AP Syllabus focus:
‘Alleles adaptive in one environmental condition may be harmful in another because selective pressures differ.’
Natural selection does not label alleles as permanently “good” or “bad.” Whether an allele increases reproductive success depends on the environment, because environmental conditions determine which phenotypes are favored.
Core idea: environment-dependent selection
An allele’s effect on evolutionary fitness can change when selective pressures change. The same genotype may produce different fitness outcomes across habitats, seasons, diets, pathogens, or climates.
Adaptive allele: an allele that increases fitness (survival and/or reproductive success) in a particular environment compared with alternative alleles.
Context dependence means an allele can be adaptive under one set of conditions yet reduce fitness under another set.
Why an “adaptive” allele can become harmful
Selection favors traits that match current constraints and opportunities, such as resource availability, predators, parasites, temperature, salinity, or toxins. When those conditions change, the fitness landscape shifts, and so does which allele is favored.
Key implications:
No universal best genotype: advantage is measured relative to alternatives in the same environment.
Trade-offs are common: improving one function can reduce another.
Local adaptation can occur: different environments favor different alleles in different places.
Geographic clustering of hemoglobin-disorder ancestry (CDC graphic): The shaded regions highlight where ancestry associated with sickle cell disease and thalassemia is more common. While not a selection model by itself, it helps connect the idea of local selective pressures (like malaria historically) to geographic differences in allele frequencies. Source
Fitness trade-off: a situation where an allele increases one component of fitness (e.g., pathogen resistance) while decreasing another (e.g., growth rate), making its net effect depend on the environment.
Biological mechanisms that create context-dependent effects
Gene-by-environment interactions (G×E)
Reaction norm (G×E) diagram: Each line represents a genotype’s phenotype across an environmental gradient. Non-parallel (or crossing) reaction norms show that the phenotypic effect of an allele depends on the environment, which can cause its fitness advantage to change as selective pressures shift. Source
The same allele can produce different phenotypes depending on conditions (e.g., temperature-dependent enzyme performance), so fitness changes as the environment changes.
Changing costs and benefits
Many adaptations carry costs that only matter in certain contexts:
Defense alleles may reduce growth when predators/parasites are absent (resources diverted to defense).
Detoxification alleles may help in polluted habitats but impose metabolic costs in clean habitats.
Heterozygote advantage can be conditional
Malaria vs. sickle-cell trait distribution map: The left panel shows malaria distribution, while the right shows where the sickle-cell trait is common. The geographic overlap supports the idea that a genotype’s fitness effect depends on the local selective agent (here, malaria), consistent with conditional heterozygote advantage. Source
An allele may be harmful in homozygotes yet beneficial when present in heterozygotes, and the net benefit can depend on the presence of a selective agent (such as a pathogen).
Pleiotropy and antagonistic pleiotropy
One allele can influence multiple traits. If those traits are favored in opposite directions under different conditions, the allele’s overall fitness effect becomes context-dependent.
Frequency dependence and ecological interactions
The advantage of an allele can depend on how common it is or on community context (competitors, mutualists, microbiome), so “adaptive” can shift as the biotic environment shifts.
How to think about “harmful in another environment”
In AP Biology terms, “harmful” means lower relative fitness, not necessarily immediate death. An allele can:
Decrease mating success (behavioral or signal changes)
Reduce fertility (fewer viable offspring)
Slow development (missing breeding windows)
Lower survival only under stress (heat, drought, low food)
Common pitfalls
Individuals do not evolve; allele frequencies shift because environments change which individuals leave more offspring.
“Adaptive” is not a permanent label; it is always environment-specific.
Short-term advantages can persist even with long-term costs if the selective pressure is strong and consistent.
FAQ
By measuring survival and reproductive output for different genotypes across controlled environments (e.g., common-garden or reciprocal-transplant designs).
Evidence includes crossing reaction norms (genotype rankings change) and consistent differences in offspring number rather than only survival.
Protein variants with different stability or catalytic efficiency can perform well only within certain ranges.
Small amino-acid changes can alter folding, membrane fluidity interactions, or binding kinetics, shifting performance with abiotic conditions.
If heterozygotes have the highest fitness only when a stressor is present, selection can maintain the allele despite homozygote costs.
When the stressor varies over space or time, allele frequencies can fluctuate rather than fixing or disappearing.
Many alleles change allocation among growth, maintenance, defence, and reproduction.
A benefit (e.g., immune activity) may require ATP and nutrients, reducing growth or fertility when resources are limited or when the threat is absent.
Yes. If conditions shift (heat wave, food shortage), the same genotype may express different phenotypes via plasticity, changing immediate survival or mating success.
However, evolutionary change requires consistent differences in reproductive success across generations.
Practice Questions
Explain why an allele that increases fitness in one habitat may decrease fitness in a different habitat. (2 marks)
States that selective pressures differ between habitats/environments (1)
Explains that the allele’s phenotype has a cost or reduced performance under the other conditions, lowering reproductive success relative to alternatives (1)
A population carries allele A, which increases resistance to a parasite but reduces growth rate. In Environment 1 the parasite is common; in Environment 2 it is absent. Describe how natural selection could act on allele A in each environment and why. (5 marks)
Environment 1: parasite present creates selection favouring resistance; allele A increases survival and/or reproductive success (1)
Environment 1: allele A therefore increases in frequency over generations (1)
Environment 2: parasite absent removes the benefit of resistance (1)
Environment 2: growth-rate cost reduces fitness; selection acts against allele A relative to other alleles (1)
Environment 2: allele A decreases in frequency, showing context-dependent fitness effects due to differing selective pressures (1)
