AP Syllabus focus:
‘Artificial selection parallels natural selection, but selective pressures come from human choices rather than the natural environment.’
Artificial and natural selection both change populations across generations by favoring heritable variation, but they differ in what sets the “rules” for which traits are favoured. Comparing them clarifies how evolution can be directed or emergent.
Core idea: same evolutionary logic, different drivers
Both artificial selection and natural selection can produce evolutionary change because they act on heritable variation and cause differences in reproductive output among individuals with different phenotypes.
Shared requirements (why both can cause evolution)
Variation: individuals differ in traits (phenotypes).
Heritability: at least some trait differences reflect genetic differences that can be passed on.
Differential reproduction: individuals with certain phenotypes contribute more offspring (and therefore more alleles) to the next generation.
Generational change: trait/allele frequencies shift over multiple generations.
What “selective pressure” means in both contexts
Selective pressure: an environmental or external factor that makes some heritable phenotypes more likely than others to survive and/or reproduce, changing trait and allele frequencies over generations.
In both types of selection, the “pressure” determines which phenotypes become more common, but the source of that pressure differs.
Diagram showing how selection changes allele frequencies across generations under different selection patterns (e.g., directional vs. balancing/disruptive). It visually links “selective pressure” to measurable evolutionary outcomes: allele-frequency shifts from to . Source
The key difference: who/what determines what is “favoured”
Natural selection: environment-driven “filter”
In natural selection, selective pressures arise from the natural environment, including factors that affect survival and reproduction. Which phenotypes are favoured depends on how well they function under current conditions, so “advantage” is typically context-dependent rather than fixed.
Selection targets traits that increase fitness in that environment
The “goal” is not perfection; it is increased reproductive success relative to other individuals
Environmental conditions can vary, so the favoured phenotype can shift over time
Artificial selection: human-defined “goal”
In artificial selection, selective pressures arise from human choices about which individuals are allowed to reproduce.
Comparative images of dog head shapes spanning dolichocephalic (long-snouted), mesaticephalic (intermediate), and brachycephalic (short-snouted) forms. The figure illustrates how strong, human-directed selection can drive rapid morphological divergence within a single species. Source
Humans define which traits are desirable (e.g., appearance, yield, behaviour), so “favoured” traits may not increase survival or reproduction in the wild.
Selection is directed toward human preferences (often narrow, trait-specific criteria)
Traits can be favoured even if they reduce wild fitness, as long as humans maintain breeding and survival
Selection can be unusually strong because breeders can control mating and cull or exclude individuals
Practical consequences of the difference
Speed and directionality
Artificial selection can produce rapid, directional change because humans can:
Choose only a small fraction of breeders
Enforce repeated selection on the same trait each generation
Natural selection can also be strong, but its direction and intensity depend on fluctuating ecological conditions, trade-offs, and chance events affecting survival and reproduction.
Trait outcomes and trade-offs
Natural selection often yields phenotypes shaped by multiple competing demands (trade-offs), because organisms must function across many challenges simultaneously.
Artificial selection often prioritises one or a few traits, increasing the chance of:
Correlated changes in other traits (because genes can influence multiple traits)
Reduced overall robustness if the selected trait conflicts with health or performance
Genetic diversity and vulnerability
Artificial selection frequently uses small breeding pools or repeated use of a few high-value parents, which can reduce genetic diversity.
Lower diversity can make populations less able to respond if conditions change or if a new disease/pest appears, because there may be fewer alternative alleles available.
How to recognise each process in evidence
Patterns consistent with artificial selection:
Rapid trait shifts aligned with human preference
Strong divergence from wild-type forms under controlled breeding
Patterns consistent with natural selection:
Trait frequencies tracking environmental differences over space or time
Fitness advantages that depend on local conditions rather than a fixed target phenotype
FAQ
Yes. If humans provide resources, protection, or controlled mating, traits that lower wild survival (e.g., reduced defences or inefficient movement) can persist and spread.
They may:
Track pedigrees and avoid close crosses
Maintain larger breeding populations
Introduce unrelated lines periodically
Selecting one trait can unintentionally shift others because:
the same genes influence multiple traits, or
genes for different traits are inherited together.
Human-managed conditions can mask weaknesses, allowing narrow optimisation. If management stops or conditions change, limited genetic options may constrain adaptation.
Yes. If a human-favoured trait reduces survival or mating success outside controlled breeding, natural selection can push trait frequencies back in the opposite direction.
Practice Questions
State one similarity and one difference between artificial selection and natural selection. (2 marks)
Similarity: both change allele/trait frequencies over generations by favouring heritable variation / differential reproductive success (1)
Difference: artificial selection is driven by human choice of breeders, whereas natural selection is driven by environmental selective pressures (1)
Explain how artificial selection can cause evolutionary change, and compare its likely effects on genetic diversity and adaptation with natural selection. (5 marks)
Explains that selected individuals are chosen to breed based on a heritable trait, so alleles for that trait increase in frequency across generations (1)
States that artificial selection pressure comes from humans rather than the natural environment (1)
Predicts reduced genetic diversity due to restricted breeding pool/intense selection (1)
Links reduced diversity to lower capacity to respond to new conditions (e.g., disease or environmental change) (1)
Compares with natural selection: favoured traits depend on environmental context and typically reflect survival/reproductive advantage in that environment (1)
