Although significant progress has been made toward understanding the genomic factors influencing gene evolution, most of these observations have come from studies of model organisms. Also, the majority of these comparisons have been among evolutionary distant species, and the relative importance of these genomic parameters at a microevolutionary time scale is less clear . This is especially true for plant studies describing the causes of evolutionary rate variation among nuclear genes; they are almost entirely restricted to relatively divergent species in the genus Arabidopsis or to evolutionary distant crop species such as Sorghum bicolor, Zea mays and Oryza sativa . As a consequence, previous studies have estimated genetic divergence in terms of nucleotide substitutions rather than FST, since the latter will be close to one in genetically distant species.