AP Syllabus focus:
‘Through artificial selection, humans choose which individuals breed, shaping variation in other species over generations.’
Artificial selection explains how human choices can drive rapid evolutionary change. By repeatedly breeding organisms with desired, heritable traits, humans can shift trait distributions and allele frequencies across generations in domesticated populations.
Core idea: human choice as a selective force
What artificial selection does
Artificial selection occurs when humans decide which individuals reproduce based on traits humans value (e.g., size, behavior, yield, appearance). Those choices create non-random differences in reproductive success among individuals, so certain heritable variants become more common over time.
Artificial selection: human-directed breeding in which individuals with preferred, heritable traits are more likely to reproduce, causing those traits (and underlying alleles) to increase in frequency across generations.
Because offspring inherit genes from selected parents, artificial selection shapes variation in other species over generations, aligning trait change with human goals rather than environmental demands.
How it differs from simply “raising organisms”
Not all human influence is artificial selection. Artificial selection specifically requires:
Variation in a trait among individuals
Human preference that determines who breeds
Heritability, so offspring resemble parents for that trait
Generational change, producing a detectable shift in the population
The mechanism: selective breeding and inherited variation
Heritable variation is essential
Artificial selection can only produce sustained change if the targeted trait has a genetic basis.
Heritability: the extent to which differences in a trait among individuals in a population are due to genetic differences (as opposed to environmental effects).
A trait can be strongly affected by environment (nutrition, training, climate), but artificial selection is most effective when the trait shows consistent parent-to-offspring resemblance under similar conditions.
Common approaches used by breeders
Artificial selection is implemented through selective breeding, often using one or more of these strategies:
Choosing breeders with extreme trait values (e.g., largest fruits, calmest animals)
Linebreeding/inbreeding to “fix” desired traits within a lineage
Outcrossing to introduce new variation, then re-selecting for the target trait
Culling or excluding individuals from breeding when they lack desired traits
Each approach changes which alleles are passed on, so the population’s genetic makeup becomes progressively biased toward the chosen traits.
Outcomes: how populations change over generations
Shifting trait distributions
Over repeated generations, artificial selection typically:
This plot shows how the frequency of a favored allele can increase over many generations under consistent directional selection, eventually approaching fixation. In artificial selection, human breeding choices create the same kind of differential reproductive success, so favored alleles can rise rapidly when selection is strong and repeated. Source
Increases the frequency of alleles contributing to the preferred phenotype
Shifts the population average toward the selected trait
Often reduces overall genetic diversity near genes linked to the selected trait, especially under strong selection
Trade-offs and unintended consequences
Selecting strongly for a visible trait can also change other traits due to genetic relationships:
Correlated traits may change if the same genes affect multiple traits (pleiotropy) or if genes are inherited together (linkage)
Health or fertility costs can appear if selection pushes traits beyond what the organism’s physiology can easily support
Reduced adaptability can result if selection narrows variation and future conditions change
Domestication as an extended form of artificial selection
When artificial selection persists for many generations, populations can become domesticated, showing suites of traits that fit human-controlled environments (predictable behavior, altered life history, or modified morphology).
This diagram summarizes the “domestication syndrome” idea: selection (often for tameness) can indirectly shift multiple traits because they share underlying developmental pathways. The labels connect behavioral selection to correlated morphological and physiological changes (e.g., pigmentation changes, floppy ears, shorter snout), illustrating how unintended consequences can emerge during sustained artificial selection. Source
These changes reflect sustained human control over breeding and survival.
What AP Biology expects you to be able to do
Key skills and understandings
Explain that artificial selection works because humans choose which individuals breed, causing inherited traits to become more common.
Connect “human preference” to differential reproduction and generational change in trait frequencies.
Identify the necessary conditions: variation, heritability, and repeated selection across generations.
Distinguish artificial selection from short-term environmental effects that do not alter inherited variation.
FAQ
They compare parent–offspring resemblance and/or use structured breeding designs.
Evidence includes:
Consistent similarity between selected parents and offspring across environments
Repeatability across multiple breeding cycles
Statistical estimates of heritability (e.g., from pedigree data), while controlling for shared environment
Genes can influence multiple traits, or be inherited together.
Key reasons:
Pleiotropy: one gene affects several characteristics
Linkage: nearby genes are transmitted together more often than not
Selection on a visible trait can “drag along” linked alleles affecting health, behaviour, or fertility
Programmes often balance selection intensity with genetic diversity.
Common practices:
Rotating breeding lines and avoiding repeated close-relative matings
Using larger numbers of breeding individuals
Monitoring pedigrees and genetic markers to maintain heterozygosity
Introducing controlled outcrosses, then re-selecting for the target trait
Genomic selection uses many DNA markers across the genome to predict breeding value.
It can speed progress by:
Selecting breeders earlier (before full adult traits are expressed)
Increasing accuracy for complex traits influenced by many genes
Improving selection in traits that are hard/expensive to measure directly
Yes, if programmes deliberately introduce and maintain diversity.
Examples include:
Crossing distinct lines or breeds to bring in new alleles
Maintaining multiple selected lines in parallel
Selecting for heterozygote advantage in specific contexts
Using managed mating schemes that keep rare alleles in the breeding pool
Practice Questions
Define artificial selection and state one requirement for it to cause evolutionary change in a population. (2 marks)
1 mark: Correct definition: humans choose which individuals breed, increasing preferred heritable traits over generations.
1 mark: One valid requirement (e.g., heritable genetic variation in the trait; repeated breeding over multiple generations; differential reproductive success caused by human choice).
A breeder selects only the fastest individuals from a population to produce the next generation for 10 generations. Explain how this can change the population over time, and describe two potential biological drawbacks of strong artificial selection. (5 marks)
1 mark: Selection means fastest individuals contribute more offspring (differential reproductive success).
1 mark: Offspring inherit alleles influencing speed, so those alleles increase in frequency over generations.
1 mark: Population trait distribution/mean shifts towards faster speed across generations.
1 mark: One drawback explained (e.g., reduced genetic diversity; increased incidence of harmful recessive alleles due to inbreeding; correlated undesirable traits due to pleiotropy/linkage).
1 mark: Second distinct drawback explained (as above, must include biological reasoning).
