RAPID EVOLUTION. “Sex determination is a rapidly evolving trait in many lineages, and sometimes closely related species, or populations of the same species, have different modes of sex determination. Houseflies, for example, normally have XY sex chromosomes, but dominant masculinizing and feminizing alleles on other chromosomes exist in some populations that override sex determination by the XY chromosomes.”
http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899