Artificial Selection Lab
Simulate selective breeding experiments by choosing which organisms reproduce based on a trait threshold. Track how the population mean shifts over generations and verify the breeder's equation R = h²S. Compare two populations with different heritabilities side by side.
Guided Experiment: Breeder's Equation
If you select only organisms with trait values above the population mean for breeding over 10 generations, how will the population mean change? How well will R = h²S predict the actual response?
Write your hypothesis in the Lab Report panel, then click Next.
Controls
Results
Response = Heritability × Selection Differential
Data Table
(0 rows)| # | Generation | Mean | Variance | S (sel. diff.) | R (response) | h²×S (predicted) |
|---|
Reference Guide
Artificial Selection
Artificial selection (selective breeding) is the process where humans choose which organisms reproduce based on desired traits. Unlike natural selection, the selection pressure is imposed by the breeder rather than the environment.
In truncation selection, a threshold is set and only individuals with trait values above (or below) that threshold are allowed to breed. This is the simplest and most common form of artificial selection in breeding programs.
Examples include breeding dogs for size, selecting crop plants for yield, and improving dairy cattle for milk production.
Heritability (h²)
Narrow-sense heritability (h²) is the proportion of phenotypic variance attributable to additive genetic variance.
When h² = 1, all phenotypic variation is genetic and selection is maximally effective. When h² = 0, all variation is environmental and selection has no effect on the next generation.
Typical heritabilities in livestock: milk yield (0.25), growth rate (0.35), litter size (0.10). Plant traits often range from 0.3 to 0.8.
Breeder's Equation (R = h²S)
The breeder's equation predicts the evolutionary response to selection in one generation.
R is the response to selection, the change in the population mean between parent and offspring generations. S is the selection differential, the difference between the mean of the selected parents and the overall population mean.
You can estimate realized heritability by measuring R and S across generations and computing R/S.
Selection Differential
The selection differential (S) measures the intensity of selection applied to the population. A larger S means stronger selection pressure.
In truncation selection, S depends on the threshold position relative to the population distribution. A higher threshold selects fewer but more extreme individuals, increasing S.
As the population mean approaches the threshold over multiple generations, fewer individuals fall below it and S decreases. This explains why selection response typically diminishes over time even with a fixed threshold.
The selection intensity (i = S/σ) standardizes the differential by the phenotypic standard deviation, allowing comparison across populations with different variances.