- The rate of genetic improvement describes how quickly the average genetic merit of a population changes in a desired direction over time as a result of selection. It is a fundamental concept in quantitative genetics, animal breeding, plant breeding, and genetic improvement programs. While genetic gain often describes the amount of improvement achieved per generation, the rate of genetic improvement commonly emphasizes how much genetic progress is achieved per unit of time, such as per year.
- A population can experience substantial genetic gain per generation but still have a relatively slow rate of improvement if generations take a long time to complete. Conversely, a breeding program may achieve a smaller response per generation but produce generations rapidly enough to achieve greater annual progress. This distinction makes the rate of genetic improvement particularly important when comparing breeding strategies with different generation intervals.
- A useful simplified expression for annual genetic improvement is:
- ΔGyear≈irσAL\Delta G_{\text{year}} \approx \frac{i r \sigma_A}{L}
- where ii is selection intensity, rr is selection accuracy, σA\sigma_A is the additive genetic standard deviation, and LL is the generation interval. This relationship shows that the rate of genetic improvement increases when selection is stronger, genetic merit is predicted more accurately, additive genetic variation is greater, or generations can be completed more quickly.
- The expression can also be understood as a modification of the classic Breeder’s Equation, R=h2SR=h^2S. The Breeder’s Equation describes expected selection response per generation under appropriate assumptions. To convert genetic response into improvement per unit of time, the generation interval must be considered. Therefore, selection response and annual genetic improvement are related but should not be treated as identical concepts.
- The first major component is selection intensity. Selection intensity describes how strongly individuals are selected relative to the amount of phenotypic variation in the population. If a breeding program selects only a small proportion of candidates as parents, selection intensity is generally higher. Stronger selection can increase expected genetic improvement per generation, provided that sufficient genetic variation exists and other constraints are not limiting.
- However, increasing selection intensity indefinitely is not necessarily desirable. Very strong selection can reduce the number of breeding individuals contributing genes to the next generation. This can decrease the effective population size, increase inbreeding, and reduce genetic diversity. A breeding program therefore needs to balance short-term genetic improvement against long-term population health and genetic sustainability.
- The second major component is selection accuracy. Accuracy measures how reliably the information used for selection predicts an individual’s true breeding value or genetic merit. Higher accuracy means that superior candidates are more likely to be selected because of genuinely favorable genetic differences rather than environmental advantages.
- Accuracy can be improved by combining information from individual phenotypes, relatives, progeny, repeated records, pedigrees, biomarkers, and genomic data. Modern genomic selection has become particularly important because genome-wide markers can be used to estimate genetic merit before individuals have accumulated extensive phenotypic or progeny information.
- A genomic estimated breeding value (GEBV) can allow breeders to identify promising individuals at a young age. This can increase selection accuracy for some traits while also allowing breeding decisions to be made earlier. The combination of accurate genomic prediction and earlier selection can substantially increase the rate of genetic improvement in appropriate breeding systems.
- The third component is additive genetic variation. Additive genetic variation represents genetic differences that can be transmitted predictably from parents to offspring. It provides the material on which directional selection acts. If a population contains little additive genetic variation for a trait, selection has limited potential to produce sustained improvement regardless of how strong or accurate selection is.
- Additive genetic variation is commonly represented by additive genetic variance, VAV_A, or its square root, the additive genetic standard deviation, σA\sigma_A. A population with greater additive genetic standard deviation generally has greater potential for genetic change under a given selection strategy. However, the amount of variation can change as selection proceeds because favorable alleles may become more common and genetic diversity can decline.
- The fourth component is generation interval. Generation interval is the average age of parents when their offspring are produced. A shorter generation interval allows genetic improvement to accumulate more rapidly per unit of time, assuming that selection intensity, accuracy, and genetic variation remain adequate.
- For example, consider two breeding programs that each achieve a genetic response of 2 units per generation. If the first program has a generation interval of 4 years, its approximate rate of improvement is 0.5 units per year. If the second program completes a generation every 2 years, its rate is approximately 1 unit per year. Both programs have the same improvement per generation, but the second achieves genetic improvement twice as quickly.
- This is one reason why genomic selection can have such a large effect on breeding efficiency. Conventional evaluation may require individuals to mature, express a trait, reproduce, or produce progeny before sufficient information is available. Genomic prediction can allow selection decisions much earlier, reducing generation interval while retaining useful prediction accuracy.
- The relationship between selection intensity, accuracy, additive genetic variation, and generation interval creates an important optimization problem. Increasing selection intensity can increase improvement, but excessive intensity can reduce genetic diversity. Increasing accuracy can improve selection decisions, but obtaining additional information may require more time and resources. Reducing generation interval can increase annual improvement, but selecting individuals too early may reduce accuracy if insufficient information is available.
- Modern breeding programs therefore attempt to optimize the entire breeding system rather than maximizing a single parameter. The goal is often to maximize the rate of genetic improvement while maintaining acceptable levels of inbreeding, genetic diversity, reproductive efficiency, animal or plant health, and economic sustainability.
- Heritability is closely related to the rate of genetic improvement because it describes the proportion of phenotypic variance attributable to genetic variance under a particular population and environment. Narrow-sense heritability, h2h^2, specifically describes the proportion attributable to additive genetic variance. Traits with higher narrow-sense heritability may respond more predictably to individual phenotypic selection. However, heritability alone does not determine the rate of genetic improvement because selection accuracy, selection intensity, additive genetic variation, and generation interval also matter.
- A trait with low heritability can still achieve substantial genetic improvement when breeders use appropriate information sources. Family records, progeny testing, repeated measurements, genomic information, and selection indices can increase the accuracy of genetic evaluation. Therefore, low heritability should not automatically be interpreted as meaning that a trait cannot be improved genetically.
- Breeding value is central to genetic improvement because breeders ultimately seek individuals whose genetic contributions will increase the desired characteristics of future generations. Breeding values can be estimated using phenotypic observations, pedigree relationships, relatives, progeny, and genomic information. The reliability of these estimates affects selection accuracy and therefore influences the expected rate of genetic improvement.
- Modern genetic evaluation commonly uses BLUP and related mixed-model methods to estimate breeding values. These methods can account for systematic environmental effects and relationships among individuals while separating genetic effects from other sources of variation. Accurate genetic evaluation is especially important when environmental conditions differ substantially among individuals or management groups.
- The rate of genetic improvement becomes more complex when several traits are considered simultaneously. Most breeding objectives include multiple characteristics rather than a single trait. A program may seek increased growth or yield while maintaining fertility, disease resistance, product quality, longevity, and adaptability.
- In such cases, genetic covariance and genetic correlation become important. Selection for one trait can cause correlated changes in another trait. Favorable genetic correlations can increase overall improvement, whereas unfavorable genetic correlations can create trade-offs. A breeding program may therefore need to sacrifice some improvement in one trait to achieve a more desirable overall response across the breeding objective.
- A selection index can combine information from multiple traits and sources into a single selection criterion. By incorporating economic or biological weights together with genetic and phenotypic relationships, selection indices allow breeders to optimize expected improvement in an overall breeding objective. The accuracy of the index influences the efficiency of selection and therefore the rate of genetic improvement.
- Genotype–environment interaction (G×E) can also affect genetic improvement. The genetic ranking of individuals may change across locations, climates, management systems, or production environments. If selection is conducted in an environment that differs from the target production environment, the observed genetic improvement may not translate directly to the intended population.
- For this reason, breeding programs often evaluate candidates in representative environments or use statistical models that account for environmental variation and genetic differences across environments. Understanding phenotypic plasticity and G×E can help breeders determine whether genetic improvement is broadly applicable or specific to particular conditions.
- The rate of genetic improvement can be measured by tracking changes in estimated genetic merit across generations or years. This is commonly referred to as a genetic trend. A positive genetic trend indicates that the average genetic value of the population is increasing for the trait of interest. Genetic trends provide an important way to evaluate whether a breeding program is achieving its objectives.
- Genetic trends must be distinguished from trends in observed phenotypic performance. Improvements in nutrition, management, technology, disease control, housing, or environmental conditions can produce large phenotypic changes without equivalent genetic improvement. Proper genetic evaluation attempts to separate these environmental changes from changes in genetic merit.
- The rate of genetic improvement can also change over time. Early in a selection program, abundant genetic variation may allow relatively rapid progress. As favorable alleles increase in frequency, the remaining genetic variation may decline and the response to selection may become smaller. This process can contribute to diminishing genetic gain over long periods of directional selection.
- Maintaining genetic diversity is therefore important for sustainable genetic improvement. Diverse populations retain more genetic variation that can potentially contribute to future progress. Excessive selection intensity, small breeding populations, and repeated use of a limited number of superior individuals can increase inbreeding and reduce genetic diversity.
- Effective population size provides a useful measure of the genetic size of a breeding population. A breeding strategy that produces rapid short-term improvement but dramatically reduces effective population size may compromise long-term progress. Modern breeding programs increasingly use genomic relationship information to control the accumulation of inbreeding while continuing to select individuals with high genetic merit.
- The rate of genetic improvement is also important in plant breeding. Plants often differ substantially from animals in reproductive biology and generation interval. Some crops can produce multiple generations per year, while perennial crops may require several years before reproduction. Technologies such as rapid generation advancement, controlled environments, doubled haploids, marker-assisted selection, and genomic selection can reduce the time required to produce improved generations.
- In animal breeding, the generation interval can be influenced by age at sexual maturity, reproductive rate, reproductive technologies, and the timing of genetic evaluation. Genomic selection has been especially valuable in species where traditional progeny testing requires several years. Earlier selection can increase the rate of genetic improvement if prediction accuracy is sufficiently high.
- The same general principles apply to natural selection and evolutionary change. Natural selection changes populations when heritable differences in traits influence survival or reproduction. The rate of evolutionary change depends on the amount of genetic variation, the strength and direction of selection, the relationship between phenotype and genetic merit, and the generation time of the organism.
- In evolutionary genetics, environmental changes can alter both selection pressures and the expression of genetic variation. Genotype–environment interaction, mutation, migration, recombination, genetic drift, and changing population structure can all influence the rate at which genetic characteristics change over time.
- The economic value of genetic improvement can be substantial because genetic changes can accumulate across generations. Improvements in productivity, disease resistance, feed efficiency, survival, fertility, or product quality may continue to influence performance long after the original selection decision. This makes the rate of genetic improvement an important consideration in long-term breeding strategy and investment.
- However, faster genetic improvement is not automatically better in every situation. Rapid change may produce undesirable correlated responses, increase inbreeding, reduce diversity, or create welfare and management challenges. A well-designed breeding program therefore aims for sustainable genetic improvement, balancing the speed of progress with biological fitness, genetic diversity, economic objectives, and long-term population health.
- The rate of genetic improvement can therefore be viewed as the outcome of several interacting forces: selection intensity, selection accuracy, additive genetic variation, and generation interval. Improving any one of these factors can potentially increase progress, but the greatest benefits often arise when they are optimized together. Modern genomic technologies have expanded the possibilities by allowing breeders to increase prediction accuracy and shorten generation intervals simultaneously.
- Understanding the rate of genetic improvement provides a framework for connecting many major concepts in quantitative genetics, including heritability, selection differential, selection response, breeding value, genetic gain, genetic correlation, genomic selection, and generation interval. Together, these concepts explain how populations can be changed systematically through selection and why some breeding strategies produce faster, more reliable, and more sustainable genetic progress than others.
