Topic 7.3 Notes – Artificial Selection

1. What Artificial Selection Is

Artificial selection is human-directed evolution. Humans choose organisms with desirable traits and allow them to reproduce more than others. That choice changes the population’s allele frequencies over time.

Quick connection to what you already know:

• Evolution = change in allele frequencies in a population over generations.
• Selection requires
  • Heritable variation (traits must be genetic)
  • Differential reproduction (some individuals leave more offspring)

In artificial selection:

• Humans are the selective pressure
• Individuals with preferred phenotypes reproduce more
• Alleles for those traits increase in frequency

This is still evolution. The mechanism is the same as natural selection. The only difference is who is doing the selecting.

A key AP idea here is that artificial selection directly affects genetic diversity within a population. It can increase certain alleles while decreasing others at the same time.

2. How Artificial Selection Changes Genetic Diversity

Artificial selection doesn’t just “add traits.” It reshapes variation in multiple ways.

Increasing the Frequency of Specific Traits

When breeders repeatedly choose individuals with a certain phenotype:

• The alleles underlying that trait increase in frequency.
• The trait becomes more common each generation.
• Change can happen quickly because mating is controlled.

Examples:

• Cows selected for higher milk production
• Plants bred for larger fruit size
• Dogs bred for specific behaviors like herding

If a question gives you data showing a steady increase in one phenotype across generations after selective breeding, that’s artificial selection changing allele frequency.

Reducing Overall Genetic Variation

Here’s the trade-off.

If only a small subset of individuals are allowed to reproduce:

• The gene pool shrinks
• Some alleles are lost entirely
• Inbreeding becomes more likely

This can create:

• Reduced heterozygosity
• Increased expression of harmful recessive alleles
• Less ability to respond to environmental change

On FRQs, if you’re asked how artificial selection might affect a population’s ability to adapt, the answer often connects to reduced genetic diversity.

Maintaining Traits That Would Not Survive in Nature

Humans often protect organisms from environmental pressures.

That allows traits to persist that might reduce survival in the wild:

• Very large fruits that can’t disperse naturally
• Dog breeds with extreme body shapes
• Livestock bred for rapid growth but poor mobility

In nature, those traits might be selected against. Under human care, they increase.

Combining Traits from Different Populations

Breeders sometimes cross individuals from different populations to introduce new alleles.

This can:

• Increase variation for a specific trait
• Still narrow overall diversity if only select offspring are bred

So artificial selection can both increase and decrease variation depending on what humans choose.

3. Examples of Artificial Selection Over Time

Seeing real examples makes the mechanism clearer.

Dogs from Wolves

All modern dog breeds descended from wolves.

Descent of modern dog breeds from the gray wolf

The branching pattern above shows the gray wolf as the common ancestor, with different regional lineages giving rise to many modern breeds.

Humans selected for:

• Size
• Behavior (herding, guarding, companionship)
• Coat type and color
• Skull and facial structure

Result:

• Huge phenotypic diversity
• Often very low genetic diversity within individual breeds

Purebred dogs are a classic example of how strong selection plus small breeding pools can increase health problems.

Crop Plants

Corn from Teosinte

Teosinte compared with modern maize (corn)

The image compares the small wild teosinte plant and its hard, few-seeded ears with modern maize, which has large ears packed with exposed kernels.

Teosinte was a small wild grass. Through repeated selection for:

• Larger kernels
• More kernels per cob
• Softer seed coverings

Humans transformed it into modern corn.

Wild Mustard to Many Vegetables

One wild mustard species was selectively bred into:

• Cabbage (leaves)
• Broccoli (flower buds)
• Cauliflower (flower clusters)
• Kale (leaves)
• Brussels sprouts (lateral buds)

Different plant parts were targeted. Same ancestor, very different phenotypes.

If the AP gives you a diagram of multiple vegetables from one ancestor, they’re testing your understanding that selection on different traits can cause divergence within a species.

4. Artificial Selection vs Natural Selection

Both processes change allele frequencies. The difference is the source of selection pressure.

Both require:

• Heritable variation
• Differential reproductive success
• Multiple generations

If a question asks whether artificial selection is “evolution,” the answer is yes because allele frequencies change over time.

5. Consequences and Trade-Offs of Artificial Selection

Artificial selection demonstrates how powerful selection can be.

Rapid Evolution

Because humans:

• Control mating
• Prevent unwanted individuals from reproducing
• Apply consistent selection each generation

Populations can change dramatically in relatively few generations.

Trade-Offs and Health Problems

Selecting strongly for one trait can affect others.

Examples:

• Dog breeds with breathing problems due to skull shape
• Crops bred for yield but more susceptible to disease
• Livestock with reduced fertility

Reasons include:

• Gene linkage
• Pleiotropy (one gene affects multiple traits)
• Energy trade-offs

Increased Dependence on Humans

Some domesticated species:

• Cannot disperse seeds effectively
• Cannot compete in the wild
• Require assistance to reproduce

Artificial selection can reduce fitness outside of human-managed environments.