Factors influencing the efficacy of natural selection, particularly how the dominance level of selected mutations interacts with the ploidy level of organisms, remain poorly understood. A key prediction is that recessivity should reduce selection efficacy in diploids but not haploids. However, this is challenging to test directly in species with a diploid-dominant life cycle. An under-explored phylogenetic clade ideal for studying this question is Bryophytes. Their haplo-diplontic life cycles are characterised by alternating between a long haploid phase (gametophyte)and a sh ort diploid phase (sporophyte). The relative lengths of the haploid and diploid phases can be appreciably different across different species with a gradient of sporophyte complexity observed from liverworts to mosses. The PhD project will investigate how ploidy affects the efficacy of adaptation and purifying selection within species in haplo-diplontic organisms. Population transcriptomic data will be produced for various species of Bryophytes to identify phase-biased genes and characterize their diversity patterns.