 These differences in gene expression are the result of either natural selection on reproductive potential and survival traits of either sex or sexual selection on traits relevant to intra-sexual competition and inter-sexual mate choice.Sex biased genes could either be male or female biased and sequence analysis of these protein coding genes have revealed their faster rate of evolution which has been attributed to their positive selection vs. reduced selective constraint.Apart from sex specific natural selection and sexual selection that includes both intersexual and intrasexual selection, a third phenomenon also explains the differences in gene expressions between two sexes – sexual antagonism.