POPULATION STUDIES IN PREDOMINANTLY SELF‐POLLINATED SPECIES. II. ANALYSIS OF QUANTITATIVE GENETIC CHANGES IN A BULK‐HYBRID POPULATION OF BARLEY

Abstract

Populations of predominantly self-fertilized species are generally considered to be highly uniform and to lack the genetic flexibility necessary to respond to long-term changes in environment. Yet a large proportion of flowering plants practice mixed selfing and outcrossing, and among these species many are both highly successful and versatile. Apparently, therefore, adopting self-fertilization need not place undue restrictions on the genetic system and some evidence is available that populations of inbreeders are not as uniform as has often been imagined. This is perhaps because selfing is rarely if ever complete and when regular or sporadic failures of the selfing mechanism occur, gene exchange can take place among members of the different partially isolated families that constitute the population. In fact, a theoretical case can be made that the mating system of mixed selfing and random outcrossing is only one of many possible variants of the model described by Wright (1931 and later) as optimum for evolution, namely, division of the population into a series of sub-groups among which isolation and intercommunication are kept in proper balance. It is only recently that detailed studies of variability have been undertaken in populations of self-pollinated species and that experimental evidence has been presented on selection and sub-speciation, reproductive method and colonization, and other aspects of evolution in these species (reviews in Stebbins 1950, 1957; Baker 1953, 1959; Grant 1958; Morley 1959). Theoretical work (Hayman 1953; Reeve