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.
SUMMARYRecessive lethals on the second chromosome were extracted from genetically isolated populations in Australia and the U.K. The frequency of allelism, used in a manner analogous to capture-recapture ot animal populations, indicated that the number of genes capable of mutating to lethal had a 95 per cent probability of being in the range 247 to 1140, although excluding possible heterotic and synthetic lethals altered this to 309 to 3568. Possible sources of bias are discussed. The disagreement between these values and those obtained the direct measurements of DNA (more than 10 times greater) is clear. It is suggested that many of the genes of eukaryotes have been duplicated by unequal exchange during recombination and occur in functionally related groups or in supergenes. Some consequences of this hypothesis to population genetics are noted.
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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