Stabilizing Selection for Heading Time in a Bulk-Hybrid Population of Barley

Abstract

SEVERAL workers^1–3 have recently pointed out the importance of the stabilizing type of natural selection, in which the intermediate phenotype has an adaptive advantage over the deviants from such a centrally located norm, thus resulting in a rather stable population mean and a continually diminishing population variance. Robertson⁴ has analysed the theoretical implications of two different models of stabilizing selection: the intermediate optimum model of Wright⁵, where the more fit individuals are phenotypically intermediate, and the homeostatic model of Lerner⁶, in which the favoured individuals are more fit since they are heterozygotes. The latter model involves true overdominance at one or more loci governing fitness or its various components. Hence, it would be of interest to investigate an actual example of stabilizing form of selection for its fit to one or the other model. The present communication summarizes evidence for stabilizing selection for heading time (recorded as the number of days before the first spike comes out of the flag leaf) in a bulk-hybrid population of barley that was derived from a composite cross and propagated over 18 generations under natural self-fertilization without conscious selection, and it further provides some information on the discriminating choice between the two models.