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, raises inbreeding 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 percentage and bag appeal 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.
