Selection Response in Subdivided Target Regions

In a small target region, it may be possible to exploit local adaptation to increase gains from selection. However, in a large region more extensive testing is usually possible, resulting in more precise estimation of genotype means. A correlated response model was adapted to determine if division of a large target region is likely to increase gains. Genotypic value in a large region and constituent subregions are considered correlated traits. Correlated response in a subregion to indirect selection across the undivided region, relative to direct response to selection within the subregion, is expressed as a function of heritability in the undivided region ( H ) and in the subregion ( H i ), and of the genotypic correlation between region and subregion means ( r G′ ). r G′ depends on the magnitude of the genotype × subregion interaction (σ 2 GS ) relative to the genotypic variance (σ 2 G ). σ 2 GS is the portion of the genotype × location interaction (σ 2 GL ) caused by local adaptation, rather than by random site‐to‐site variability in genotype means. Subdivision can increase heritability through the addition of σ 2 GS to the numerator of H i , but this may be offset by reduced replication across locations within the subregion. Modeling using variance estimates from several cereal programs indicated that, unless σ 2 GL is large relative to σ 2 G and at least 30% of σ 2 GL is due to σ 2 GS , subdivision is unlikely to increase response. These results help explain the success of breeding programs that test broadly.