A semi-natural Drosophila melanogaster population was twice forced through a genetic bottleneck and allowed to recover naturally. In one case additional variation was introduced to the recovering population. The percentage of lethal chromosomes, the level of allelism between these lethals, and the effective population size calculated from the allelism of these lethals all rose sharply in the few generations following each bottleneck, though this was not t~,e case in the very first generation. Thereafter this rise decelerated rapidly and never returned to pre-bottleneck levels. Additional introduced variation had little effect. The reasons for and implications of this pattern have been considered.
A semi-natural Drosophila melanogaster population was twice forced through a genetic bottleneck and allowed to recover naturally. In one case additional genetic variation was introduced to the recovering population. Variation in chromosomal fitness was drastically reduced by both bottlenecks but actual fitness values were greatly influenced by the founder effect. This variation reappeared rapidly at first and then gradually reverted to pre-bottleneck levels. However, there were more low-fitness and fewer high-fitness chromosomes than before the first bottleneck. Additional introduced variation had a negligible effect.
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