Integrating natural and sexual selection across the biphasic life cycle

An alternation between diploid and haploid phases is universal among sexual eukaryotes. Across this biphasic cycle, natural selection and sexual selection occur in both phases. Together, these four stages of selection act on the phenotypes of individuals and influence the evolutionary trajectories of populations, but are rarely studied holistically. Here, we provide a conceptual framework that transcends taxonomic groups, and unifies the entire selection landscape within and across the diploid and haploid phases. Within a single cycle, our synthesis produces within-phase trade-offs, and across-phase one-way bridges where selection in one phase is tethered to a given genotype’s phenotypic experience in the previous phase. From this, we define four types of parental effect across a wide conceptual context, expanding on the conventional definition of a connection across cycles from diploid to diploid. Examples across sexual eukaryotes for each trade-off and bridge are presented with the aim to highlight knowledge gaps and inspire new experimentation in different taxa. We argue that knowledge of the complex and intertwined opportunities for selection within biphasic life cycles will offer clearer insights into key ecological and evolutionary processes, with benefits to applied science.