3.7.3 Evolution and speciation — Atlas

Evolution is what happens when allele frequencies in a population shift across generations under natural selection. Speciation is what happens when the shift goes far enough — geographically or behaviourally — that two parts of the population can no longer interbreed to produce fertile offspring.

Natural selection changes allele frequencies when variation, differential survival, and heritability all hold.

Natural selection is the mechanism that shifts allele frequencies in a population across generations in response to environmental pressures. Three conditions must hold simultaneously. The result, repeated over many generations, is evolution at the population level.

The three conditions for natural selection

Variation — individuals in the population differ in phenotype. Differential survival and reproduction — some phenotypes do better in current conditions and leave more offspring. Heritability — the advantageous traits are encoded in alleles passed from parent to offspring. All three must hold simultaneously; remove any one and the mechanism stops working across generations.

Mutation is random and pre-existing

Mutations occur randomly. They are not produced in response to environmental demand. The environment selects from variation that already exists; it does not generate that variation. The selection pressure does not cause the mutation. The mutation pre-exists; the selection pressure decides whether its carriers spread.

Write random mutation pre-exists the selection pressure. Don't write mutate to adapt or mutation caused by [environmental change]. Both reverse the causal direction and are explicit rejects.

Pitfall — The anti-teleology rule: don't write mutate to adapt

Mutations do not happen because they would be useful.

Variants of this error — mutate to adapt, mutation caused by drinking milk, mutate in response to, mutation is not harmful — all reverse the causal direction the topic depends on. Mutations are random events. The selection pressure does not generate them.

The correct framing is two-step: a random pre-existing mutation creates an allele, and the environment then determines whether carriers survive and reproduce more than non-carriers. Any teleological phrasing — any wording implying mutations happen because they would be useful — is rejected across every selection question on this topic.

The survival chain runs at the level of individuals, not alleles. Individuals carrying advantageous alleles survive at higher rates; those individuals reproduce more; their offspring inherit the alleles; the frequency of the advantageous allele rises in the next generation's gene pool. Across many generations, allele frequencies shift in favour of whatever the current conditions reward.

Write individuals carrying the allele survive and reproduce, not alleles survive and reproduce. And write alleles, not genes. Gene frequency, advantageous gene, and alleles surviving all misplace the level at which the mechanism operates.

Selection pressure is not the isolating feature

The selection pressure is the environmental difference — temperature, food type, predator, drought, antibiotic exposure. Altitude, distance, or a mountain range is the isolating mechanism that produces the difference; it is not the selection pressure itself. Two distinct mark targets in any speciation answer.

Directional, stabilising, and disruptive selection are three patterns of natural selection.

Three patterns of natural selection are distinguished by which phenotypes are favoured within the population's range of variation. AQA awards a dedicated mark for naming the selection type, even when the surrounding mechanism is correctly described. Naming is its own mark target.

The three selection patterns.

Selection type	Distribution shift	AQA exemplar
Directional	Mean shifts toward one extreme	Antibiotic resistance in bacteria
Stabilising	Mean stays; extremes culled; distribution narrows	Human birth weight
Disruptive	Both extremes favoured; distribution bifurcates	Body size in some large mammals

Name the selection type — directional, stabilising, or disruptive — as a discrete term. Describing the mechanism without naming the type forfeits its dedicated mark.

A random pre-existing mutation creates an allele in the gene pool.
An environmental change (a new selection pressure) makes the allele advantageous.
Individuals carrying the allele survive and reproduce more than non-carriers.
Name the selection type — directional selection.
The allele frequency increases in the next generation's gene pool.

On directional answers, the two most-dropped marks are random pre-existing mutation (Step 1) and naming directional selection (Step 4). Lead with the random mutation and name the type explicitly.

Dominance and the rate of spread

Dominant advantageous alleles spread quickly because heterozygotes express them from the first generation. Recessive advantageous alleles spread slowly because most copies are hidden in heterozygotes, invisible to selection. For rate-of-spread questions, name dominance as the explanation — not selection alone.

Allopatric speciation requires a geographical barrier; the two halves diverge with no gene flow.

Speciation is divergence into two reproductively isolated populations that can no longer interbreed to produce fertile offspring. Allopatric speciation requires a physical, geographical barrier dividing a single population. The barrier may be a mountain range, sea, river, desert, or continental drift; the function is the same — individuals from the two groups can no longer meet to mate, and gene flow ceases.

What happens after the barrier rises

With no gene flow, the two populations accumulate different mutations independently. The two environments expose them to different selection pressures, favouring different alleles in each. Allele frequencies in the two populations diverge progressively over generations. Eventually the divergence reaches reproductive incompatibility — the two groups cannot interbreed to produce fertile offspring.

When the stimulus names a specific feature (river, mountain, ocean), reference it by name. Generic a physical barrier does not earn the geographical isolation mark when a specific feature is given in the question.

The AQA exemplar is the camel and the llama. A common ancestor was split by continental drift; the two groups evolved independently on their separated continents over millions of years; the descendants can no longer produce viable fertile hybrids together. Two genera, one ancestor, one slow geological split.

Sympatric speciation occurs without geographic separation; reproductive isolation arises by polyploidy, behaviour, or anatomy.

Sympatric speciation produces new species from a single ancestral population occupying the same geographic area, without a physical barrier to movement. The opening step of every sympatric answer is the statement that the two groups are in the same area, or are not geographically isolated. This step is the single most-omitted mark on sympatric questions.

Open every sympatric answer with same area or not geographically isolated. This is its own mark and the most-omitted step on the topic's hardest question type.

Polyploidy

A chromosomal error producing more than the normal diploid chromosome set. A tetraploid cannot produce viable fertile offspring with a diploid: the cross gives triploid offspring whose chromosomes cannot pair correctly in meiosis. Polyploidy generates reproductive isolation in a single generation — the fastest known speciation mechanism. Particularly common in plants.

Behavioural isolation

A mutation alters courtship signals or mate recognition — a pheromone profile, a song pattern, a visual display, a timing cue. Mutant individuals no longer attract or are attracted to wild-type partners. Reproduction becomes limited to individuals sharing the mutation, even within the same shared habitat.

Anatomical isolation

A structural change to reproductive anatomy or to the structures involved in pollination physically prevents successful mating or fertilisation. A different flower shape locks out the original pollinator; incompatible reproductive organs prevent mating between mutant and wild-type. The mechanism is mechanical, not geographic.

Disruptive selection often plays a role in sympatric speciation. Where the same-area population already shows two phenotypic clusters with different fitness, disruptive selection sharpens the distinction and reinforces the developing reproductive isolation. Name disruptive selection explicitly in sympatric sequences where the population bifurcates.

Name disruptive selection explicitly in sympatric sequences where the population bifurcates. It is a credit-bearing step in its own right.

Pitfall — Read the named mode before writing

The single largest mark-loser on this topic is writing an allopatric answer to a sympatric question, or vice versa.

When the stem names sympatric speciation, an allopatric-only answer caps at three marks out of five — the geographical isolation step is explicitly rejected. When the stem names allopatric speciation, sympatric for the geographical isolation step is also rejected. Allopatric and sympatric are not interchangeable.

Read the named mode first. Anchor the opening step to the right mode — same area for sympatric, the specific named geographical feature for allopatric. Then run the remaining five steps as standard.

The full speciation sequence is six ordered steps; each step is its own mark.

Every speciation answer in the corpus follows the same six-step structure. The first step varies with mode; the remaining five are shared. Each step is its own mark target, so skipping any step costs that mark.

Geographic context. For allopatric, name the specific isolating feature from the stimulus. For sympatric, state same area or not geographically isolated.
Reproductive isolation. The two groups stop interbreeding; gene flow ceases; the gene pools become separate.
Different selection pressures and different mutations accumulate independently in the two populations.
Name the selection type — directional, stabilising, or disruptive — where it fits the stimulus.
Change in allele frequency. The two populations' gene pools diverge as advantageous alleles rise and disadvantageous ones fall.
Cannot interbreed to produce fertile offspring. The two populations are now distinct species.

Write reproductive isolation as a discrete step, and close with cannot interbreed to produce fertile offspring. AQA accepts no gene flow or gene pools remain separate for step 2. The terminal step closes the chain; allele frequencies diverged or different species evolved alone do not earn the final mark.

Key terms

mutation
alleles
allele frequency
selection pressure
directional selection
disruptive selection
allopatric speciation
sympatric speciation
geographical isolation
gene flow
reproductive isolation
fertile offspring