Natural Selection and Adaptation
4. Selection Pressures
Learning outcomes
- I can define selection pressure.
- I can identify environmental factors that act as selection pressures.
- I can explain how selection pressures influence populations.
- I can predict how populations may respond to changing conditions.
- I can analyze examples of selection pressures in nature.
What Is a Selection Pressure?
A selection pressure is an environmental factor that affects an organism's chances of surviving and reproducing.
Selection pressures are important because individuals within a population are not identical. They have different inherited characteristics, and some of these characteristics may provide an advantage under particular environmental conditions.
If individuals with an advantageous inherited characteristic reproduce more successfully, that characteristic may become more common over many generations.
A simplified sequence is:
Variation → selection pressure → differences in survival and reproduction → inheritance → population change
Selection pressures therefore help drive natural selection and evolutionary change.
Selection Pressures Come From the Environment
A population can experience many selection pressures at the same time.
Examples include:
- Predation.
- Competition.
- Disease.
- Parasites.
- Food availability.
- Water availability.
- Temperature.
- Drought.
- Salinity.
- Oxygen availability.
- Pollution.
- Antibiotics.
- Pesticides.
- Competition for mates.
Some selection pressures involve other organisms, while others involve non-living environmental conditions.
Biotic Selection Pressures
Biotic selection pressures involve living organisms or biological interactions.
Examples include:
- Predators.
- Competition.
- Disease-causing organisms.
- Parasites.
- Availability of prey.
- Availability of mates.
Consider a population of mice hunted by birds.
If some mice are better camouflaged than others, predation may favor the better-camouflaged individuals.
Predation is therefore acting as a selection pressure.
Abiotic Selection Pressures
Abiotic selection pressures involve non-living environmental conditions.
Examples include:
- Temperature.
- Rainfall.
- Drought.
- Light.
- Salinity.
- Soil conditions.
- Oxygen concentration.
- pH.
- Fire.
Suppose a region becomes much drier.
Water becomes scarce.
Plants with inherited characteristics that improve water conservation may have an advantage.
Over generations, these characteristics could become more common.
Selection Pressures Do Not Affect Every Individual Equally
A selection pressure becomes evolutionarily important when individuals vary in ways that affect their response to it.
Imagine a population of insects containing green and brown individuals.
They live on brown tree bark.
Birds hunt them visually.
Brown insects may be more difficult to see.
Therefore:
Predation
→ green insects are detected more frequently
→ more brown insects survive
→ brown insects reproduce more successfully
→ genes associated with brown coloration become more common.
The predator is the selection pressure, while camouflage is the characteristic providing an advantage.
Selection Pressure and Natural Selection
Selection pressure is one part of natural selection.
For natural selection to cause evolutionary change, several conditions are important:
- Individuals vary.
- At least some of the variation is inherited.
- A selection pressure affects survival or reproduction.
- Some inherited characteristics provide an advantage.
- Individuals with those characteristics have greater reproductive success.
- Their offspring inherit the characteristics.
Over generations, the population changes.
Predation as a Selection Pressure
Predators can strongly influence prey populations.
Imagine a population of rabbits with inherited variation in running speed.
A new predator enters the ecosystem.
Faster rabbits may escape more successfully.
The sequence could be:
Variation in speed
→ predator creates selection pressure
→ faster rabbits escape more often
→ faster rabbits survive and reproduce more successfully
→ genes contributing to greater speed become more common.
Predators can therefore influence the evolution of prey.
Prey Can Also Create Selection Pressures
Selection works in both directions.
Suppose a predator population varies in running speed.
If faster predators catch more prey:
Fast prey
→ selection pressure on predators
→ faster predators obtain more food
→ greater survival and reproductive success
→ characteristics contributing to speed may become more common.
Predators and prey can therefore exert selection pressures on each other.
Competition as a Selection Pressure
Competition occurs when organisms require the same limited resources.
Organisms may compete for:
- Food.
- Water.
- Light.
- Space.
- Territory.
- Nesting sites.
- Mates.
Suppose food becomes scarce.
Individuals that can obtain food more efficiently may reproduce more successfully.
Competition can therefore act as a selection pressure.
Worked Example: Competition for Seeds
Imagine a population of birds with different beak sizes.
Most birds normally eat small, soft seeds.
A drought occurs and many of the small seeds disappear.
Larger, harder seeds remain.
Birds with larger, stronger beaks may be able to open these seeds more effectively.
The sequence is:
Drought
→ fewer soft seeds
→ increased competition for food
→ stronger-beaked birds obtain more food
→ greater survival and reproduction
→ genes associated with stronger beaks become more common.
The drought and resulting change in food availability create important selection pressures.
Disease as a Selection Pressure
Disease can strongly affect populations.
Individuals may vary genetically in their resistance to a pathogen.
Suppose a disease enters a population.
Some individuals may be more resistant.
These individuals may:
- Survive more frequently.
- Remain healthy enough to reproduce.
- Produce more surviving offspring.
If disease resistance has a heritable component, resistance may become more common over generations.
The disease is acting as a selection pressure.
Parasites as Selection Pressures
Parasites obtain resources from a host and usually reduce the host's fitness.
Examples include:
- Ticks.
- Fleas.
- Parasitic worms.
- Disease-causing microorganisms.
Hosts with inherited characteristics that improve resistance to parasites may have greater reproductive success.
At the same time, hosts can create selection pressures on parasites.
This can produce continuing evolutionary change in both populations.
Temperature as a Selection Pressure
Temperature can strongly affect organisms.
Suppose a climate becomes colder.
Individuals within a mammal population vary in:
- Fur thickness.
- Body size.
- Metabolic characteristics.
Individuals with characteristics that reduce heat loss may survive and reproduce more successfully.
Over many generations, the population may become better adapted to cold conditions.
Drought as a Selection Pressure
Drought reduces water availability.
This can affect both plants and animals.
Plants may vary in characteristics such as:
- Root depth.
- Leaf size.
- Waxy coverings.
- Ability to close stomata.
- Water-storage capacity.
Individuals that conserve or obtain water more successfully may have an advantage.
If these differences are inherited, the population may change over generations.
Salinity as a Selection Pressure
Salinity is the concentration of dissolved salts in an environment.
Organisms living in aquatic or coastal environments may experience changes in salinity.
Individuals may differ in their ability to:
- Maintain water balance.
- Control salt concentrations.
- Function under high or low salinity.
If salinity changes, individuals with better tolerance may reproduce more successfully.
Food Availability as a Selection Pressure
Changes in food availability can alter which characteristics are advantageous.
For example, a bird population might normally have access to:
- Small seeds.
- Large seeds.
- Insects.
If environmental change removes most small seeds, birds capable of using other food sources may gain an advantage.
Food availability can therefore influence:
- Feeding structures.
- Feeding behavior.
- Competition.
- Reproductive success.
Antibiotics as a Selection Pressure
Antibiotics provide a particularly clear example of natural selection.
A bacterial population contains genetic variation.
Some bacteria may already possess resistance to a particular antibiotic.
When the antibiotic is used:
Antibiotic applied
→ susceptible bacteria die
→ resistant bacteria survive
→ resistant bacteria reproduce
→ resistance becomes more common.
The antibiotic acts as a selection pressure.
Antibiotics Do Not Create Resistance Because Bacteria "Need" It
This distinction is very important.
Incorrect:
"The bacteria became resistant because the antibiotic forced them to adapt."
Better:
Genetic variation already existed or arose through mutation. The antibiotic killed susceptible bacteria, while resistant bacteria were more likely to survive and reproduce.
The antibiotic selects among variants.
It does not deliberately produce the useful variation.
Pesticides as Selection Pressures
Pesticides can create similar selection pressures in pest populations.
Suppose most insects are susceptible to a pesticide, but a few possess inherited resistance.
After spraying:
Pesticide applied
→ susceptible insects die
→ resistant insects survive
→ resistant insects reproduce
→ resistance becomes more common.
Repeated pesticide use can therefore result in populations that are increasingly difficult to control.
Human Activities Can Create Selection Pressures
Humans can change environments rapidly.
Human-created selection pressures can include:
- Antibiotics.
- Pesticides.
- Pollution.
- Hunting.
- Fishing.
- Habitat modification.
- Artificial light.
- Urban environments.
- Climate change.
Populations may respond evolutionarily when these pressures consistently affect reproductive success and relevant variation is heritable.
Fishing as a Selection Pressure
Fishing can sometimes remove particular types of individuals from a population.
Suppose fishing methods disproportionately catch large fish.
If body size has a heritable component, smaller individuals may have a greater chance of surviving long enough to reproduce.
Over many generations, this could influence characteristics of the population.
The important sequence is:
Variation
→ selective removal
→ differences in reproductive success
→ inherited population change.
Selection Pressures Can Change
Environments are not constant.
Selection pressures can change because of:
- Climate change.
- Seasonal changes.
- New predators.
- New diseases.
- New competitors.
- Habitat destruction.
- Changes in food availability.
- Human activities.
A characteristic that was once advantageous may become neutral or disadvantageous.
Worked Example: Environmental Change
Imagine a population of insects living on green vegetation.
Most insects are green, while some are brown.
Green coloration provides good camouflage.
A prolonged drought kills much of the vegetation, leaving brown stems and soil exposed.
Now:
Before drought
Green insects may be better camouflaged.
After drought
Brown insects may be better camouflaged.
The environmental change has altered the selection pressure.
Predation may now favor a different coloration.
Selection Pressures Can Reverse Direction
Suppose thick fur is advantageous during a long period of cold weather.
If the climate becomes much warmer:
- Thick fur may increase the risk of overheating.
- Thinner fur may become advantageous.
The same characteristic can therefore be:
Advantageous under one condition
but
disadvantageous under another.
Natural selection depends on the current environment.
Populations Can Respond in Different Ways
When conditions change, populations do not always respond in the same way.
Possible outcomes include:
- Evolutionary change.
- Behavioral changes.
- Movement to another habitat.
- Reduced population size.
- Local extinction.
- Complete extinction.
Evolutionary adaptation is only one possible response.
Migration Versus Evolution
Suppose temperatures increase.
Some animals may move into cooler regions.
This is a behavioral or distributional response, not necessarily evolutionary change.
Evolution requires changes in inherited characteristics within populations across generations.
This distinction is important.
Acclimatization Versus Evolution
Individual organisms may sometimes adjust physiologically to changing conditions.
For example, an individual may adjust to:
- Higher temperatures.
- Lower oxygen levels.
- Changes in light.
These changes during an individual's lifetime are not necessarily evolutionary adaptations.
For evolution to occur, inherited characteristics within the population must change across generations.
Selection Pressure Does Not Guarantee Adaptation
A population may experience a strong selection pressure but still fail to adapt.
Evolutionary adaptation requires suitable heritable variation.
If useful variation is absent, or environmental change occurs too quickly, the population may decline.
For example:
Rapid environmental change
→ insufficient useful variation
→ high mortality
→ population decline
→ possible extinction.
Natural selection therefore does not guarantee survival.
Strength of Selection Pressure
Some selection pressures have relatively small effects on reproductive success.
Others have very strong effects.
For example, if a toxin kills almost every individual lacking resistance, the selection pressure may be extremely strong.
Strong selection can sometimes cause rapid changes in populations, particularly in organisms with:
- Short generation times.
- Large populations.
- High reproductive rates.
This is one reason evolutionary change can occur quickly in bacteria.
Multiple Selection Pressures
Organisms usually experience several selection pressures simultaneously.
A rabbit might experience:
- Predation.
- Competition for food.
- Disease.
- Temperature changes.
- Competition for mates.
A characteristic that helps with one pressure might create disadvantages under another.
For example, a large body may help an animal compete for mates but require more food.
Natural selection therefore involves complex combinations of advantages and disadvantages.
Trade-Offs
An evolutionary trade-off occurs when a characteristic provides both benefits and costs.
For example, bright feathers might:
- Increase success in attracting mates.
but also:
- Make an animal easier for predators to detect.
The evolutionary outcome depends on the combined effect on reproductive success.
Sexual Selection
Competition for mates can also create selection pressures.
Individuals may differ in:
- Coloration.
- Calls.
- Courtship behavior.
- Body size.
- Antlers or horns.
Characteristics that increase mating success may become more common, even when they have some survival costs.
This process is called sexual selection.
Worked Example: Deer Antlers
Male deer may use antlers during competition for mates.
Suppose larger antlers increase success in obtaining mates.
If antler characteristics are heritable:
Variation in antler size
→ competition for mates
→ some males reproduce more successfully
→ genes contributing to advantageous antler characteristics are passed on
→ these characteristics may become more common.
Competition for mates acts as a selection pressure.
Selection Pressures and Adaptations
Over many generations, selection pressures can contribute to the development of adaptations.
Examples include:
Cold temperature
→ thick insulation.
Predation
→ camouflage.
Water shortage
→ water-conserving characteristics.
Hard food
→ feeding structures capable of processing it.
Disease
→ resistance.
Antibiotics
→ antibiotic resistance in bacteria.
The selection pressure does not directly create the adaptation. Instead, it influences which inherited variants reproduce most successfully.
Analyzing Selection Pressures in Nature
When analyzing an example, ask five questions:
What is the environmental factor?
This identifies the possible selection pressure.
What variation exists?
Identify differences between individuals.
Which individuals have an advantage?
Determine which variation works better under the conditions.
How does this affect reproductive success?
Explain why those individuals contribute more offspring.
How might the population change?
Predict which inherited characteristics may become more common.
Worked Example: Arctic Rabbits
Suppose a rabbit population contains variation in fur thickness.
Temperatures become colder.
Selection pressure
Low temperature.
Variation
Some rabbits have thicker fur than others.
Advantage
Thicker fur reduces heat loss.
Reproductive success
Rabbits with thicker fur may survive and reproduce more successfully.
Population response
If fur thickness is heritable, genes contributing to thicker fur may become more common over generations.
This is how to construct a complete natural-selection explanation.
Worked Example: Changing Rainfall
A plant population contains variation in root depth.
Rainfall decreases over several decades.
Selection pressure
Reduced water availability.
Variation
Some plants have deeper roots.
Advantage
Deeper-rooted plants can access water unavailable to shallow-rooted plants.
Reproductive success
Deep-rooted plants may survive and produce more seeds.
Population response
Inherited characteristics associated with deeper roots may become more common.
Worked Example: New Predator
A lizard population contains variation in leg length and running speed.
A fast predator enters the ecosystem.
If longer legs contribute to greater running speed:
New predator
→ stronger predation pressure
→ faster lizards escape more successfully
→ greater survival and reproduction
→ inherited characteristics contributing to speed may become more common.
However, this prediction depends on whether leg length and speed actually affect reproductive success and have heritable components.
Predicting Population Responses
When predicting how a population might respond to environmental change, avoid simply saying:
"The organisms will adapt."
Instead, identify the mechanism.
A stronger answer is:
If inherited variation exists that improves survival or reproductive success under the new conditions, individuals possessing those characteristics may reproduce more successfully. Over generations, those characteristics may become more common.
This explains natural selection rather than assuming adaptation automatically occurs.
Environmental Change and Extinction
Sometimes populations cannot respond successfully.
For example:
Temperature rises rapidly
→ suitable habitat disappears
→ individuals cannot tolerate the new conditions
→ reproduction decreases
→ population declines.
If the population cannot:
- Adapt.
- Move.
- Change behavior sufficiently.
it may become extinct.
Selection Pressures Can Maintain Variation
Selection pressures do not always cause one characteristic to completely replace all others.
Different environments may favor different characteristics.
For example:
- One coloration may be advantageous in forests.
- Another may be advantageous in grasslands.
If environments vary across an organism's range, multiple forms may remain within the species.
Selection Pressures Change Over Time
Evolution does not produce a permanently "perfect" population.
Environmental conditions continually change.
A population well adapted to today's conditions may face very different pressures in the future.
This means natural selection is an ongoing process.
Common Mistakes
Thinking a Selection Pressure Is a Characteristic
A predator is a selection pressure.
Camouflage is a characteristic that may provide an advantage against that pressure.
Saying Selection Pressures Create Useful Mutations
Selection pressures select among genetic variation. They do not produce mutations because organisms need them.
Saying Individuals Evolve
Evolutionary changes occur in populations over generations.
Assuming Every Environmental Factor Is a Selection Pressure
An environmental factor acts as a selection pressure only when it influences reproductive success in a way that can affect population characteristics.
Assuming Organisms Always Adapt
Populations may migrate, decline, or become extinct instead.
Forgetting Reproduction
Survival matters evolutionarily because it can influence reproductive success.
Assuming a Trait Is Always Advantageous
Advantages depend on environmental conditions.
Confusing Acclimatization With Evolution
Changes occurring within an individual's lifetime are not necessarily inherited evolutionary changes.
Assuming Only Natural Factors Create Selection Pressures
Human activities can create powerful selection pressures, including antibiotics, pesticides, fishing, pollution, and habitat modification.
Check Your Understanding
1. Define selection pressure.
2. Give four examples of biotic selection pressures.
3. Give four examples of abiotic selection pressures.
4. Explain how predation can act as a selection pressure.
5. Explain how competition can act as a selection pressure.
6. How can disease influence natural selection?
7. Explain how drought could act as a selection pressure on plants.
8. Why might temperature act as a selection pressure?
9. Explain how antibiotics act as a selection pressure on bacteria.
10. Why is it incorrect to say that bacteria become resistant because they need to survive antibiotics?
11. Explain how pesticides can lead to resistant insect populations.
12. Give three examples of selection pressures created by human activities.
13. Why might a characteristic that is advantageous today become disadvantageous in the future?
14. Explain why selection pressure does not guarantee that a population will adapt.
15. Distinguish between an evolutionary response and an individual response to environmental change.
16. Explain what is meant by an evolutionary trade-off.
17. A drought causes hard seeds to become more common than soft seeds. Predict how this could affect a bird population with variation in beak size.
18. A new predator enters an ecosystem. Explain how this could influence a prey population over many generations.
19. A population of plants experiences increasing temperatures and decreasing rainfall. Identify two possible selection pressures and predict characteristics that might provide an advantage.
20. Explain the complete sequence connecting variation → selection pressure → reproductive success → inheritance → population change.
Key Terms
- Selection pressure – environmental factor that affects survival or reproductive success and can influence which inherited characteristics become more common.
- Biotic selection pressure – selection pressure involving living organisms or biological interactions.
- Abiotic selection pressure – selection pressure involving non-living environmental conditions.
- Variation – differences between individuals within a population.
- Heritable variation – differences with a genetic component that can be passed to offspring.
- Natural selection – process through which inherited characteristics affecting reproductive success become more or less common over generations.
- Adaptation – inherited characteristic that increases reproductive success in a particular environment.
- Predation – interaction in which one organism captures and consumes another.
- Competition – interaction occurring when organisms require the same limited resources.
- Fitness – reproductive success in a particular environment.
- Antibiotic resistance – inherited ability of bacteria to survive exposure to an antibiotic that would normally kill or inhibit them.
- Pesticide resistance – inherited ability to survive exposure to a pesticide.
- Sexual selection – selection resulting from differences in success at obtaining mates.
- Trade-off – situation in which a characteristic provides both benefits and costs.
- Acclimatization – adjustment occurring within an individual's lifetime in response to environmental conditions.
- Extinction – permanent disappearance of a species.
Key Takeaways
- A selection pressure is an environmental factor that affects survival or reproductive success.
- Selection pressures can be biotic or abiotic.
- Predation, competition, disease, and parasites are common biotic selection pressures.
- Temperature, drought, salinity, and other physical conditions can act as abiotic selection pressures.
- Human activities can create powerful selection pressures.
- Antibiotics select for resistant bacteria; they do not cause bacteria to develop resistance because they need it.
- Pesticides can similarly select for resistant pests.
- Selection pressures act on variation already present or arising within populations.
- Individuals possessing advantageous inherited characteristics may reproduce more successfully.
- These characteristics can become more common over generations.
- Changing environmental conditions can change the direction or strength of selection.
- A characteristic that is advantageous in one environment may be disadvantageous in another.
- Organisms often experience several selection pressures simultaneously.
- Adaptations can involve trade-offs.
- Populations do not automatically adapt when conditions change.
- Populations may instead migrate, decline, or become extinct.
- Evolutionary change requires differences in inherited characteristics across generations.
- A useful framework for analyzing selection pressure is: environmental factor → variation → advantage → reproductive success → inheritance → population change.