A framework for integrating natural and sexual selection within- and across-generations of eukaryotic biphasic life cycles

An alternation between diploid and haploid phases is universal among sexual eukaryotes, with three kinds of biphasic life cycle distinguished by the presence or absence of mitosis in each phase. Across these biphasic cycles, 'narrow sense' natural selection and sexual selection occur in both phases. These opportunities for selection arise from variation in the phenotypes of individuals and influence the evolutionary trajectories of populations. However, selection concepts are not equally understood or applied to different cycles, and the same terms have different meanings for universal processes. 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 biphasic cycle, selection opportunities are modified by within-phase two-way interactions, 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 throughout a wide conceptual context, expanding on the conventional definition of a connection crossing two circuits of the cycle from diploid to diploid. Examples traversing sexual eukaryotes for each interaction and bridge are presented with the aim to highlight knowledge gaps and inspire new research in different taxa.