Mutation and selection are responsible for major evolutionary changes in all organisms. Whether evolutionary processes are mainly driven by selection on preexisting genetic variation or by selection on new mutations is still debatable. In this research, by making use of a highly interbred Drosophila melanogaster stock, the role of new mutations in evolution was investigated in two experiments addressing the following questions: 1) does fitness significantly decline due to the accumulation of new deleterious mutations in heterozygotes? 2) Do new advantageous mutations quickly improve adaptation to a novel environment?
In the first experiment, the second and third chromosomes of Drosophila melanogaster were maintained as heterozygotes in males without recombination for forty-nine generations. We observed that in smaller population sizes fitness decreased significantly as a consequence of new deleterious mutations accumulated in heterozygotes, whereas in larger populations, fitness was not significantly changed. In the second experiment, a homozygous stock of D. melanogaster evolved quickly to resist a previously toxic level of dietary salt as the result of newly arisen beneficial mutations. Hence, the outcomes of these studies demonstrated that the accumulation of new mutations (both deleterious and advantageous) have significant consequences for the persistence of small populations and the process of adaptation to new environments.