To assess the fitness effects of ploidy changes, Zörgö et al. tested the growth properties of 51 strains of Saccharomyces cerevisiae and Saccharomyces paradoxus, each in haploid and diploid forms, under 33 distinct environmental conditions. For each condition, there was strong conservation among strains for whether haploidy or diploidy was advantageous. However, the fitness effects of ploidy varied in an unexpectedly complex manner across the specific environmental conditions. In particular, the expected patterns of haploid advantages in nutrient-poor conditions (owing to bioenergetic efficiency) and diploid advantages in mutagenic environments (owing to mutation buffering) were not observed.