AP Syllabus focus:
‘Biotic and abiotic environments fluctuate, altering which genetic variations are favored and the direction and rate of evolution.’
Natural selection is not fixed because environments are not fixed.
Three classic selection models (stabilizing, directional, and diversifying) are shown as changes in the distribution of phenotypes before vs. after selection. The directional-selection panel is especially useful for visualizing how an environmental shift can change which end of a trait spectrum has higher fitness, thereby changing allele frequencies over generations. Source
When conditions change, the traits that improve survival and reproduction can change too, shifting which alleles increase, decrease, or remain stable in a population.
Why selection pressures shift
Environmental conditions vary across time and space, so the “best” phenotype in one context may be average or harmful in another. Shifts can be gradual (climate trends) or abrupt (fires, storms, pollution events), and organisms experience these changes at the scale of their local habitat.
Key idea: selection depends on context
A trait’s effect on reproductive success is always tied to the environment in which the organism lives.
A reaction norm plots phenotype (y-axis) against an environmental gradient (x-axis) for different genotypes, showing how the same genotype can express different phenotypes across conditions. Differences in line slopes illustrate genotype-by-environment interaction (G×E), meaning the effect of genotype depends on the environment—exactly the kind of context dependence that can cause selection to reverse, weaken, or intensify after environmental change. Source
If that environment changes, the relationship between phenotype and fitness can reverse, weaken, or intensify.
Sources of environmental fluctuation
Selection pressures come from both abiotic and biotic factors, and both can fluctuate.
Abiotic changes (nonliving)
Temperature shifts (seasonal extremes, heat waves, cold snaps) can alter enzyme performance, membrane fluidity, and water balance.
Water availability changes (drought, flooding) can favor traits affecting stomatal regulation in plants or osmoregulation in animals.
Light and UV exposure can change across seasons or due to canopy loss, affecting photosynthesis or pigmentation.
Soil or water chemistry (salinity, pH, nutrient availability, toxins) can alter which physiological variants function best.
Biotic changes (living)
Predator abundance can rise or fall, changing the benefit of camouflage, warning coloration, or defensive structures.
Disease and parasites can emerge or shift in prevalence, changing which immune-related variants are favored.
Competitor density can fluctuate, changing selection on resource use, growth rate, or timing of reproduction.
Mutualists (pollinators, microbiomes) can change, altering the advantage of traits that influence attraction or symbiosis.
What is a selection pressure?
Selection pressure: An environmental factor that causes differences in survival or reproductive success among phenotypes, leading to changes in allele frequencies over generations.
Selection pressures often act together, so organisms face trade-offs (a benefit under one condition paired with a cost under another).
How shifting pressures change evolutionary direction and rate
Direction: which variants are favored
When conditions change, the favored phenotype can shift:
From one extreme to another (e.g., selection for cold tolerance replaced by selection for heat tolerance).
From a specialized phenotype to a more generalist phenotype if the environment becomes more variable.
Toward different trait combinations if multiple pressures change simultaneously (predation plus resource limits).
Rate: how quickly allele frequencies change
The speed of evolutionary change depends on:
Strength of selection: large fitness differences accelerate change.
Speed and predictability of change: rapid, unpredictable shifts can prevent consistent increases of any one phenotype.
Generation time: shorter generation times allow faster evolutionary responses.
Standing genetic variation: more pre-existing variation increases the chance that some individuals already possess advantageous traits when conditions shift.
Temporal and spatial variation in selection
Fluctuating selection over time
Seasonal or multi-year cycles can cause alternating selection, where different traits are favored at different times. This can:
Maintain multiple alleles if no single allele is best across all conditions.
Favor phenotypic plasticity (the ability of one genotype to produce different phenotypes in different environments) when cues reliably predict conditions.
Heterogeneous selection across space
When habitats differ (sun vs shade, wet vs dry), different phenotypes can be favored in different locations. If individuals move between habitats, the local advantage of a trait may be diluted, but spatial variation can still maintain diversity if environments remain patchy.
Interactions and feedbacks
Shifting environments can change population density, which then changes selection:
After a disturbance, low density can reduce competition, weakening selection on competitive ability.
As populations rebound, competition intensifies, potentially strengthening selection on resource efficiency or reproductive timing. Biotic evolution can also shift pressures: if prey evolve better defenses, predators impose new or stronger selection on different prey traits, and vice versa.
FAQ
They compare fitness-related outcomes across time periods or habitats.
Common approaches include:
long-term mark–recapture studies of survival and breeding success
measuring trait distributions before and after environmental events
tracking allele-frequency changes at candidate loci across years
If different alleles are favoured at different times or places, no single allele is consistently best.
When the advantage alternates, selection can act like a balancing force, keeping multiple alleles in the population rather than driving one to fixation.
Plasticity can buffer organisms against change by allowing one genotype to produce different phenotypes.
It tends to be most beneficial when environmental cues are reliable and the plastic response improves reproductive success under the new conditions.
Yes. Examples include sudden changes in temperature regimes (urban heat), water chemistry (runoff), or light environments (deforestation).
These can abruptly change which traits improve survival and reproduction, even over just a few generations.
Adaptation can be constrained by:
insufficient heritable variation for the needed trait
selection changing faster than generations turn over
correlated trade-offs, where improving one trait worsens another
small population size increasing the chance of losing beneficial alleles by chance
Practice Questions
Explain how a change in an abiotic factor can shift which phenotypes are favoured by natural selection in a population. (2 marks)
Identifies an abiotic factor that changes (e.g., temperature, water availability, salinity) (1)
Explains that the change alters relative reproductive success so different phenotypes/alleles increase or decrease in frequency over generations (1)
A population experiences alternating wet and dry years. Describe how fluctuating selection pressures could affect (i) the direction of selection and (ii) the rate of evolutionary change in this population. (5 marks)
States that different phenotypes may be favoured in wet vs dry years (direction changes over time) (1)
Explains that alleles advantageous in one year type may be disadvantageous in the other, potentially preventing fixation (1)
Links environmental predictability/alternation to maintenance of variation or favouring flexibility in traits (1)
Describes factors affecting rate, e.g., strength of selection, generation time, amount of existing variation (1)
Explains that rapid alternation can slow net allele-frequency change compared with constant selection (1)
