- Selection Response is the change in the average phenotype of a population from one generation to the next as a result of selection. In genetics and quantitative genetics, selection response describes how much a population changes when individuals with particular phenotypic or genetic characteristics are preferentially chosen as parents of the next generation. It is one of the central concepts connecting genetic variation, heritability, selection, and evolutionary change.
- Selection response occurs because individuals in a population differ in their phenotypes and some of these differences have a genetic basis. When individuals with desirable characteristics are selected to reproduce, their offspring tend to inherit genetic variants associated with those characteristics. As a result, the population mean can shift in the direction favored by selection. The magnitude of this change depends on the strength of selection, the amount of additive genetic variation, and the relationship between phenotype and breeding value.
- The classic quantitative-genetic relationship between selection and response is expressed by the Breeder’s Equation:
- R = h²S
- where R is the selection response, h² is narrow-sense heritability, and S is the selection differential. The selection differential is the difference between the mean phenotype of the selected parents and the mean phenotype of the original population. The equation shows that response is greater when selection is stronger and when a larger proportion of phenotypic differences is attributable to additive genetic differences.
- The selection differential measures the intensity of selection on the parental generation, whereas selection response measures the resulting change in the offspring generation. These concepts are related but are not identical. A large selection differential does not necessarily produce a large response if the trait has low narrow-sense heritability. Conversely, a moderate selection differential can produce substantial response when the trait has abundant additive genetic variation and high heritability.
- Narrow-sense heritability is important because selection response depends primarily on additive genetic effects that can be transmitted predictably from parents to offspring. It is defined as:
- h² = VA / VP
- where VA is additive genetic variance and VP is phenotypic variance. A high value of narrow-sense heritability means that a relatively large proportion of phenotypic variation is associated with additive genetic variation under the specific population and environmental conditions being studied. It does not mean that a fixed percentage of an individual’s phenotype is genetic.
- The relationship between selection response and additive genetic variance can also be expressed as:
- R = iσA
- where i is the selection intensity and σA is the additive genetic standard deviation. This formulation emphasizes that genetic change depends directly on the amount of additive genetic variation available for selection and on how strongly individuals are selected.
- Selection response is therefore fundamentally different from phenotypic change caused entirely by environmental factors. If a population mean changes because of improved nutrition, temperature, management, disease control, or another environmental influence, this does not necessarily represent genetic selection response. A genetic response requires a change in the population’s inherited genetic composition.
- The distinction between phenotypic selection and genetic selection is important. Phenotypic selection chooses individuals based on observable traits. When those traits are heritable, phenotypic selection can generate a genetic response. However, environmental effects can reduce the accuracy of phenotype-based selection because individuals may have high or low phenotypes for environmental rather than genetic reasons.
- Selection response is closely related to breeding value. The breeding value represents the additive genetic contribution expected to be transmitted from an individual to its offspring. When selection is based on accurate estimates of breeding value, the expected response can be predicted more effectively than when selection relies only on phenotype, especially for traits with low heritability or traits that are difficult to measure directly.
- In animal and plant breeding, selection response is a primary measure of genetic improvement. Breeders may select individuals for increased growth rate, improved milk production, greater yield, disease resistance, fertility, drought tolerance, feed efficiency, or other economically and biologically important traits. Over successive generations, repeated selection can produce cumulative genetic change.
- Selection response can be positive or negative depending on the direction of selection. If individuals with larger trait values are favored, the population mean may increase. If smaller values are favored, the mean may decrease. Selection can also target an intermediate phenotype, in which case directional change in the mean may be limited even though genetic composition changes.
- The response to selection depends strongly on the amount of genetic variation present in the population. If a trait has little additive genetic variation, strong selection may produce only a small response. Once favorable alleles become common or additive genetic variation is depleted, continued selection may become less effective unless new genetic variation enters the population through mutation, recombination, gene flow, or other processes.
- The relationship between selection response and genetic variance also explains why populations can differ in their capacity to respond to the same selection pressure. Two populations may experience the same selection differential but show different responses because they differ in additive genetic variance, heritability, genetic architecture, allele frequencies, or environmental conditions.
- For traits controlled by many genes, selection response usually reflects the combined effects of numerous loci. Such traits are often described as polygenic traits and may exhibit continuous variation. The underlying genetic architecture can influence how rapidly a population responds to selection and whether response remains approximately predictable across generations.
- Selection response can be complicated when traits are genetically correlated. Genetic correlation occurs when genetic effects influencing one trait are statistically associated with genetic effects influencing another trait. Selection on one trait can therefore cause correlated changes in other traits. These changes may be favorable, unfavorable, or neutral depending on the biological relationship between the traits.
- For example, selection for increased growth may also affect reproductive performance, body composition, disease resistance, or longevity if the traits share genetic influences. Such correlated responses are particularly important in breeding programs where multiple traits must be improved simultaneously.
- Multiple-trait selection can be described using selection indices, which combine information from several traits into a single criterion for selection. Selection indices can incorporate economic values, phenotypic measurements, genetic relationships, breeding values, and genetic covariances. They are widely used when breeding objectives involve several traits rather than a single characteristic.
- Environmental conditions also influence selection response. The expression of genetic differences can depend on the environment through genotype–environment interaction (G×E). If different genotypes perform differently across environments, selection in one environment may not produce the same response in another. This is especially important for traits affected by temperature, nutrition, disease pressure, soil conditions, or management systems.
- Phenotypic plasticity can also influence observed selection response. A genotype may produce different phenotypes under different environmental conditions, meaning that changes in population phenotype can reflect both genetic change and environmental change. Separating these effects is essential when evaluating long-term genetic improvement.
- Selection response is also affected by common environmental effects, maternal effects, and permanent environmental effects. These factors can create similarities or persistent differences among individuals that may be incorrectly interpreted as genetic differences if they are not properly accounted for. Quantitative-genetic models therefore often partition phenotypic variation into genetic and environmental components when estimating expected response.
- Repeated measurements can improve the accuracy of selection decisions. For traits measured several times, repeatability describes the consistency of measurements on the same individual. Multiple records can help distinguish persistent individual differences from temporary environmental effects and can improve estimates of an individual’s genetic merit.
- In modern breeding programs, selection increasingly uses genomic information. Genomic selection uses genome-wide genetic markers to estimate genomic estimated breeding values (GEBVs). By predicting genetic merit before an individual’s phenotype or reproductive performance is fully known, genomic selection can increase selection accuracy and shorten the generation interval, potentially increasing the rate of genetic improvement.
- The rate of genetic change can be summarized more broadly by the breeder’s equation for response to selection and related expressions involving selection intensity, accuracy, additive genetic standard deviation, and generation interval. In breeding programs, rapid genetic improvement generally requires not only strong selection but also accurate evaluation, sufficient genetic variation, and an appropriate generation interval.
- A useful conceptual expression for the rate of genetic gain per unit time is:
- ΔG/year ≈ (i × r × σA) / L
- where i is selection intensity, r is the accuracy of selection, σA is additive genetic standard deviation, and L is generation interval. This highlights why modern breeding programs focus on improving selection accuracy while reducing the time between generations.
- Selection response can also be studied experimentally. In a selection experiment, individuals are selected for a particular phenotype over multiple generations and the resulting change in the population mean is measured. Comparing selected and control populations can help estimate realized genetic responses and provide evidence about the genetic basis of a trait.
- Realized heritability can be estimated from the relationship between cumulative response and cumulative selection differential in some selection experiments. This approach provides an empirical estimate of how strongly a population responds to selection under the particular experimental conditions.
- Selection response is not always linear across generations. Early generations may show relatively rapid change, followed by slower response as favorable genetic variants become more common. Changes in allele frequencies, depletion of genetic variation, linkage, inbreeding, genetic correlations, and changes in selection criteria can all influence the trajectory of response.
- Inbreeding can also affect long-term selection response. Intensive selection from a limited number of individuals can reduce effective population size and increase relatedness among breeding individuals. Increased inbreeding can expose deleterious recessive alleles, reduce genetic diversity, and potentially limit future response. Maintaining sufficient genetic diversity is therefore important for sustainable long-term selection.
- Selection response is also relevant to natural populations. Natural selection changes the frequencies of genetic variants when individuals with different phenotypes differ in survival or reproductive success. When phenotypic differences are heritable, selection can produce evolutionary change in population means and distributions. In this sense, selection response provides an important quantitative-genetic framework for understanding evolution.
- The response to selection should not be confused with simple phenotypic change. A population may become taller, heavier, more productive, or more resistant to disease because of environmental improvement without undergoing corresponding genetic change. Demonstrating genetic response requires evidence that the change is transmitted across generations or is associated with changes in genetic composition or breeding values.
- Selection response is also related to genetic gain, although the terms emphasize slightly different perspectives. Selection response generally refers to the change produced by a particular selection episode or generation, whereas genetic gain often describes accumulated or expected improvement in a breeding objective over time. Both concepts are central to breeding-program evaluation.
- For traits with categorical outcomes, such as affected versus unaffected states, selection response can be more complicated. Threshold traits may have an underlying continuous liability even though the observed phenotype falls into discrete categories. Selection can change the underlying genetic liability and consequently alter the proportion of individuals crossing the phenotypic threshold.
- The prediction of selection response therefore depends on the trait’s genetic architecture, the accuracy of phenotypic or genomic evaluation, environmental conditions, selection intensity, population structure, and the availability of additive genetic variation. No single heritability value or response estimate applies universally across populations or environments.
- Selection response is a central concept in quantitative genetics, animal breeding, plant breeding, conservation genetics, and evolutionary biology. It provides a quantitative connection between variation within a population, the choice of individuals that reproduce, and the genetic change observed in subsequent generations.
- Understanding selection response also provides a foundation for related concepts such as selection differential, heritability, additive genetic variance, breeding value, genetic covariance, genetic correlation, genetic gain, and genomic selection. Together, these concepts explain why populations respond differently to selection and how genetic improvement can be predicted and managed across generations.
