Genetic mechanisms of sex determination display great variety among groups of organisms. Yet amid this diversity, sex chromosomes show remarkably similar features in the disparate groups that have independently evolved a chromosomal mechanism of sex determination. Gender specific selection pressures on genes linked to the sex-determining locus may represent one cause of this uniformity. In this talk, I will show how genome rearrangements present among species of Drosophila provide a temporal measure of change in the sex chromosomes and represent an ideal substrate to identify forces that shape the convergence in structure and content of the sex chromosomes. Fusion of an autosome to the X, to the Y, or to both sex chromosomes has occurred many times to generate new sex-linked genomic regions subject to diversification as neo-sex chromosomes. Ancient additions to the sex chromosomes, such as present in D. pseudoobscura and D. robusta, have undergone complete diversification of the acquired neo-sex chromosomes—but with different fates for the ancestral Y chromosome. More recent additions to the sex chromosomes, such as present in D. miranda, show intermediate patterns of diversification. The very earliest stage of sex chromosome evolution, involving suppression of recombination, is represented in D. americana. A centromeric fusion between the X and an autosome (element B or melanogaster 2L) has established a sex-linked genomic region with active exchange between X- and Y-linked alleles. Selection pressures unique to females and males have the potential to influence gene function and genetic variation differentially along the length of the neo-X and neo-Y chromosomes. Our studies reveal selection favoring the suppression of recombination to protect the contents of the neo-X. Selection to optimize female fitness may represent a general force that shapes the contents of X chromosomes.