Selection on Quantitative Traits and Predicting the Response
When selection acts on a continuous trait, the response over one generation is the strength of selection multiplied by the heritable share of variation. That single relation covers artificial breeding and natural selection alike.
What a learner can do afterwards
- Predicts the mean of the next generation from a selection differential and a heritability.
- Distinguishes directional, stabilising and disruptive selection by what happens to mean and spread.
- Explains why the response stalls after many generations of the same selection.
1 · Read
Picture a herd of wheat with heights spread in a curve. Pick the tallest as parents and the next field averages taller, but only as far as height passes down. Multiply the pick gap by the inherited share to predict the move. A huge push on a barely passed trait still shifts the middle only slightly.
Now read pressure shapes the way you read weather. Pushing one side shifts the whole middle up or down. Trimming both ends keeps the middle and squeezes the width. Favouring both ends keeps the middle yet stretches the width, sometimes splitting the curve in two. Middle plus width names the pressure.
Two numbers make your predictions exact. The pick gap is the picked parents average minus the whole herd average. The inherited share is the passed fraction of the spread, and it caps how fast the middle can travel. Measure both before you promise gains, and check variation, inheritance, and success links first.
Expect plateaus and read them as news. After many rounds of top picks the remaining spread shrinks and each round gains less. Health costs of extremes and shifting conditions cancel further moves. A stalled line tells you where variation or trade offs set the limit.
You multiply the pick gap by the passed share to predict the next middle.
2 · Watch
Take it off screen
Where it sits
Learn first
This opens up
Nothing builds on it yet.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.