Final answer:
Before the discovery of induced mutations by Hermann Muller, Drosophila melanogaster was a fortuitous choice for genetic studies due to their large number of visible phenotypes, which made it easier to track genetic changes and study their outcomes on physical traits.
Step-by-step explanation:
Before Hermann Muller's discovery that radiation induces mutation, scientists like Thomas Hunt Morgan and his colleagues used organisms that demonstrated spontaneous mutations. The use of Drosophila melanogaster, or fruit flies, was fortuitous due to their large number of visible phenotypes, which include traits such as body color and eye color. Specifically, Drosophila were chosen for their short life cycle, ease of husbandry, and the relatively simple genetic model they provided, all of which were conducive to genetic studies. These traits allowed Morgan's group to identify and track genetic changes across generations. Each trait, such as a wild-type phenotype with grey body and red eyes, or mutations leading to a black body and/or cinnabar eyes, could be easily observed and mapped.
Mutations are essential for genetic diversity and are the ultimate source of new alleles in any population. They can be beneficial, harmful, or neutral, affecting an organism's survival, reproduction, and longevity. Following Muller's work, it became clear that mutations could be artificially induced, allowing scientists to study genetic changes more systematically, but prior to this, reliance was mainly on naturally occurring variations that could be readily observed in Drosophila.