- 1 The concept of repeatability, the measurement of consistent individual differences, has become an increasingly important tool in evolutionary and ecological physiology. Significant repeatability facilitates the study of selection acting on natural populations and the concept has several practical implications for identifying traits.
- 2 When properly defined and measured, repeatability can set the upper limit to heritability. This is potentially a very useful interpretation of the repeatability of traits measured on natural populations because often, estimates of heritability cannot be obtained. Many recent reports of repeatability of individual differences for traits have made this interpretation.
- 3 However, repeatability estimates may not set an upper limit to heritability if: (a) measured traits are not genetically identical, (b) common environmental effects work in opposition to direct genetic effects, (c) the temporary environments for each trait are negatively correlated, (d) significant genotype–environment interaction is present, or (e) the traits are influenced by maternal effects.
- 4 The quantitative genetic theory that defines the concept of repeatability is reviewed and implications of violations of the five assumptions are discussed in the context of interpreting repeatability as an upper estimate to heritability.