Theory 05/Selection across levels/Evolution
Variance Transfer Between Levels
Selection at one level of organisation generates variance that becomes the raw material for selection at the next, and the coupling is strong enough that the rate of adaptation at the higher level is set by the variance produced below rather than by selection pressure above.
01What is already known
Multi-level selection is well established in principle. What is less developed is the accounting: how much of the variance generated at one level is actually available at the next. Microbiologists see it in strain replacement, ecologists in community assembly, and economists in firm heterogeneity.
02The mechanism
Within-level selection reduces variance at that level while producing variation in the aggregate traits that higher levels see. The transfer efficiency — how much lower-level variance becomes higher-level variance — depends on the ratio of between-group to within-group variance. When within-group variance is high, higher-level selection has little to act on regardless of how strong it is.
03Predictions
- Adaptation rate at a higher level should correlate with lower-level variance more strongly than with higher-level selection differential.
- Reducing within-group variance experimentally should increase the efficacy of between-group selection.
- Systems with strong within-level selection should show slower higher-level adaptation, even under strong higher-level pressure.
04Refutation
If higher-level adaptation proceeds at a rate set by selection differential independent of the variance supplied from below, the transfer account is redundant.
Artificial selection experiments with controlled variance structure, plus comparative analyses where within-group variance can be estimated independently of selection strength.