Multilevel selection on offspring size and the maintenance of variation

Multilevel selection on offspring size occurs when offspring fitness depends on both absolute size (hard selection), and size relative to neighbours (soft selection). We examined multilevel selection on egg size at two biological scales: within clutches and among females, for an external fertilising tubeworm. We exposed clutches of eggs to two sperm environments (limiting and saturating) and measured their fertilisation success. We then modelled environmental (sperm) differences in hard and soft selection on individual eggs, as well as selection on clutch-level traits (means and variances). Hard and soft selection differed in strength and form depending on sperm availability – hard selection was consistently stabilising; soft selection was directional and favoured eggs relatively larger (sperm limitation) or smaller (sperm saturation) than the clutch-mean. At the clutch-level, selection on mean-egg size was largely stabilising; whilst selection on within-clutch variance was weak, but generally negative – although some correlational selection occurred between these two traits. Importantly, we found that the optimal clutch-mean egg size differed for mothers and offspring, suggesting some antagonism between the levels of selection. We thus identify several pathways that may maintain offspring-size variation: environmentally (sperm)-dependent soft selection; antagonistic multilevel selection; and correlational selection on clutch-means and variances. Multilevel approaches are powerful, but seldom used, tools for studies of offspring size and we encourage their future use.