Balanced vs. slightly unbalanced selection.

Balanced selection, defined as that all parents have equal contributions to the next generation, is expected to minimize the loss of gene diversity and thereby to allow a steady increase of selection response even at very advanced generations. In this study, balanced selection was compared with two degrees of slightly unbalanced selection (equal contributions from most of the families) and with unrestricted selection (unequal contributions) in terms of the retrieved response and genetic diversity consumed (group coancestry) over 30 generations of selection. A simulation was used varying the heritability and considering two population sizes and two family sizes. Results showed that a slight unbalance can bring a favourable relation between retrieved response and genetic diversity when compared to balanced selection at low and moderate heritabilities, and when small families and population size were assumed. The implicit constraint of a constant number of contributing families imposed in balanced and slightly unbalanced selection reduced the likelihood of gene loss compared to unrestricted selection. I conclude that a slight unbalance often could be more favourable than complete balance regarding selection efficiency, and that gain retrieved per unit of group coancestry declined with further unbalance.