Artificial
Selection
Explore how humans shape the evolution of species by selectively breeding organisms with desirable traits across generations.
AP Biology
What Is Artificial Selection?
Artificial selection is the process by which humans intentionally breed organisms to promote specific desirable traits. Unlike natural selection, where the environment determines which traits are advantageous, in artificial selection humans act as the selective agent.
Why Artificial Selection Matters
Agriculture
Crop yields have increased dramatically through selective breeding for disease resistance, size, and nutritional content.
Medicine
Model organisms like lab mice are bred with specific genetic traits to study human diseases and test treatments.
Evolutionary Evidence
Darwin used artificial selection as evidence that traits can change over generations, supporting his theory of natural selection.
Essential Vocabulary
1
Phenotype: The observable physical or biochemical characteristics of an organism, determined by both genotype and environment.
2
Genotype: The genetic makeup of an organism: the specific set of alleles it carries.
3
Heritable Variation: Differences among individuals that are passed from parents to offspring through DNA.
4
Selective Pressure: In artificial selection, the human breeder determines which trait confers "fitness" for reproduction.
5
Allele Frequency: The relative proportion of a particular allele in a population, which shifts over generations during selection.
How Artificial Selection Works
The Core Principle
If any one of these conditions is missing, artificial selection cannot occur. For example, if a trait is caused entirely by environmental factors (not heritable), selecting for it will not change the next generation's phenotype distribution.
Natural vs. Artificial Selection
| Natural Selection | Artificial Selection | |
|---|---|---|
| Selective Agent | Environment | Humans |
| Goal | Survival & reproduction | Desired phenotype |
| Speed | Slow (many generations) | Rapid (few generations) |
| Genetic Diversity | Maintained or increased | Often reduced |
| Direction | Unpredictable, varies | Intentional, consistent |
| Example | Peppered moth coloration | Dog breed development |
How to Apply Artificial Selection
1
Identify the target trait: Determine which phenotypic characteristic you want to enhance or diminish in the population.
2
Survey variation: Examine the population for individuals that express the desired trait more strongly than others.
3
Select parents: Choose only individuals with the most extreme expression of the desired trait to breed.
4
Breed and evaluate offspring: Cross selected parents, then evaluate the offspring for the desired trait.
5
Repeat across generations: Continue selecting and breeding the best performers. Each generation should show increased frequency of the desired trait.
Example: Teosinte to Maize
Example: Dog Breeds from Wolves
Example: Brassica oleracea Diversification
Selection on different Brassica organs
Artificial selection has emphasized different organs of Brassica oleracea: flower clusters in broccoli, the terminal bud in cabbage, an enlarged stem in kohlrabi and leaves in kale.Selection emphasizes different organs
Artificial selection has emphasized different organs of Brassica oleracea: flower clusters in broccoli, the terminal bud in cabbage, an enlarged stem in kohlrabi and leaves in kale.Consequences: Reduced Genetic Diversity
Unintended Effects: Linkage & Inbreeding
Genetic Linkage
Selecting for one trait may inadvertently carry along linked genes on the same chromosome. Domesticated foxes bred for tameness also developed floppy ears and curly tails.
Inbreeding Depression
Mating closely related individuals increases homozygosity, which can expose harmful recessive alleles. Many purebred dogs suffer breed-specific genetic disorders.
Modern Applications
Crop Improvement
Wheat varieties have been selectively bred over centuries for higher yields, shorter stalks, and disease resistance, as seen in the Green Revolution.
Livestock Breeding
Dairy cows have been selectively bred to produce over 10 times more milk than their wild ancestors.
Lab Organisms
Drosophila (fruit flies) and E. coli strains are bred for specific traits to study gene function and disease.
Common Mistakes
✕ Confusing with GMOs
Artificial selection uses existing genetic variation through breeding. Genetic engineering directly modifies DNA in a lab. They are related but distinct processes.
✕ Ignoring Heritability
Only heritable traits respond to selection. A plant grown larger due to fertilizer won't pass that size to offspring if the trait is not genetic.
✕ Assuming Traits Are Isolated
Due to genetic linkage and pleiotropy, selecting for one trait can change other unrelated traits simultaneously.
Summary of Key Concepts
| Concept | Key Detail | Example |
|---|---|---|
| Definition | Humans select traits for breeding | Breeding larger tomatoes |
| Requirements | Variation, heritability, selective agent | Coat color in dogs |
| Mechanism | Shifts allele frequencies | Increasing "A" allele in crops |
| Speed | Faster than natural selection | Dog breeds in ~15,000 years |
| Risk | Reduced genetic diversity | Irish Potato Famine |
| Linkage | Unintended trait co-selection | Belyaev's tame foxes |
Question 1 of 5
Which of the following is the primary difference between artificial selection and natural selection?
The defining distinction is the selective agent. In artificial selection, humans choose which traits are favored. In natural selection, environmental pressures determine which traits enhance survival and reproduction.
Question 2 of 5
A farmer selects the cows that produce the most milk and breeds them together. After several generations, the average milk production increases. Which condition was essential for this outcome?
For selection to change the trait across generations, the variation must have a genetic basis (be heritable). If high milk production were caused solely by diet (environmental), selecting those cows would not increase milk production in offspring.
Question 3 of 5
In Belyaev's silver fox experiment, foxes were selected solely for tameness. Over generations, the foxes also developed floppy ears and piebald coats. What best explains these unselected traits appearing?
Genetic linkage and pleiotropy explain this. When genes are located close together on a chromosome (linkage) or a single gene affects multiple traits (pleiotropy), selecting for one trait can inadvertently alter others.
Question 4 of 5
Broccoli, kale, cauliflower, and cabbage all belong to the species Brassica oleracea. How did artificial selection produce such different-looking vegetables from one species?
From the same ancestral species, different human populations selected for different heritable structures. Selecting for enlarged flower clusters produced broccoli, selecting for large leaves produced kale, and so on: all from the same genetic starting material.
Question 5 of 5
A major risk of intensive artificial selection in crop species is reduced genetic diversity. Why does this make populations more vulnerable?
When a population is genetically uniform, a single disease or environmental change can affect every individual equally. With no resistant variants present, the entire population can be devastated: as seen in the Irish Potato Famine, where genetically uniform potato crops were wiped out by blight.