Constraints on adaptation: explaining deviation from optimal sex ratio using artificial neural networks.

Determining processes constraining adaptation is a major challenge facing evolutionary biology, and sex allocation has proved a useful model system for exploring different constraints. We investigate the evolution of suboptimal sex allocation in a solitary parasitoid wasp system by modelling information acquisition and processing using artificial neural networks (ANNs) evolving according to a genetic algorithm. Theory predicts an instantaneous switch from the production of male to female offspring with increasing host size, whereas data show gradual changes. We found that simple ANNs evolved towards producing sharp switches in sex ratio, but additional biologically reasonable assumptions of costs of synapse maintenance, and simplification of the ANNs, led to more gradual adjustment. Switch sharpness was robust to uncertainty in fitness consequences of host size, challenging interpretations of previous empirical findings. Our results also question some intuitive hypotheses concerning the evolution of threshold traits and confirm how neural processing may constrain adaptive behaviour.

Eusociality and the success of the termites: insights from a supertree of dictyopteran families.

Sociality in insects may negatively impact on species richness. We tested whether termites have experienced shifts in diversification rates through time. Supertree methods were used to synthesize family-level relationships within termites, cockroaches and mantids. A deep positive shift in diversification rate is found within termites, but not in the cockroaches from which they evolved. The shift is responsible for most of their extant species richness suggesting that eusociality is not necessarily detrimental to species richness, and may sometimes have a positive effect. Mechanistic studies of speciation and extinction in eusocial insects are advocated.

Herbivore host choice and optimal bad motherhood.

When theory predicts which phenotypes are well adapted to a given environment, the data do not always match the predictions. Host-plant selection by herbivorous insects is one such example. Herbivorous insects often appear to make poor choices about where their offspring should develop. New evidence presented by Scheirs et al. suggests that adult insects can choose oviposition sites that enhance their own long-term fitness at the expense of their individual offspring. This suggests that herbivorous insects might be genuinely bad mothers, that host choice is nonetheless adaptive, and that theory needs to incorporate new assumptions about host effects on adult performance.

Nonsiblicidal behavior and the evolution of clutch size in bethylid wasps.

Parent-offspring conflict over clutch size may lead to siblicidal behavior between juveniles. In parasitoid wasps, selection for siblicide in small broods is predicted to produce a dearth of gregarious broods with few eggs. Here we document the clutch size distribution in the Bethylidae, a large family of aculeate parasitoids. Small gregarious clutches are the most common. Further data suggest that the most common gregarious clutches in the parasitoid Hymenoptera as a whole contain only a few eggs. Across bethylid species, both clutch size and wasp size increase with host size. Within genera clutch size is more closely related to host size, but between genera or larger clades wasp size is more closely related to host size. The volume of the emerging wasp brood does not depend on whether a species lays single- or multiple-egg clutches once host size is taken into account. These data suggest that clutch size in bethylid wasps is best described by traditional optimality models and that siblicide plays little role in this and possibly other families. We propose several ecological reasons for the rarity of siblicide in bethylids: ectoparasitism, idiobiosis, and a suite of characteristics associated with high within-brood relatedness.

Mixed sex allocation strategies in a parasitoid wasp.

The sex allocation strategy of the parasitoid Laelius pedatus (Hymenoptera: Bethylidae) on different-sized hosts was investigated. The wasp lays from one to five eggs, and clutch size increases with host size. On the smallest hosts, single male eggs are laid, while on slightly larger hosts single female eggs are laid. On still larger hosts, gregarious clutches are laid which nearly always consist of a single male and one or more female eggs. The sex ratio strategy of the wasp appears to be influenced by a combination of local mate competition and conditional sex expression based on host quality.