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

Abstract

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.

Comparing parasitoid life histories

“…The sex ratio might be considered a core life‐history trait (Charnov, ), and in parasitoids can sometimes be linked quite closely to some other general life‐history traits (Griffiths & Godfray, ; Hardy & Mayhew, ; West & Sheldon, ; but see Lewis et al., ). However, it is generally not included in comparative studies involving several of the above traits.…”

Influence of natal host and oviposition experience on sex allocation in a solitary egg parasitoid, Anastatus disparis (Hymenoptera, Eupelmidae)

“…In conclusion, sex allocation in A. disparis females fitted with the predictions of condition-dependent sex allocation theory in parasitoids (Trivers and Willard 1973, West and Sheldon 2002, Lewis et al 2010. It also provided a nice test of experience (i.e.…”

“…This suggests that information females obtain from the environment influences their sex allocation. The asymmetry in response suggests that female-biased sex ratios on the larger host were optimal come what may, whilst if only presented with the smaller host, female biased sex ratios are optimal, but in the presence of (or experience of ) larger hosts, then more males are produced in the smaller hosts (Lewis et al 2010).…”

A chalcid wasp acts chiefly as a hyperparasitoid by mostly using small uncommon hosts

“…For solitary parasitoids, in which only one offspring successfully develops on each host, the size of the host is usually an important determinant of the final size of the adult parasitoid offspring that develops on it (Charnov et al, 1981;King, 1989;Godfray, 1994). Because fitness is usually strongly determined by body size in parasitoids (Visser, 1994;West et al, 1996), but more strongly in females than in males (Jones, 1982;van den Assem et al, 1989;Heinz, 1991), it is expected that female solitary parasitoid wasps will allocate large, high quality hosts to female offspring and small, lower quality hosts to male offspring (Charnov et al, 1981;Lewis et al, 2010), a prediction that has received widespread support (King, 1987;West & Sheldon, 2002). This robust observation allows us to use the sex allocation strategy of solitary parasitoids on a given host as an indicator of relative host quality (especially for idiobionts, which permanently paralyze their hosts) without having to directly estimate wasp fitness, an example of the wider principle that sex allocation can be used as a useful proxy for other parameters in behavioural research (West & Rivero, 2000;West et al, 2001;West & Herre, 2002).…”