AP Syllabus focus:
‘Some phenotypic variations increase or decrease an organism’s fitness, depending on the particular environmental conditions it experiences.’
Phenotypic differences only matter evolutionarily when they affect fitness in a particular setting. The same trait can be beneficial, neutral, or harmful depending on abiotic conditions, biotic interactions, and resource availability.
Core idea: fitness is environment-specific
Natural selection favors phenotypes that increase fitness in a given environment; when the environment differs, the ranking of which phenotypes do best can change.
Phenotype: the observable characteristics of an organism (e.g., morphology, physiology, behavior) produced by interactions between genotype and environment.
Fitness is not a “best” phenotype in general; it is a match between phenotype and local conditions.
Evolutionary fitness: an organism’s relative ability to leave viable, fertile offspring in a particular environment.
Because fitness is measured by reproductive output, any factor that changes survival to reproduction, mating success, or number/quality of offspring can shift which phenotypes are favored.
How specific environments change which phenotypes are favored
Environmental features that determine fitness effects
Different environments impose different constraints and opportunities, so the same phenotypic variation can change performance:
Abiotic factors (nonliving)
Temperature, water availability, pH, salinity, oxygen level, light, nutrient availability
These can alter enzyme activity, membrane fluidity, gas exchange, and water balance, changing growth and reproduction
Biotic factors (living)
Predators, pathogens, parasites, competitors, mutualists, and mates
These can change the value of traits tied to defense, immune function, resource acquisition, or courtship
A phenotype that improves performance under one combination of abiotic and biotic factors may reduce performance under another combination.
Trade-offs: why “better” in one place can mean “worse” in another
Phenotypes often involve trade-offs, so improvements in one function may reduce another:
Energy invested in rapid growth may reduce investment in immune defense
Greater pigmentation may increase UV protection but also increase heat absorption or visibility to predators
Higher metabolic rate may increase activity and resource capture but raise food requirements
These trade-offs make fitness strongly dependent on which challenges dominate the environment.
Context dependence and “conditional” advantages
Some phenotypes only increase fitness when a specific condition is present:
A defensive trait is beneficial where predators are common, but may carry unnecessary costs where predators are rare
A competitive trait is valuable when resources are scarce, but less beneficial when resources are abundant
A tolerance trait (e.g., drought tolerance) increases fitness only when water limitation is a key constraint
Connecting phenotype to fitness in a specific environment
Fitness differences emerge through measurable components:
Viability selection: phenotypes differ in survival to reproductive age
Sexual selection: phenotypes differ in mating success (mate choice or competition)
Fecundity selection: phenotypes differ in number or quality of offspring produced
In practice, biologists often compare relative fitness among phenotypes within the same environment to determine which is favored.
Relative fitness: the reproductive success of a phenotype compared with other phenotypes in the same population and environment (often scaled so the highest is 1).
Genotype × environment effects on phenotype and fitness
Phenotype is shaped by both genotype and environment, so environment can change not only which phenotype is best, but also which phenotype is expressed.
A norm of reaction shows how phenotype changes across an environmental gradient for different genotypes. Non-parallel reaction norms indicate genotype × environment (G×E) interactions, meaning the effect of the environment depends on genotype and the phenotype expressed can differ across conditions. This provides a visual framework for understanding phenotypic plasticity and why “best” phenotypes can shift between environments. Source
Phenotypic plasticity: the ability of a single genotype to produce different phenotypes when exposed to different environmental conditions.
Plasticity can increase fitness when environments vary across space (different habitats) because individuals can shift traits (e.g., behavior, physiology, morphology) toward a locally beneficial state. However, plastic responses can be limited by developmental constraints, time lags, or energetic costs, meaning plasticity is not always advantageous.
Illustrative examples of environment-dependent fitness (conceptual)
Camouflage coloration: A coat color that blends into dark substrate reduces predation risk in dark habitats, but the same color increases visibility in light habitats, decreasing fitness.
These images compare light and dark color morphs against bark backgrounds, illustrating how visibility to predators can depend on environmental context. When a morph matches the substrate, detection (and thus predation) tends to decrease, increasing survival and expected reproductive success. When the substrate changes, the same morph can become conspicuous and experience reduced fitness. Source
Water-use traits in plants: Traits that reduce water loss (e.g., lower stomatal opening) can improve survival and reproduction in dry environments but may limit CO₂ uptake and growth where water is plentiful.
Beak size and food type: Larger, stronger beaks can increase feeding efficiency where hard seeds dominate, but may reduce efficiency on small, soft seeds or impose higher energetic costs when food is scarce.
Hemoglobin variants and oxygen availability: Variants that increase oxygen binding can improve performance at low oxygen (e.g., high altitude) but may reduce oxygen unloading in tissues at sea level, lowering overall performance.
The oxygen–hemoglobin dissociation curve relates partial pressure of oxygen to percent hemoglobin saturation, showing how oxygen loading and unloading change across environments. Right/left shifts illustrate how factors like pH, temperature, and 2,3-BPG alter hemoglobin’s affinity for oxygen, changing performance under different conditions. This is a mechanistic example of how an environment can alter which physiological phenotype is advantageous. Source
These examples reflect the syllabus focus: phenotypic variations can increase or decrease fitness depending on the particular environmental conditions experienced.
FAQ
They often use common garden and reciprocal transplant experiments.
Common garden: raise different genotypes/phenotypes in the same controlled environment to reduce environmental variation.
Reciprocal transplant: move individuals between habitats and compare survival and reproduction.
If rank order of reproductive success changes between habitats, fitness is environment-dependent.
A reaction norm describes how a genotype’s phenotype changes across environments.
It helps separate whether environment-dependent fitness is due to:
different phenotypes being expressed (plasticity), or
the same phenotype having different success across environments.
Yes.
This can happen if:
benefits and costs balance (trade-offs),
resources are abundant so differences don’t affect reproduction, or
different strategies yield similar reproductive output (alternative stable strategies).
Equal fitness can be temporary and may shift if conditions change.
Costs reduce fitness when the environmental benefit is small or absent.
Common costs include:
higher energy use,
increased predation risk (e.g., conspicuous displays),
reduced growth or fertility due to resource allocation.
A trait is favoured only when benefits outweigh costs in that environment.
Fitness involves lifetime reproductive success, which can be hard to track.
Challenges include:
long lifespans and dispersal,
hidden mating events,
variable offspring survival,
environmental heterogeneity within a habitat.
Researchers may use proxies (e.g., offspring number, survival to breeding age), but each proxy has limitations.
Practice Questions
Explain why a phenotype that increases body insulation could have higher fitness in a cold environment but lower fitness in a warm environment. (3 marks)
States that fitness depends on reproductive success in a specific environment (1)
Cold environment: insulation reduces heat loss, improving survival to reproduction and/or reproductive output (1)
Warm environment: insulation can cause overheating or increased energetic costs, reducing survival, mating success, and/or fecundity (1)
A lizard species shows two phenotypes: Phenotype A runs faster but dehydrates more quickly; Phenotype B runs slower but conserves water. Habitat 1 is hot and dry with scarce water; Habitat 2 is cooler with frequent water sources and many predators. Use the idea of environment-dependent fitness to predict which phenotype would be favoured in each habitat and justify your predictions using fitness components. (6 marks)
Identifies that selective advantage depends on environmental conditions (1)
Predicts Phenotype B favoured in Habitat 1 (hot/dry) (1)
Justifies with higher survival to reproduction due to reduced dehydration / improved water balance (viability component) (1)
Predicts Phenotype A favoured in Habitat 2 (cooler, predators) (1)
Justifies with higher survival and/or mating opportunities due to escape from predators via speed (viability and/or sexual selection component) (1)
Mentions trade-off/cost: benefit in one habitat becomes a disadvantage in the other (e.g., dehydration cost vs speed cost) (1)
