The Salicaceae family, including Populus and Salix, is a powerful model system for studying the influence of genome structure on the evolution of adaptive traits. A comparison of synonymous nucleotide substition rates between Populus and Salix demonstrates that the Salix lineage has accumulated more polymorphisms. This elevated evolutionary rate is recapitulated in higher rates of genome fractionation in Salix following the shared Salicoid genome duplication. The two genera have numerous contrasting phenotypic characterisitics, including growth form, pollination mode, and generation time. Furthermore, although both species are primarily dioecious, they appear to have different sex determination loci. Sex determination in Populus is controlled primarily by loci on chromosome 19 across all species that have been studied thus far. In contrast, it is now clear that sex determination occurs on Chromosome 15 in Salix purpurea, based on mapping in an F2 intraspecific cross as well as in an association population of unrelated individuals,. This is quite surprising, especially given the high collinearity of the vast majority of the Populus and Salix genomes. Furthermore, examination of the genotypes of sex determination loci in male and female trees suggests that Populus trichocarpa has a predominantly XY sex determination system, while Salix purpurea has a ZW system. A comparative analysis of the sex determination regions of the two genera reveals many shared characteristics, including structural complexity, high repeat density, and suppressed recombination. Ongoing questions include whether there are shared mechanisms of sex determination in the two species, the extent of sex dimorphism, the role of pollinator attraction and defense in sex chromosome evolution, and the presence of sexually antagonistic genes in the sex determination regions.