Artificial Selection
~8 min read · Tier toggle changes depth (T1 plain · T2 standard · T3 rigour)
You should understand the basic mechanism of natural selection, specifically how heritable variation leads to differential reproductive success, and be comfortable with the concept of a fitness landscape.
Why this matters
Artificial selection demonstrates that evolutionary change is not merely a theoretical abstraction but a predictable, observable process driven by differential reproduction.
While natural selection operates through environmental pressures that are often complex and slow, artificial selection provides a clear, human-directed analogue that isolates the core mechanism of evolution: the non-random retention of heritable traits. By examining how humans have shaped the diversity of domesticated species, we gain a powerful empirical foundation for understanding how selection coefficients operate in wild populations. This lecture bridges the gap between the abstract logic of natural selection and the tangible reality of biological change, showing that the same principles governing the evolution of finch beaks also apply to the breeding of crop plants and livestock. Understanding artificial selection is therefore essential for appreciating the universality of evolutionary theory and the profound impact of human agency on the biosphere.
The Core Mechanism in Plain Language
At its heart, artificial selection is the process by which humans act as the selective agent, choosing which individuals reproduce based on specific, desired traits. Unlike natural selection, where the environment determines which variants survive and pass on their genes, in artificial selection, the "environment" is effectively the human breeder. This distinction is crucial because it allows us to observe evolutionary change in accelerated timeframes. For instance, the vast diversity of dog breeds, from the small Chihuahua to the large Great Dane, all descended from a common wolf ancestor, is a direct result of thousands of years of selective breeding. Humans identified individuals with desirable characteristics, such as size, coat texture, or temperament, and ensured that these individuals contributed more offspring to the next generation than those without those traits. Over many generations, the frequency of the genes associated with these traits increased in the population, leading to the distinct phenotypes we see today. This process is not random; it is driven by a specific goal or preference. However, the underlying biological mechanism remains identical to natural selection: heritable variation exists within the population, and individuals with certain genotypes are more likely to be selected for reproduction. The key difference lies in the direction and intensity of the selection pressure. In nature, selection is often stabilising or directional based on ecological niches, whereas in artificial selection, it can be extreme, pushing traits to levels that might not be sustainable in the wild. This human-driven process highlights the power of selection to reshape populations, providing a clear demonstration that evolution is a real, ongoing process that can be harnessed and directed. By studying artificial selection, we can better understand the constraints and possibilities of evolutionary change, as well as the ethical and ecological implications of human intervention in biological systems.
Selection Coefficients and Response to Selection
To quantify the strength of artificial selection, biologists use the concept of the selection coefficient, denoted as \( s \). This coefficient measures the relative fitness difference between genotypes or phenotypes. In the context of artificial selection, we often look at the selection differential, \( S \), which is the difference in the mean phenotype of the selected parents compared to the mean phenotype of the entire population. The response to selection, \( R \), is the change in the mean phenotype of the next generation. These three variables are linked by the breeder's equation, a fundamental tool in quantitative genetics. The equation states that the response to selection is equal to the heritability of the trait, \( h^2 \), multiplied by the selection differential. This relationship highlights that evolutionary change depends not only on the strength of selection but also on the genetic variation available in the population. If a trait has low heritability, meaning that much of the variation is due to environmental factors rather than genetics, then even strong selection will result in a small response. Conversely, if a trait is highly heritable, a moderate selection differential can lead to a significant change in the population mean. This principle is evident in the rapid evolution of crop yields over the past century. By selecting for plants with higher grain production, breeders have been able to increase yields dramatically, and long-term experiments show how durable such responses can be: in the Illinois maize experiment, selection for high and low oil content begun in 1896 was still producing change after more than a hundred generations. Understanding these quantitative relationships allows us to predict the outcomes of selection programs and to design more effective strategies for improving agricultural and livestock traits.
Constraints and Trade-offs in Artificial Selection
While artificial selection can drive rapid evolutionary change, it is not without limitations. One major constraint is the genetic trade-off between traits. Selecting for one trait often comes at the cost of another, a phenomenon known as antagonistic pleiotropy. For example, in dairy cattle, selection for high milk yield has historically been associated with a decrease in fertility and an increase in susceptibility to certain diseases. This occurs because the genes that promote high milk production may also divert resources away from reproductive functions or immune system maintenance. As a result, breeders must balance the desire for high production with the need for animal health and longevity. Another constraint is the loss of genetic diversity. Intense selection for specific traits can reduce the overall genetic variation in a population, making it more vulnerable to environmental changes, such as new pathogens or climate shifts. This is a significant concern in modern agriculture, where many crop varieties are genetically uniform. While this uniformity can be beneficial for efficient farming, it also creates a risk of catastrophic failure if a disease or pest evolves to target the specific genetic makeup of the crop. Therefore, sustainable artificial selection programs must carefully manage genetic diversity, often by maintaining seed banks or breeding lines that preserve rare alleles. By understanding these constraints, we can develop more resilient and sustainable breeding strategies that account for the complex interplay of traits and the long-term health of the population.
Comparison with Natural Selection
Although artificial and natural selection share the same underlying mechanism, they differ in several key aspects. The most obvious difference is the direction of selection. In natural selection, the direction is determined by environmental pressures, such as predation, climate, or resource availability. In artificial selection, the direction is determined by human preferences, which can be arbitrary or even counter to natural fitness. For example, humans may select for traits that reduce an animal's ability to survive in the wild, such as the flattened faces of bulldogs and pugs, which cause breathing problems, or the short legs of dachshunds. This can lead to the evolution of traits that are maladaptive in a natural context but desirable in a domesticated one. Another difference is the intensity of selection. Artificial selection can be much more intense than natural selection, as humans can eliminate individuals with undesirable traits entirely, whereas in nature, selection is often a matter of degree. This intense selection can lead to rapid evolutionary change, as seen in the domestication of crops and livestock. However, this rapid change can also lead to genetic bottlenecks and reduced adaptability. Despite these differences, the principles of natural selection still apply to domesticated species. For instance, if a domesticated animal is released into the wild, it will be subject to natural selection, and those with traits that enhance survival in the wild will be favoured. This interplay between artificial and natural selection is an important area of research, particularly in the context of invasive species and the potential for domesticated animals to adapt to new environments.
The concept
Natural selection
Differential survival and reproduction of heritable variation.
A common misconception is that artificial selection is the opposite of natural selection. In reality, it is a subset of the same process. Both rely on heritable variation and differential reproduction. The difference lies in the agent of selection, not the mechanism. Confusing the two can lead to a misunderstanding of how evolution works, as it implies that human intervention creates a new type of evolutionary force, when in fact it simply redirects the existing force of selection. C
Worked example
Problem. A population of chickens has a mean egg weight of 50 \(\mathrm{g}\). The heritability of egg weight is 0.6. A breeder selects the top 10% of hens by egg weight for reproduction and mates them with cockerels chosen from families of equally superior egg weight, so both sexes carry the same selection differential. The mean egg weight of these selected hens is 60 \(\mathrm{g}\). Calculate the expected mean egg weight of the next generation.
- Identify the selection differential, \( S \), which is the difference between the mean of the selected parents and the mean of the population. \( S = 60\ \mathrm{g} - 50\ \mathrm{g} = 10\ \mathrm{g} \).
- Identify the heritability, \( h^2 \), which is given as 0.6.
- Apply the breeder's equation, \( R = h^2 S \), to calculate the response to selection. \( R = 0.6 \times 10\ \mathrm{g} = 6\ \mathrm{g} \).
- Add the response to selection to the original population mean to find the new mean. \( 50\ \mathrm{g} + 6\ \mathrm{g} = 56\ \mathrm{g} \).
- Verify that the units are consistent and that the result is biologically plausible, given the heritability and selection intensity.
Units check. The selection differential and response are both in grams, and the heritability is a dimensionless ratio, so the final result is correctly expressed in grams.
Three quick tests
- What is the primary difference between artificial and natural selection?
Answer
The agent of selection: humans in artificial selection, the environment in natural selection. - How does heritability affect the response to selection?
Answer
Higher heritability leads to a greater response to selection for the same selection differential. - What is a potential downside of intense artificial selection?
Answer
Loss of genetic diversity, which can reduce adaptability and increase susceptibility to disease.
Back to BU-104 →
Every lecture and every concept in this unit, with prerequisites and threads.
Lecture notes checked line by line by the editor; every number in the worked example was recomputed.
Concepts cited: T-017 · Threads: evolution