Thomas Hunt Morgan used Drosophila melanogaster for studying heredity. Morgan used the Drosophila melanogaster, known as the fruit fly, because it feeds on decaying fruit. It is small, about 3 mm long, and easy to raise in the laboratory; a thousand can be collected in a one-quart glass milk bottle. Moreover, it is fertile all year long and very prolific, producing a new generation every twelve days, or thirty generations per year.

Not only are male and female offspring easy to distinguish, but embryonic development occurs outside the body, making it a simple matter to study the effects of mutations on development.Finally, Drosophila melanogaster has only four pairs of chromosomes. Morgan began working seriously with Drosophila in 1907, with the intention of breeding many generations of flies, and perhaps producing one that looked different from the rest. In short, he hoped to find an occasional fly that had undergone a mutation, sudden change in body form, a phenomenon that had recently been discovered in plants by the Dutch biologist Hugo de Vries.

But despite much effort and the breeding of successive generations, Morgan initially failed to detect a single mutation. Finally, in April 1910 he suddenly had a breakthrough. In one of his bottles filled with Drosophila was a male fly with rather than the normal red eyes. Morgan realized the implications of this immediately; the birth of this single spontaneous mutant—this one male fly with white eyes—allowed him to begin addressing some key questions in heredity: How did this white eye color originate?What determines eye color? The low frequency of spontaneous mutation and the perpetuation of mutations that did occur indicated that genetic material is constant. The observation was soon confirmed in many other organisms, from Drosophila to man and from bacteria to yeast, offering proof both of inheritance and of the capacity for mutation to allow for evolutionary change in spite of the general constancy of genetic material.