Drosophila Fruit Flies
Selection Experiments And Quantitative Trait Loci
Current theory suggests that aging exists due to the decreasing force of natural selection as a function of age, and that it has an underlying genetic basis. Much of the experimental support for this theory has come from study of D. melanogaster. When an appropriate population of D. melanogaster is cultured in the laboratory using only the oldest individuals to reproduce the next generation, the force of selection now acts on the older individuals. Over many generations, this selection results in populations with significantly increased fertility at older ages and with significantly increased life span relative to control populations. In other words, by experimentally altering "natural" selection in the laboratory, D. melanogaster is forced to evolve into a long-lived strain. Such long-lived strains also exhibit increased stress resistance and an increased expression of stress response genes, suggesting that life span and stress resistance are related.
Life span varies quantitatively—either shorter or longer—and for this reason is called a quantitative trait. The chromosomal loci, or genes, that affect life span are called quantitative trait loci, or QTLs. QTLs affecting life span have been identified and genetically mapped using appropriate crosses between strains having different life spans.