Historical constraints and heterospecific interference lead to maladaptive sex ratio changes in a pollinating fig wasp

Natural selection’s power is beautifully illustrated by sex ratio variation that seems near-perfectly adapted. However, the fit of sex ratios to optimal predictions is usually tested in oversimplified ecological settings. This creates a one-sided view of evolution, where details are ignored, and evolution appears to produce optimal solutions. Entertaining maladaptation as a potential explanation allows consideration of historical contingencies and trade-offs. These become important in realistic settings where traits may need to be multifunctional, and historical contingencies may channel evolution to suboptimal phenotypes. Evaluating traits’ adaptive/maladaptive nature requires understanding the underlying mechanisms (the proximate causes). We studied clutch composition in a pollinating fig wasp where heterospecific females potentially interfere with sex ratio production and adjustment. Using an information-theoretic approach, we compared hypotheses and confirmed that heterospecific females disrupt Ceratosolen arabicus females’ sex ratios. Given the high fitness costs of failing to adjust optimally, we argue this maladaptation stems from rudimentary mechanisms that fail in realistic conditions and are thus forced to be multifunctional, creating fitness trade-offs. Our findings highlight how historical constraints and ecological interactions may limit adaptation, challenging the assumption that selection always produces optimal traits. Furthermore, they emphasize the need to incorporate ecological complexity when testing evolutionary predictions.