The productivity of four inbred strains of Drosophila melanogaster

Four inbred strains of flies have been maintained by brother × sister mating since November, 1963. Counts of production per vial were made at 14 days, and the flies from the most productive vial were used as the parents of the next generation. For analysis the data were separated into three groups: vials with no production (blanks), vials with less than 20 flies (low production), and those with over 20 flies. The number of blanks did not change significantly in strain A but did show significant decreases in strains B and D. Until generation 60 the number of blanks per generation was not distributed in a Poisson series, but after this generation it was. The number of vials with low production was high in all strains for the first 20 generations and then decreased in widely different generations for each strain.Changes took place in all four strains but by different mechanisms. In strain A there was no increase in range, but the mode moved from 40 to 150 flies per vial and the proportion of low productive vials fell from 15% to 2% per generation. In strain D the low production vials fell from 22% to 6% and there was an extension of the range. Each of the strains shows definite trends of increase and decrease which last from 3 to 30 generations, and vary in slope from 0.3 to 13.8 flies per vial per generation. While we cannot determine how much or how many of these slopes are environmental, it can be argued that since many of the slopes of the four strains are not correlated, part of the cause is genetic. The changes in the direction of the trends show that phenotypic selection in small populations was not effective in accumulating and exploiting mutations for greater productivity.