 is more than 8 million different combinations — from random orientation alone, before crossing over has added anything.
3. Random fertilisation
Meiosis has already made every gamete genetically different. At fertilisation, any male gamete can fuse with any female gamete, and which pair actually meets is a matter of chance. This creates variation between zygotes. Put the three sources together and the chance of two siblings being genetically identical is effectively zero.
Source
What happens
Variation is created between…
Mutation
A random change in the DNA base sequence generates a completely new allele
Individuals in a population (the only source of genuinely new alleles)
Crossing over (prophase I)
Non-sister chromatids exchange sections, producing new allele combinations on a chromosome
Gametes made by one individual
Random orientation (metaphase I)
Homologous pairs line up independently, so chromosomes are dealt into gametes in different combinations
Gametes made by one individual
Random fertilisation
Any male gamete can fuse with any female gamete
Zygotes, and therefore the resulting offspring
A neat way to keep these straight: mutation writes new text, meiosis reshuffles the pages, and fertilisation picks which two books get combined. Only the first one adds anything that was not there before.
Overproduction, competition and the struggle to survive
Most species produce far more offspring than can possibly survive. Some ant species lay millions of eggs at a time; even an elephant with one calf per pregnancy produces more calves over a lifetime than the habitat can support at a stable population size.
Darwin called this overproduction of offspring, and it matters because resources are limited. Food, water, space and light all set a ceiling on how many individuals a habitat can hold — its carrying capacity. When more offspring are produced than resources can support, individuals compete, and many fail to survive and reproduce.
Intraspecific vs interspecific competition
Interspecific competition is between individuals of different species.
Intraspecific competition is between individuals of the same species.
Intraspecific competition plays the bigger role in evolution, and you should be able to say why:
Individuals are most likely to interact with members of their own species.
They occupy the same niche, so they want exactly the same resources.
They are affected by the same abiotic and biotic factors, so the same selection pressures act on all of them.
Put that together: within one species, some individuals have characteristics that make them better adapted — a lion that is stronger and faster catches more prey. Those individuals are more likely to survive into adulthood, more likely to find a mate, and so produce more offspring. Less well adapted individuals often do not reach adulthood at all, so they get no chance to reproduce. This is what is loosely called survival of the fittest.
Heritable and non-heritable characteristics
Only characteristics determined by alleles can be passed on. These are heritable, and they can be physical (the length of a giraffe’s neck) or behavioural (a woodlouse moving towards a dark hiding place).
Characteristics acquired during a lifetime are not heritable. Putting on weight after a good autumn, losing a limb to a predator, or building up muscle through exercise all change the individual but not its DNA, so none of it reaches the offspring.
Lamarckism and the paradigm shift
Before Darwin, the leading idea was Lamarckism: that an organism could pass on characteristics it acquired during its own lifetime. Under Lamarck, giraffes stretched their necks reaching for leaves and their calves inherited the longer neck.
Darwin’s On the Origin of Species (1859) replaced that with a mechanism that actually works, and it was extremely controversial at the time. When new evidence contradicts the assumptions a whole field was built on, and the field has to reorganise around a new explanation, that is a paradigm shift. Nearly two centuries of genetics has since supported Darwin rather than Lamarck.
Worked examples
WORKED EXAMPLE 1
A population of beetles lives on pale tree bark. Most beetles are pale, but a few are dark. The trees become covered in dark soot from a nearby factory. Explain how the beetle population changes over the following decades. [5]
Step 1: variation
There is variation in colour in the population, caused by different alleles.
Step 2: the selection pressure
On dark bark, dark beetles are better camouflaged, so predators find them less easily.
Step 3: differential survival
Dark beetles are more likely to survive to adulthood and reproduce; pale beetles are eaten more often.
Step 4: inheritance
Surviving dark beetles pass the allele for dark colour to their offspring.
Step 5: the outcomeThe frequency of the dark allele increases over many generationsnever write that the beetles turned dark — individuals do not change colour, the population changes composition
WORKED EXAMPLE 2
Explain why natural selection cannot occur in a population in which every individual is genetically identical. [3]
Step 1: what selection needs
Natural selection requires variation so that some individuals differ from others.
Step 2: what happens without it
If all individuals are identical, none has an advantageous characteristic, so no individual is more likely to survive and reproduce than any other.
Step 3: the consequenceNo alleles increase in frequency, so the population cannot adapt to environmental changethis is exactly why loss of genetic diversity is dangerous for endangered species
WORKED EXAMPLE 3
A weightlifter develops large muscles through years of training. Explain why this characteristic will not be inherited by their children. [3]
Step 1: classify the characteristic
Large muscles built by training are acquired during the lifetime of the individual, so they are non-heritable.
Step 2: explain the genetics
The training does not change the DNA base sequence in the gametes, so no altered alleles are passed on.
Step 3: name the discredited ideaInheritance of acquired characteristics is Lamarckism, which the evidence does not supportonly a change in the alleles of gametes can reach the next generation
💡 Exam tip
Always say more likely to survive, never “will survive”. This is one of the most commonly lost marks in the whole topic.
Finish with allele frequency. Most natural selection answers are only complete when you mention the change in frequency over generations.
Name the selection pressure explicitly — predation, drought, a pesticide, competition for mates.
Use the word variation early. It is usually the first marking point.
Say populations evolve, not individuals.
For “sources of variation” questions, give mutation plus meiosis plus random fertilisation, and remember mutation is the only one making new alleles.
⚠ Common mix-up
Saying organisms adapt to survive, as if it were deliberate. Adaptation is the result of selection, not a decision.
Confusing evolution with natural selection. One is the change; the other is the cause.
Claiming mutations happen because they are needed. Mutations are random, and most are neutral or harmful.
Thinking meiosis creates new alleles. It only recombines existing ones.
Forgetting somatic mutations do not count. Only mutations in gamete-forming cells matter for evolution.
Mixing up intraspecific and interspecific. Intra means within one species; inter means between species.
Describing an individual as “fit” when you mean strong. Fitness in biology means reproductive success.
Up next: Selection Pressures — a closer look at exactly what does the selecting, including the strange case of traits that make an animal more likely to be eaten.
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