* In addition, evolution cannot explain the many complex sex-determining systems. For example, in most mammals, females have two X chromosomes whereas males have an X and Y chromosome, but in birds, many insects, and other organisms, the situation is reversed. In flies, sex is determined the ratio of non-sex chromosomes to X chromosomes (so that males have only one X whereas females have two). It is impossible for evolution to create these new sex-determining systems ''ex nihilo''. | * In addition, evolution cannot explain the many complex sex-determining systems. For example, in most mammals, females have two X chromosomes whereas males have an X and Y chromosome, but in birds, many insects, and other organisms, the situation is reversed. In flies, sex is determined the ratio of non-sex chromosomes to X chromosomes (so that males have only one X whereas females have two). It is impossible for evolution to create these new sex-determining systems ''ex nihilo''. |