Abstract
Annual surveys of a natural population of Drosophila melanogaster have been made by setting up a survey station in Hiroshima City from 1961 to 1971. The surveys made through the 11years disclosed an increasing trend in frequencies of lethal plus semilethal second chromosomes and a decreasing trend in the frequency of allelism between lethals. The lethal plus semilethal frequency increased almost three fold (from 13% to 37%), and the allelism frequency decreased from about 5% to 1%. For this directional change; 1) the infection with a killing agent did not appear to be associated, since the frequency of infected flies was negatively correlated with the lethal and semilethal frequency, 2) the same correlation was observed concerning the frequency of delta-associating second chromosomes; hence, delta may not be associated, and 3) the effects of weather factors were not evident. The increase in lethal and semilethal frequencies and the decrease in allelism frequencies were interpreted to be due to an increase in population size during this decade, and the environmental change produced by man which might affect the increase was discussed.
