Increased selection response in larger populations. II. Selection for ethanol vapor resistance in Drosophila...

The effect of large population size on selection response was investigated using Drosophila melanogaster, with four "small" lines of 160 selected parents/generation compared to two "large" lines of 1,600 selected parents/generation. All lines were selected under similar conditions at a selection intensity of approximately 0.55 standard deviations, for 65 generations, for increased ethanol vapor resistance (measured in minutes required to become anesthetized). Two unselected control lines of 320 parents/generation were also maintained. A significant effect of population size was found. The final treatment means and standard errors were: 27.91 +/- 1.28 min (two "large" lines); 19.40 +/- 1.54 min (four "small" lines); and 4.98 +/- 0.35 min (two control lines). To estimate the mutation rate for the trait, two isogenic lines of about 400 selected parents were selected for 29 generations. The mean increase in additive genetic variance per generation was 0.0009 times the initial environmental variance of the outbred lines. This is comparable to other reported mutation rates. Mutation can explain part of the difference in evolved resistance between treatments, but it appears that even at rather large population sizes, a large difference in long-term response can be obtained in larger outbred lines, from more complete utilization of the initial genetic variation.