The characteristics we described above suggest that sex-biased genes are dispensable. However, that is not to say that indispensable genes are not subject to sexual antagonism, rather that sexual antagonism for these genes is less likely to produce sex-biased gene expression. If sex-biased genes are less critical, they would be subject to less powerful purifying selection, and would show elevated rates of functional protein change through neutral processes alone. Additionally, this reduction in purifying selection allows for two additional processes to occur. First, the lack of purifying selection associated with some degree of dispensability allows for flexibility in the fitness landscape of sex-biased genes. This then allows positive selection, sexual or otherwise, to influence the evolution of sex-specific expression patterns when genes are sexually antagonistic. It may be that indispensable genes have sex-specific fitness optima in some of their functionalities; however, constraints acting from other pathways or functions prevent them from realizing independent male and female fitness peaks. Dispensability may therefore allow for sex-specific selection to produce sex-biased gene expression patterns, and this has implications to the evolution of sexually selected phenotypes. If the evolution of sex-biased expression is limited to those genes that are non-critical, dispensability of the underlying genes may act as a brake on the evolution of sexually dimorphic traits.
Testing these questions will require an integrated approach. Global dispensability is best measured in a systems biology framework, and systems biology data will be required to understand its role in metazoan evolution. Metazoan systems biology is a nascent field, and time is needed to develop network maps and functional genomics. In the meantime, we can focus on two persisting gaps. First, we need a better understanding of the actual loci under sexual selection in order to determine how sexual selection shapes gene expression patterns. Additionally, we need more extensive molecular data on divergence and standing polymorphisms in an array of animals in order to delineate the beacon of positive selection from relaxed constraint (McDonald & Kreitman 1991
). These are crucial if we want to understand how selection shapes expression patterns.