Getting Started
For thousands of years, humans have actively shaped the evolution of other species, from the crops in our fields to the animals in our homes. This process, known as artificial selection, is a powerful demonstration of how selection can drive rapid evolutionary change. It operates on the genetic variation within a population, fundamentally altering the course of that species' development to suit human needs or desires.
What You Should Be able to Do
After completing this section, you should be able to:
Define artificial selection and describe its fundamental mechanism.
Explain how human choices act as a selective pressure that alters allele frequencies in a population's gene pool.
Analyze the effects of artificial selection on the genetic diversity of a population.
Compare and contrast the process and outcomes of artificial selection with those of natural selection.
Key Concepts & Mechanisms
Artificial selection is a process driven by human intervention. By choosing which individuals in a population will reproduce based on the presence of desirable traits, humans act as the primary selective agent, causing significant and often rapid changes in the genetic makeup of that population over generations.
Inputs & Preconditions
For artificial selection to occur, several conditions must be met:
Genetic Variation: A population must exhibit a range of different traits. This variation is the raw material for selection; without different options to choose from, no selection can occur. This variation arises from random mutations and genetic recombination during sexual reproduction.
Heritability: The desired traits must be heritable, meaning they are passed from parents to offspring through genes. Selecting for a trait that is not genetically determined (e.g., a scar) will have no effect on the next generation.
Human Intent: There must be a specific goal or desired outcome. Humans must identify a particular trait (a specific characteristic of an organism) that they wish to amplify or diminish in the population.
Key Steps / Mechanism
The process of artificial selection, also known as selective breeding, follows a deliberate, cyclical pattern:
Observation & Selection: Humans observe the natural variation within a population and identify individuals that possess the most desirable version of a trait (e.g., the sweetest fruit, the thickest wool, the most docile temperament).
Controlled Breeding: These selected individuals are isolated and intentionally bred with one another. This prevents individuals with less desirable traits from contributing their genes to the next generation.
Evaluation of Offspring: The offspring from this controlled breeding are evaluated. Those that best express the desired trait are chosen to become the parents for the subsequent generation.
Repetition: This cycle of selection and breeding is repeated over many generations. With each cycle, the frequency of the alleles responsible for the desired trait increases within the population's gene pool (the total collection of genes and their different versions, or alleles, in a population).
Outputs & Effects
The consistent application of this process has profound and predictable consequences:
Directional Change in Phenotype: The observable characteristics of the population shift dramatically in the direction of the selected trait. For example, the size of corn kernels has increased over 10-fold from its wild ancestor, teosinte.
Shift in Allele Frequencies: The genetic foundation of the population is altered. Alleles that code for the desired traits become more common, while alleles for undesired traits become rare or may even be eliminated from the gene pool entirely.
Reduction in Genetic Variation: A significant and often unintended consequence of artificial selection is a loss of overall genetic diversity. By focusing on a narrow set of traits, many other alleles that are not selected for are lost. This can make the population more susceptible to diseases or environmental changes, as it lacks the genetic toolkit to adapt to new challenges.
Regulation
Unlike natural selection, where the "regulator" is the environment (e.g., predators, climate, resource availability), the regulating force in artificial selection is conscious human choice. The direction, intensity, and duration of the selective pressure are determined entirely by human goals, which can be for agricultural productivity, aesthetic preference, or scientific research.
Key Models & Diagrams
The process of artificial selection can be modeled as a simple, iterative flowchart that leads to a change in the population's genetic makeup.
| Step | Action | Genetic Consequence |
|---|---|---|
| 1. Initial State | A population exhibits natural genetic variation for a trait (e.g., milk yield). | The gene pool contains a wide variety of alleles related to the trait. |
| 2. Selection | Humans identify and select individuals with the highest milk yield. | Individuals with alleles for low yield are prevented from reproducing. |
| 3. Breeding | The selected high-yield individuals are bred together. | The alleles for high yield are passed on to the next generation. |
| 4. Outcome | The offspring generation has a higher average milk yield than the parent generation. | The frequency of high-yield alleles increases in the population's gene pool. |
| 5. Repetition | The process is repeated, always selecting the highest-yield offspring to be parents. | Over generations, high-yield alleles become fixed or near-fixed in the population. |
Key Components & Evidence
Domestication of Dogs: All modern dog breeds, from Great Danes to Chihuahuas, descended from a common wolf ancestor. Humans selected for traits like temperament, size, and coat color, creating immense phenotypic diversity between breeds but low genetic diversity within each breed.
Agricultural Crops: The development of modern wheat, corn, and rice from wild grass ancestors is a primary example. Selection for large seeds, high yield, and disease resistance has transformed our food supply.
Teosinte to Maize: The evolution of modern corn (maize) from the wild grass teosinte is one of the most well-documented cases. Archaeological and genetic evidence shows clear, directed selection for larger, softer kernels on a single, non-shattering stalk.
Brassica oleracea: A single plant species, wild mustard, has been selectively bred to produce a variety of common vegetables. Selecting for leaves produced kale, for terminal buds produced cabbage, for flowers produced broccoli and cauliflower, and for lateral buds produced Brussels sprouts.
Livestock: Dairy cattle have been selected for maximum milk production, while beef cattle have been selected for muscle mass. These represent divergent selection pathways from a common ancestral bovid.
Reduced Genetic Diversity: The Cavendish banana, the primary banana in global trade, is a monoculture propagated by cloning. Its extreme lack of genetic diversity makes the entire species exceptionally vulnerable to a single pathogen, such as the fungus causing Panama disease.
Skill Snapshots
Causation
Cause: Humans consistently select and breed corn plants with the largest number of kernels.
Effect: The average number of kernels per cob in the corn population increases over generations.
Cause: A dog breeder selects for a specific "desirable" flat-faced skull shape in pugs.
Effect: The alleles for brachycephaly increase in frequency, leading to a population with common respiratory health problems.
Cause: Intense selection for a single trait (e.g., rapid growth in chickens).
Effect: A reduction in overall genetic diversity, potentially leading to the loss of alleles for disease resistance.
Comparison
Selective Agent: In artificial selection, the agent is human choice; in natural selection, the agent is the environment.
Speed of Change: Artificial selection can produce dramatic changes in a relatively short time (tens or hundreds of generations), whereas natural selection often operates on much longer evolutionary timescales.
Goal-Orientation: Artificial selection is goal-directed and purposeful (teleological), while natural selection is a non-random process with no end goal or purpose.
Change and Continuity Over Time (CCOT)
Baseline: The ancestral wild wolf population possessed high genetic diversity and a phenotype adapted for survival in its natural environment.
Change 1: Early humans began selecting for tameness and less aggressive behavior, leading to the initial domestication of the dog.
Change 2: Over the last few centuries, humans have applied intense, specialized selection to create distinct breeds with specific morphologies and behaviors (e.g., herding, retrieving, companionship).
Continuity: Despite the vast phenotypic differences, all dog breeds share a common genetic ancestry with the wolf and retain fundamental biological systems and many instinctual behaviors.
Common Misconceptions & Clarifications
Misconception: Artificial selection creates new traits out of thin air.
- Clarification: Selection can only act on the genetic variation that is already present in a population. It increases the frequency of existing alleles but does not create new ones on demand. New traits ultimately arise from random mutation, which may then be selected for.
Misconception: The traits selected for are always beneficial for the organism's survival.
- Clarification: Many traits selected by humans are detrimental to an organism's health or survival in the wild. The bulldog's breathing difficulties or the inability of some domestic turkeys to mate naturally are prime examples of traits that would be quickly eliminated by natural selection.
Misconception: Artificial selection is a modern scientific invention.
- Clarification: While modern genetic knowledge has made the process more efficient, humans have practiced artificial selection for over 10,000 years, since the dawn of agriculture. Early farmers and herders practiced it intuitively by saving seeds from the best plants and breeding the best livestock.
One-Paragraph Summary
Artificial selection is a human-directed process that causes evolutionary change by intentionally selecting individuals with desired heritable traits to reproduce. This mechanism of selective breeding, repeated over generations, leads to a significant increase in the frequency of alleles associated with the chosen traits, resulting in dramatic phenotypic shifts within the population. While it has been instrumental in developing modern agriculture and domesticating animals like dogs, this process also typically leads to a reduction in overall genetic diversity. This loss of variation can make the selected population more vulnerable to environmental changes and disease, highlighting the powerful and lasting impact humans have on the genetic makeup of other species.