Data from: Sexual selection on male size drives the evolution of male-biased sexual size dimorphism via the prolongation of male development

Sexual size dimorphism (SSD) arises when the net effects of natural and sexual selection on body size differ between the sexes. Quantitative SSD variation between taxa is common, but directional intraspecific SSD reversals are rare. We combined micro- and macroevolutionary approaches to study geographic SSD variation in closely related black scavenger flies. Common garden experiments revealed stark intra and interspecific variation: Sepsis biflexuosa is monomorphic across the Holarctic, while S. cynipsea (only in Europe) consistently exhibits female-biased SSD. Interestingly, S. neocynipsea displays contrasting SSD in Europe (females larger) and North America (males larger), a pattern opposite to the geographic reversal in SSD of S. punctum documented in a previous study. In accordance with the differential equilibrium model for the evolution of SSD, the intensity of sexual selection on male size varied between continents (weaker in Europe) whereas fecundity selection on female body size did not. Subsequent comparative analyses of 49 taxa documented at least six independent origins of male-biased SSD in Sepsidae, which is likely caused by sexual selection on male size and mediated by bimaturism. Therefore, reversals in SSD and the associated changes in larval development might be much more common and rapid and less constrained than currently assumed.

性大小二态性（Sexual size dimorphism, SSD）指自然选择与性选择对生物体体型的净效应在两性间存在差异的现象。不同类群间的定量SSD差异较为普遍，但种内定向SSD逆转事件却十分罕见。我们结合微观进化与宏观进化研究手段，对近缘黑食腐蝇的地理SSD差异展开研究。同质园实验（common garden experiment）结果显示出显著的种内与种间差异：双刺黑食腐蝇（Sepsis biflexuosa）在全北界（Holarctic）均表现为体型单态（无大小二态性）；而S. cynipsea（仅分布于欧洲）则始终呈现雌性偏向的SSD。有趣的是，新胞黑食腐蝇（S. neocynipsea）在欧洲与北美展现出截然相反的SSD模式：欧洲种群为雌性体型更大，北美种群则为雄性体型更大，这一模式与此前研究中记录的斑点黑食腐蝇（S. punctum）的SSD地理逆转模式恰好相反。依据SSD进化的差异平衡模型（differential equilibrium model），雄性体型所受性选择的强度在不同大陆间存在差异（欧洲地区强度较弱），而雌性体型所受繁殖力选择的强度则无此类地理分化。后续对49个类群的比较分析表明，腐蝇科（Sepsidae）中至少存在6次雄性偏向SSD的独立起源事件，该现象大概率由雄性体型所受的性选择驱动，并由双成熟发育（bimaturism）介导。因此，SSD的逆转及其伴随的幼虫发育变化，或许比当前学界所假设的更为普遍、迅速，且受约束更少。