elimination of Y to occur, it was necessary to develop an alternative way of determining sex (for example, by determining sex by the ratio of the X chromosome to autosomes), and any genes necessary for male function had to be moved to other chromosomes. In the meantime, modern data demonstrate the complex mechanisms of Y chromosome evolution and the fact that the disappearance of the Y chromosome is not guaranteed. 1:1 sex ratio Fisher's principle outlines why almost all species using sexual reproduction have a sex ratio of 1:1. W. D. Hamilton gave the following basic explanation in his 1967 paper on "Extraordinary sex ratios", given the condition that males and females cost equal amounts to produce: Suppose male births are less common than female. A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring. Therefore, parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them. Therefore, the genes for male-producing tendencies spread, and male births become more common. As the 1:1 sex ratio is approached, the advantage associated with producing males dies away. The same reasoning holds if females are substituted for males throughout. Therefore, 1:1 is the equilibrium ratio. Non-therian Y chromosome Many groups of organisms in addition to therian mammals have Y chromosomes, but these Y chromosomes do not share common ancestry with therian Y chromosomes. Such groups include monotremes, Drosophila, some other insects, some fish, some reptiles, and some plants. In Drosophila melanogaster, the Y chromosome does not trigger male development. Instead, sex is determined by the number of X chromosomes. The D. melanogaster Y chromosome does contain genes necessary for male fertility. So XXY D. melanogaster are female, and D. melanogaster with a single X (X0), are male but sterile. There are some species of Drosophila in which X0 males are both viable and fertile. ZW chromosomes Other organisms have mirror image sex chromosomes: where the homogeneous sex is the male, said to have two Z chromosomes, and the female is the heterogeneous sex with a Z chromosome and a W chromosome. For example, female birds, snakes, and butterflies have ZW sex chromosomes, and males have ZZ sex chromosomes. Non-inverted Y chromosome There are some species, such as the Japanese rice fish, in which the XY system is still developing and cross over between the X and