Figure 2.

Unguarded X effects in species with mammalian‐like, Drosophila‐like, and bird‐like sex chromosomes. Results are shown for plausible values of the haploid genomic deleterious mutation rate (U_H  = 1.1 for vertebrates, based on a relatively high estimate from humans: Keightley 2012; Dukler et al. 2021; U_H  = 0.5 for Drosophila: Haag‐Liautard et al. 2007), with typical sex chromosome sizes for mammals, flies, and birds (bird results are based on the upper end of range of Z chromosome sizes: 0.07 < P_Z  < 0.1; Stiglec et al. 2007; Sultanova et al. 2020), and equally strong purifying selection in each sex (β = 1). The gray‐shaded regions correspond to the range of $\overline{\alpha }$ estimates obtained from Drosophila studies ($0.1<\overline{\alpha }<0.3$; Charlesworth 2015), and the orange lines correspond to dominance values consistent with both theory and mutation accumulation studies from model organisms ($\overline{h}=0.25$; see Manna et al. 2011).