"d. melanogaster" is not so much studied in the field as in the laboratory. the best-known studies of "drosophila" in a natural environment was done by dobzhansky and colleagues on "drosophila pseudoobscura" from the early 1930s to 1970. reprints and discussions are in lewontin "et al" 2003, and theoretical discussions in dobzhansky, 1970.