# Selection pressure

> Selection pressure is how strongly a breeder, or the environment, favours some plants over others, set by how few are kept and how demanding the criteria are.

- Canonical: https://genetixbay.com/knowledge/glossary/selection-pressure.html
- Also written: selective pressure, selection intensity, selection differential, breeding pressure
- Glossary category: Genetics and breeding · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Selection pressure is the strength of selection: how strongly some plants are favoured over others as parents. A breeder who keeps the top 5% of 200 plants applies heavier pressure than one who keeps the top half of 10. The environment can apply pressure too, for example a mould-prone climate culling susceptible plants.

## In practice

More pressure means faster progress on the trait selected, up to a point. Keeping too few parents shrinks the [gene pool](https://genetixbay.com/knowledge/glossary/gene-pool.md), raises [inbreeding](https://genetixbay.com/knowledge/glossary/inbred-line.md) and can carry along hidden defects. Breeders balance intensity with population size, and often select for several traits at once rather than pushing one to the extreme.

Pressure is also a tool. Growing a population outdoors in a wet autumn, or under pest load, shows resistance that a clean indoor room never reveals. Breeders call that "growing under pressure" to expose weaknesses.

Market pressure counts. Selecting decade after decade mainly for [THC](https://genetixbay.com/knowledge/glossary/thc.md) percentage and [bag appeal](https://genetixbay.com/knowledge/glossary/bag-appeal.md) has pushed commercial lines towards similar profiles, a documented convergence (Smith et al. 2022).

## Units and formula

Selection intensity rises as the proportion kept falls. For a normally distributed trait, keeping the top 50% gives a selection intensity of about 0.8 standard deviations; keeping the top 5% gives about 2.06, roughly two and a half times the gain per generation (Falconer and Mackay 1996).

## Don't confuse with

- **Stress**: a growing condition; it becomes selection pressure only when you breed from the survivors.

## Related terms

[Selection](https://genetixbay.com/knowledge/glossary/selection.md), [Heritability](https://genetixbay.com/knowledge/glossary/heritability.md), [Pheno hunt](https://genetixbay.com/knowledge/glossary/pheno-hunt.md), [Genetic bottleneck](https://genetixbay.com/knowledge/glossary/genetic-bottleneck.md), [Gene pool](https://genetixbay.com/knowledge/glossary/gene-pool.md)

## Read more

- [Selection and pheno hunting: sample sizes and scoring rubrics](https://genetixbay.com/knowledge/breeding/selection-pheno-hunting.md)

## Sources

1. Falconer DS, Mackay TFC. 1996. Introduction to Quantitative Genetics, 4th edition. Longman, Harlow. ISBN 978-0-582-24302-6
2. Smith CJ, Vergara D, Keegan B, Jikomes N. 2022. The phytochemical diversity of commercial Cannabis in the United States. PLOS ONE 17(5):e0267498. https://doi.org/10.1371/journal.pone.0267498