The big idea: A selection pressure is any factor in the environment that makes some individuals survive and reproduce better than others.
It is the 'force' that decides which variations are favoured. The stronger and more consistent the pressure, the faster natural selection changes the population.
Selection pressures come in two kinds: abiotic (from the non-living environment) and biotic (from other living things).
| Type of selection pressure | What it is | Examples |
|---|---|---|
| Abiotic (non-living) factors | Pressures from the non-living environment | Temperature, drought, light, salinity, pH, oxygen level, soil minerals |
| Biotic (living) factors | Pressures from other organisms | Predators, prey availability, parasites and disease, competition for food, mates or territory |
- Selection pressure
- Any environmental factor that affects an organism's chance of surviving and reproducing, so it influences which traits are favoured.
- Abiotic factor
- A non-living part of the environment that can act as a selection pressure, e.g. temperature, drought, salinity or light.
- Biotic factor
- A living part of the environment that can act as a selection pressure, e.g. predators, parasites, or competition with other organisms.
- Competition
- A biotic selection pressure in which organisms struggle over a limited resource — food, water, territory or mates.
Abiotic vs biotic — a quick sort: Ask one question: is the factor alive?
Not alive → abiotic (cold, drought, salt, pH, light).
Alive → biotic (predators, parasites, competitors, mates).
Both can be selection pressures, because both decide who survives and breeds.
Most selection pressures favour traits that help an organism stay alive — good camouflage, disease resistance, the ability to find food. This is survival selection.
But surviving is not enough — an organism must also reproduce. Some traits spread because they raise mating success, even if they make survival harder. This is sexual selection.
Sexual selection: Sexual selection favours traits that help an individual get a mate, rather than traits that help it survive.
It works in two ways:
Mate choice (intersexual): one sex — usually females — chooses partners with a showy trait. Males with brighter colours or bigger displays mate more, so the trait spreads. This is what produces showy plumage and courtship displays.
Mate competition (intrasexual): members of one sex — usually males — compete or fight for access to mates. Bigger, stronger or better-armed individuals win, so antlers, horns and large size spread.
| Route | How it works | Classic example |
|---|---|---|
| Intersexual (mate choice) | One sex (usually females) chooses mates with a showy trait, so that trait spreads | Peacock's huge tail; bright bird plumage; elaborate courtship displays |
| Intrasexual (mate competition) | Members of one sex (usually males) fight or compete for access to mates | Stags clashing antlers; large body size; weapons such as horns or tusks |
The survival-vs-mating trade-off: Sexually selected traits are often costly.
A peacock's huge tail is heavy, conspicuous to predators and expensive to grow — it makes the bird less likely to survive.
So why does it spread? Because the boost to mating success outweighs the cost to survival. A male that lives a little less safely but fathers far more chicks passes on more alleles than a plain, safe male who rarely mates.
This is the key insight: natural selection rewards whatever maximises reproduction, not whatever maximises survival.
Survival selection
- Favours traits for staying alive
- Driven by abiotic + biotic pressures
- Camouflage, disease resistance, speed
- Usually improves survival
Sexual selection
- Favours traits for getting a mate
- Driven by mate choice + mate competition
- Bright plumage, antlers, courtship dances
- Can reduce survival, yet still spreads
Spot the pressure: If a question describes a trait that is showy, costly or used to attract or win mates (bright feathers, a display, antlers), the selection pressure is sexual selection — not camouflage or 'survival of the fittest' in the everyday sense.
Practice with real exam questions
Answer exam-style questions and get AI feedback that shows you exactly what examiners want to see in a full-marks response.
Try Practice Free7-day free trial • No card required
How this is tested: On Paper 1A a 1-mark item describes a trait — for example showy plumage and a courtship display — and asks you to identify the selection pressure behind it. The answer is sexual selection (mate choice favouring a mating trait).
On Paper 2 an Evaluate question can ask you to judge a claim about selection — for example, that natural selection can favour competitive rather than cooperative behaviour. 'Evaluate' means weigh both sides and reach a judgement: give a point for, a point against, then a short conclusion.
IB-style question — evaluate competition vs cooperation
Evaluate the claim that natural selection can sometimes favour competitive rather than cooperative behaviour. [2]
How to score both marks
- Make the point FOR the claim. When a resource (food, water, territory, mates) is limited, individuals that out-compete others survive and reproduce more, so competitive behaviour is favoured and its alleles spread — competition IS a biotic selection pressure.
- Make the point AGAINST. But cooperation can also be favoured when working together raises everyone's survival or reproduction — e.g. group defence against predators, or helping close relatives that share your alleles — so selection does not always favour competition.
- Reach a judgement. The claim is partly correct: natural selection favours whichever behaviour gives the greater reproductive success in that situation — competition when resources are scarce, cooperation when shared effort pays off. (Mark 1: a valid point for. Mark 2: a valid counter-point / judgement.)
Final answer
Partly true: when resources are limited, competitive individuals out-reproduce others, so competition is favoured; but cooperation is favoured when it raises shared survival/reproduction. Selection favours whichever behaviour maximises reproductive success in that context.
✓ Why this scores full marks: An Evaluate answer is not 'yes' or 'no'. It gives a reason the claim could be true, a reason it could be false, and a balanced judgement.
Writing only 'competition is favoured because the fittest survive' gives the point FOR but misses the counter-point — so it caps at 1 of 2 marks.
| Survival selection | Sexual selection | |
|---|---|---|
| What it favours | Traits that help the organism SURVIVE its environment | Traits that raise MATING SUCCESS (attract a mate or beat rivals) |
| Driven by | Abiotic + biotic pressures (cold, predators, food) | Mate choice and competition for mates (a biotic pressure) |
| Typical traits | Camouflage, thick fur, fast running, disease resistance | Bright plumage, antlers, courtship dances, loud calls, large size |
| Effect on survival | Usually IMPROVES survival | The trait can REDUCE survival (costly, conspicuous) but is still favoured |