Natural Selection in Varietal Mixtures of Winter Wheat¹

Abstract

T HE importance of natural selection among plants has long been recognized, but the rate at which such changes may occur in cultivated crops has only recently received attention. Harlan and Martini 3 determined the rate of natural selection in a mixture of I I easily recognized varieties of barley which was studied at IO stations for a period of 4 to 12 years. They found that at all stations there was a rapid elimination of the less-adapted sorts. At most places the variety that would eventually dominate the population was quickly evident. The leading variety varied with the location of the station. A variety dominant at one station in some cases was eliminated at another. Few varieties survived at all stations. Some varieties increased for a time and then decreased.This paper is a report on the rate at which changes in varietal ratios occurred in successive crops after two varieties of winter wheat were mixed in the proportion of equal numbers of kernels. One mixture included Kanred and Harvest Queen, the other Kanred and Curren.One portion of each mixture was sown at Manhattan, Karis., and another at Hays, Kans., in the fall of I93I. Seed from the previous crop was used to sow each succeeding crop at the respective station so that the changes in the varietal ratios of the population were c~mulative for each station. The experiment was continued for 7 years at Hays and 9 years at Mahnattan. Since Harvest Queen and Currell are awnless, there was no difficulty in distinguishing these varieties from Kanred. The rate at which natural selection occurred was determined on the basis of separations of the plants which were pulled from sample areas. The number of heads and number of kernels of each variety were determined in the tests at Manhattan and the number of heads and weight of grain at Hays. CHANGES IN VARIETAL PLANT RATIOSThe cumulative changes in the percentage of plants that survived in the varietal population when Kanred was grown in competitive mixtures with Harvest Queen and with Currell are shown for both Manhattan and Hays in Table I.

Variability in Crop Plants, Its Use and Conservation

“…Evidence on the point comes from studies of change of composition with time of 'composed' mixtures. The little information there is relates to wheat and barley in the U.S.A. and, so far as it goes, does seem to indicate that survival is proportional to agronomic value (Montgomery, 1912; Laude & Swanson, 1942;Harlan & Martini, 1938;Suneson, 1949). In Suneson's barley experiment, however, there was one rather striking apparent exception: 'Atlas' in comparison with 'Vaughn' is relatively low yielding and susceptible to disease, yet it virtually eliminated 'Vaughn ' in competition.…”

Survival of Barley and Wheat Varieties in Mixtures1

“…Laude and Swanson (5) reported cumulative changes in mixtures of Kanred with Harvest Queen and with Currell at two locations over a 9-year period. Kanred, the better adapted variety, dominated all mixtures after 9 years.…”

The estimation and use of selective values in predicting population change

“…The estimators of selective values derived earlier were applied to data from i6 separate experimental populations, among which nine have previously been reported in the literature (Harlan and Martini, 1938;Laude and Swanson, 1943;Suneson and Wiebe, i;Suneson, 1949). More than i000 estimates of selective values were made from the data, but only part of the results need be considered since a detailed account of a few experiments serves to illustrate the main findings.…”