- Natural selection and artificial selection are two important processes that can change the genetic composition of animal populations over generations. Both processes depend on genetic variation, differences among individuals, inheritance, and differences in reproductive success. The major difference is the source and direction of selection. Natural selection results from environmental conditions and biological interactions that cause some individuals to survive and reproduce more successfully than others, whereas artificial selection occurs when humans deliberately choose which animals will reproduce according to defined breeding objectives.
- Understanding the difference between natural and artificial selection is fundamental to animal breeding, evolutionary genetics, population genetics, and the management of domesticated species. Natural selection shapes populations according to their ability to survive and reproduce under particular environmental conditions, while artificial selection allows breeders to deliberately increase the frequency of traits considered desirable for production, reproduction, health, behaviour, adaptation, welfare, or economic value.
- Both processes operate on variation that already exists within a population or arises through mutation, recombination, migration, and other genetic processes. Selection itself does not create new genetic variation. Instead, it changes the relative reproductive contribution of individuals carrying different genetic variants. Over generations, this can alter allele frequencies and consequently change the genetic composition of the population.
- A useful starting point for understanding both forms of selection is the relationship between phenotype and its causes:
- P = G + E
- where P represents the observed phenotype, G represents genetic effects, and E represents environmental effects. The genetic component can include additive genetic effects, dominance, and epistasis. Because phenotype is influenced by both genes and environment, selection based on observed characteristics does not always produce the expected genetic response.
- Natural selection occurs when environmental and biological conditions create differences in survival, mating success, fertility, or offspring survival among individuals. Individuals with characteristics that improve reproductive success under a particular environment may leave more descendants. If those characteristics have a genetic component, the associated alleles may become more common in subsequent generations.
- For example, animals living in hot environments may differ genetically in heat tolerance, water-use efficiency, coat characteristics, metabolism, or behaviour. If animals possessing certain heritable characteristics survive heat stress and reproduce more successfully, natural selection can increase the frequency of associated genetic variants. Similarly, exposure to pathogens can favour animals with genetic characteristics that improve disease resistance or disease tolerance.
- Natural selection does not necessarily produce the traits that humans consider economically desirable. It favours characteristics according to their effects on biological fitness within a particular environment. A trait that improves survival may be favoured even if it reduces growth or production. Conversely, a high-production phenotype may not be favoured by natural selection if it increases metabolic demands, reduces fertility, or decreases survival under challenging environmental conditions.
- Artificial selection operates differently because the selection criterion is established by humans. Breeders identify animals with desirable characteristics and preferentially use them as parents of the next generation. The objective may be increased milk production, faster growth, improved carcass quality, greater egg production, improved fertility, disease resistance, feed efficiency, heat tolerance, temperament, longevity, or other characteristics.
- Artificial selection can therefore be described as directed genetic selection based on human-defined breeding objectives. Instead of allowing reproductive success to be determined primarily by environmental and biological competition, breeders deliberately influence which individuals contribute genes to future generations.
- Despite these differences, natural and artificial selection share several fundamental genetic principles. Both require genetic variation, both depend on inheritance, and both can change allele frequencies over generations. Both can alter population characteristics and both can produce substantial evolutionary or genetic change when selection is sustained.
- The difference can be illustrated through reproductive contribution. Under natural selection, reproductive contribution is influenced by survival, mating success, fertility, competition, predation, disease, climate, and other biological or environmental factors. Under artificial selection, reproductive contribution is strongly influenced by breeder decisions. Animals chosen as parents may contribute disproportionately more genes to the next generation than animals that are not selected.
- This difference in reproductive contribution is particularly important in modern animal breeding. A genetically superior animal may be used extensively through artificial insemination, embryo transfer, or other reproductive technologies. This can accelerate genetic improvement because desirable alleles can spread rapidly through a population. However, excessive use of a small number of animals can also increase genetic concentration, reduce effective population size, increase relatedness, and increase the risk of inbreeding.
- Natural selection generally acts through fitness, whereas artificial selection acts through breeding objectives. Fitness is a biological concept related to an individual’s contribution of viable offspring to future generations. Breeding objectives, in contrast, may include economic, production, welfare, health, reproductive, and management considerations that are not necessarily aligned with natural fitness.
- This distinction is particularly important in domesticated animals. Under natural conditions, an animal may be favoured if it can survive environmental challenges and reproduce successfully. Under intensive production systems, however, breeders may select animals for high production even when humans provide substantial environmental support through nutrition, veterinary care, housing, disease control, and reproductive management.
- As a result, artificial selection can change the genetic characteristics of domesticated populations in directions that would not necessarily be favoured by natural selection. For example, humans can select strongly for increased growth, milk yield, egg production, wool production, or carcass traits even when these traits would provide limited advantages under natural conditions.
- Heritability is important in both natural and artificial selection because selection can produce a sustained genetic response only when differences in fitness or the selected trait have some heritable basis. Narrow-sense heritability is expressed as: h² = σ²_A / σ²_P
- where σ²_A is additive genetic variance and σ²_P is phenotypic variance.
- For artificial selection, heritability helps determine how effectively phenotypic information can be used to predict additive genetic merit. For natural selection, heritable variation in traits affecting survival or reproduction allows selection pressures to produce evolutionary change.
- The expected response to artificial selection can be expressed using the breeder’s equation: R = h² × S
- where R is the response to selection, h² is narrow-sense heritability, and S is the selection differential.
- Natural selection can also produce a response when selection acts on heritable variation, although the specific selection process is usually more complex because survival and reproductive success are influenced by multiple traits and environmental factors simultaneously.
- A major difference is that artificial selection can use information that would not necessarily influence natural reproductive success. Breeders can measure traits directly, record production, analyse pedigrees, estimate breeding values, genotype animals, and use statistical models to rank individuals. This allows selection to be based on predicted genetic merit rather than simply observed survival or reproductive success.
- Modern animal breeding uses estimated breeding values (EBVs) and genomic selection to identify animals with desirable genetic potential. BLUP (Best Linear Unbiased Prediction) and animal models can combine information from the individual, relatives, progeny, pedigree relationships, and environmental effects. Genomic data can further improve selection accuracy, particularly for young animals.
- Natural selection does not use EBVs, genomic breeding values, selection indexes, or planned mating systems. Instead, reproductive success emerges from interactions among organisms and their environments. Natural selection can therefore be highly complex and may involve competition, predation, disease, climate, social structure, mate choice, sexual selection, and resource availability.
- Another important difference is that artificial selection can be deliberately multi-trait. Breeders can construct a breeding objective that combines production, fertility, health, longevity, welfare, and adaptation. Selection index methods can combine information from multiple traits and assign economic or biological weights according to breeding goals.
- Natural selection can also act on multiple traits simultaneously, but there is no central decision-maker assigning explicit weights. The effective weighting of traits emerges from their effects on survival and reproductive success under the prevailing environment.
- Natural selection and artificial selection may sometimes favour the same traits. For example, strong disease resistance may benefit both survival under natural conditions and productivity in managed systems. Heat tolerance may improve survival in hot environments and also support production under heat stress. Fertility and survival may be favoured by both natural and artificial selection because they contribute directly to reproductive success.
- However, the two forms of selection can also act in opposing directions. Artificial selection for very high production may increase metabolic demands, while natural selection under harsh environments may favour greater energy conservation, survival, or reproductive efficiency. The outcome depends on the environment, management system, genetic correlations, and breeding objectives.
- Genetic correlation is particularly important when comparing natural and artificial selection. Traits do not evolve or respond independently when they share genetic determinants. Selection for one trait can cause correlated responses in other traits.
- For example, selection for production may influence fertility, longevity, disease resistance, or behaviour if these traits are genetically correlated. If a breeding program ignores these relationships, artificial selection may produce unintended consequences. This is one reason modern breeding programs increasingly use multi-trait selection and selection indexes.
- The environment also affects both forms of selection through genotype–environment interaction (G×E). Animals may have different genetic rankings under different environmental conditions. A genotype that performs exceptionally well under high-quality nutrition and controlled housing may not have the same advantage under heat stress, disease exposure, limited feed, or extensive production systems.
- Natural selection is often highly environment-specific because reproductive success directly depends on local environmental conditions. Artificial selection can partially overcome environmental constraints through management, but the genetic response may still depend on the environments in which animals are evaluated and ultimately used.
- Another important difference concerns the speed and direction of change. Artificial selection can be highly directional because breeders deliberately choose animals according to specific criteria. Strong selection intensity and reproductive technologies can rapidly increase the frequency of desirable alleles.
- Natural selection may also produce directional change, but the direction depends on changing environmental conditions. If the environment changes, the traits favoured by natural selection may change as well. A population adapted to one environment may therefore experience new selection pressures after climate change, disease emergence, or changes in food availability.
- Artificial selection can also respond rapidly to changing objectives. Breeders can modify selection criteria when market conditions, production systems, disease threats, climate conditions, or welfare priorities change. This flexibility is one of the major advantages of managed breeding programs.
- However, rapid artificial selection can create risks. Strong selection for a small number of traits may reduce genetic diversity, increase genetic concentration, and increase the use of popular sires. When a small number of animals contribute a large proportion of the next generation, the effective population size (Ne) can decline.
- A simplified relationship is: ΔF ≈ 1 / (2Ne)
- where ΔF represents the approximate rate of increase in inbreeding and Ne represents effective population size. Smaller effective population sizes generally result in faster accumulation of inbreeding.
- This creates an important difference between the immediate objective of selection and long-term population management. Artificial selection may achieve rapid genetic gain, but sustainable breeding must also maintain sufficient diversity for future adaptation and avoid excessive inbreeding.
- The expected inbreeding of offspring can be related to parental relatedness: E(F_offspring) = φ(sire, dam) Or: E(F_offspring) = r(sire, dam) / 2
- where φ represents coancestry and r represents relationship on the conventional relationship scale.
- Modern artificial breeding programs can manage these risks using mate allocation, mean kinship, optimal contribution selection, genomic relatedness, and other population-management methods. These approaches allow breeders to select genetically superior animals while limiting excessive concentration of ancestry.
- Natural selection also affects genetic diversity, but its consequences depend on population size, migration, mutation, selection strength, environmental conditions, and genetic drift. Strong directional selection can reduce variation at particular loci, while changing environments can maintain genetic variation when different genotypes are favoured under different conditions.
- Another difference is the role of genetic drift. Genetic drift is random change in allele frequencies caused by sampling effects and is especially important in small populations. Artificial breeding systems can unintentionally increase the effects of drift when a small number of animals contribute disproportionately to reproduction.
- Natural populations can also experience genetic drift, particularly after population bottlenecks or reductions in population size. Therefore, both natural and artificial population processes can influence genetic diversity, but artificial breeding decisions provide an opportunity to manage reproductive contributions deliberately.
- Natural selection may favour traits that increase biological fitness even when those traits have little immediate economic value. Artificial selection can assign value to traits according to human objectives. This makes artificial selection especially important for domesticated animals because humans determine much of the environment and reproductive structure in which these populations exist.
- For example, a breeder may select animals for feed efficiency because reducing feed requirements improves economic and environmental sustainability. Natural selection could also favour efficient animals under resource limitation, but the exact direction and strength of selection would depend on the environment and the relationship between feed efficiency, survival, reproduction, and other traits.
- Similarly, breeders may select for disease resistance, heat tolerance, fertility, longevity, and welfare-related traits. These objectives may align with natural selection in some circumstances but differ in others.
- The distinction between natural and artificial selection is also important for animal adaptation. Natural selection can gradually produce local adaptation to temperature, pathogens, feed resources, altitude, water availability, and other environmental conditions. Artificial selection can either support or oppose this process depending on breeding objectives.
- Modern breeding programs increasingly recognize the importance of climate adaptation. Selection for heat tolerance, disease resistance, resilience, fertility, and survival can help populations remain productive under changing environmental conditions. In this context, artificial selection can deliberately incorporate traits that may also be favoured by natural selection.
- Artificial selection can therefore be viewed as a form of human-directed evolutionary change. It uses the same fundamental genetic processes of inheritance and allele-frequency change but redirects reproductive contribution according to human-defined objectives.
- There is also an important distinction between selection pressure and breeding management. Artificial selection is not limited to choosing animals with desirable phenotypes. It involves decisions about which animals reproduce, how many offspring they produce, how long they remain in the breeding population, which animals they are mated with, and how genetic diversity is managed.
- Natural selection, in contrast, emerges from the interaction between organisms and their environments. It includes survival selection, sexual selection, and selection associated with fertility and offspring survival. Mate choice and competition can also strongly influence reproductive success.
- Sexual selection is a specialized form of natural selection involving differences in mating success. Characteristics that improve an individual’s ability to obtain mates can increase in frequency even when they do not directly improve survival. Artificial breeding programs can sometimes mimic or override sexual selection because breeders determine which males and females are allowed to reproduce.
- For example, a male animal may be highly successful in obtaining mates under natural conditions because of size, behaviour, dominance, vocalization, ornamentation, or other characteristics. In an artificial breeding program, however, a breeder may select a male primarily because of his estimated breeding value for production, fertility, disease resistance, or another desired trait.
- Another difference is the role of human technology. Artificial selection can be greatly accelerated by artificial insemination, embryo transfer, in vitro embryo production, genomic selection, reproductive biotechnologies, and genetic testing. These technologies allow selected animals to contribute genes to many offspring and can shorten the time required to spread desirable genetic variants.
- However, reproductive technologies can also intensify genetic concentration. Extensive use of a small number of elite animals can increase relatedness among future generations and may increase the risk of spreading deleterious recessive variants. Therefore, artificial selection must be combined with responsible population management.
- Genomic selection provides another major distinction between modern artificial breeding and natural selection. Genomic selection uses large numbers of genetic markers to predict the breeding value of animals. This allows breeders to make selection decisions at a young age and can increase the rate of genetic improvement.
- A simplified expression for annual genetic improvement is: ΔG/year = i × r × σ_A / L
- where i is selection intensity, r is selection accuracy, σ_A is additive genetic standard deviation, and L is generation interval. Modern artificial selection can influence all of these components through selection decisions, genomic evaluation, reproductive management, and breeding program design.
- Natural selection does not deliberately optimize these parameters. Generation intervals, reproductive contribution, and selection intensity emerge from biological processes and environmental conditions.
- Despite these differences, artificial selection should not be considered completely separate from natural selection. Domesticated animals continue to experience natural selection pressures. They may face disease, heat stress, nutritional challenges, reproductive constraints, social interactions, and other environmental pressures. At the same time, artificial selection determines which animals contribute genes to future generations.
- The two processes can therefore act simultaneously. An animal may be selected by humans for high genetic merit while also experiencing natural selection through health, fertility, survival, and environmental adaptation. The combined result determines the genetic trajectory of the population.
- This interaction is particularly important when breeding for fitness-related traits. If artificial selection reduces fertility, survival, disease resistance, or adaptation, natural selection and management pressures may counteract some of the intended genetic changes. Conversely, breeding programs that include health, fertility, longevity, and resilience can align artificial selection more closely with long-term biological fitness.
- The most sustainable approach is therefore not to maximize production alone, but to develop balanced breeding objectives that combine production with health, fertility, survival, welfare, adaptation, and genetic diversity. This approach allows artificial selection to support productive animals without compromising the long-term biological sustainability of the population.
- In summary, natural selection and artificial selection share the fundamental genetic principles of variation, inheritance, differential reproductive success, and changes in allele frequencies. The major difference is the source of selection pressure. Natural selection is driven by environmental and biological conditions, whereas artificial selection is deliberately imposed by humans through breeding decisions.
- Natural selection primarily favours characteristics that improve biological fitness in a particular environment. Artificial selection favours characteristics chosen according to human-defined breeding objectives. Natural selection operates through survival and reproductive success, while artificial selection operates through deliberate selection and mating decisions.
- For animal breeding, understanding this distinction is essential because domesticated populations are influenced by both processes. Breeders can use artificial selection to accelerate genetic improvement, but they must also recognize the continuing importance of natural selection, environmental adaptation, genetic correlations, health, fertility, survival, and welfare.
- The ultimate goal of modern animal breeding is therefore not simply to make animals more productive. It is to use artificial selection responsibly to improve economically and biologically important traits while maintaining genetic diversity, controlling inbreeding, supporting adaptation, protecting animal welfare, and ensuring that genetic improvement remains sustainable across generations.
