--- title: "Selection on Quantitative Traits and Predicting the Response" description: "The breeder's equation predicts a one-generation change in a quantitative trait's mean: R = h²S, using narrow-sense heritability and the selection differential. Add R to the starting mean to predict t" canonical: https://lightmysky.com/learn/science/selection-on-quantitative-traits-and-predicting-the-response-mt_M-czTLDmfl source: https://lightmysky.com/learn/science/selection-on-quantitative-traits-and-predicting-the-response-mt_M-czTLDmfl.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Selection on Quantitative Traits and Predicting the Response The breeder's equation predicts a one-generation change in a quantitative trait's mean: R = h²S, using narrow-sense heritability and the selection differential. Add R to the starting mean to predict the offspring mean. This is an approximate prediction under stated assumptions, not a universal rule for every selection process. Subject: Science · Area: Genetics & Evolution · Ages 19 to 20 Page: https://lightmysky.com/learn/science/selection-on-quantitative-traits-and-predicting-the-response-mt_M-czTLDmfl ## Ready when they can - Predicts response and the next-generation mean from a selection differential and narrow-sense heritability under stated assumptions. - Distinguishes directional, stabilising and disruptive selection by reproductive success across trait values, and describes expected effects on mean and spread. - Explains possible causes of a selection plateau without treating the plateau alone as proof of its cause. ## Lesson: Predicting a response to selection S = selected parents' mean - population mean before selection. Narrow-sense heritability h² = V_A/V_P divides additive genetic variance by phenotypic variance. The breeder's equation predicts R = h²S, the change in mean. Add R to the starting mean for the predicted offspring mean. Under the model assumptions, positive S and positive h² predict a rise in mean. This equation does not predict the spread or identify a selection mode. **Example.** A plant population averages 40 cm, selected parents average 46 cm and h² = 0.4. S = 46 - 40 = 6 cm, so R = 0.4 × 6 = 2.4 cm. The predicted offspring mean is 42.4 cm. This approximate model assumes a roughly linear parent-offspring relationship based on additive inheritance, similar growing conditions and no strong effects from selection on other traits. *(drawing: Heights in cm. The selected parents are 6 cm above the starting mean; the predicted offspring increase is 2.4 cm.)* Selection modes describe which trait values have greater reproductive success. Directional selection favours one end of the range and can shift the mean towards that end; spread need not stay fixed. Stabilising selection favours intermediate values over both extremes and tends to narrow the spread. With a symmetric distribution and equal selection against both extremes, the mean can stay unchanged. *(drawing: An example of stabilising selection: intermediate plants leave 6 offspring each; short and tall plants leave 2 each.)* Disruptive selection favours both extremes over intermediate values. It can increase spread or produce two peaks. A symmetric pattern can leave the mean unchanged, but this is not required. These distribution effects are tendencies, not guarantees for each generation. A changed distribution alone cannot prove selection because environmental changes and other processes can also alter measured traits. Repeated selection can reach a plateau, but not at a fixed time. With phenotypic variance fixed, declining additive genetic variance lowers h². For the same positive S, the predicted gain then shrinks. Opposing selection, health trade-offs or changed conditions can also limit gains. A plateau alone cannot identify its cause. Measure S, variance components and conditions again before predicting another generation. **Recap.** Use narrow-sense h² in R = h²S, then add R to the starting mean. Identify selection by reproductive success. A plateau alone does not reveal its cause. *Lesson, questions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Optional connections - [Selective Breeding and Genetic Engineering](https://lightmysky.com/learn/science/selective-breeding-and-genetic-engineering-mt_a3zTtiYrR2) - [Quantitative Traits, Variance Components and Heritability](https://lightmysky.com/learn/science/quantitative-traits-variance-components-and-heritability-mt_orJhIlYnKM)