SYNERGISTIC SELECTION AND GRADED TRAITS

Fitness interactions where benefits are shared only between individual s with similar traits are often referred to as `synergistic'. Examples include defence characters, like insect warning colouration and plant unpalatability, and joint activities needing the active participation of all group members, such as cooperative hunting. Previous analyses, assuming discrete variation in the trait, have shown that synergistic selection can be a sufficient explanation for the evolutionary stabil ity of such traits. Here, we investigate the consequences of graded va riation in the trait responsible for synergistic effects. Classifying the synergism as unbiased when an individual receives maximum associat ional benefit by having the same trait value as its neighbours, and le tting a positive (negative) bias represent the maximum above (below) t his value, we show that only positively biased synergistic selection c an enhance a graded trait. Thus for graded traits, a synergistic benef it is not in itself sufficient for evolutionary stability. We study po ssible reasons for synergistic bias in a simple model of plant defence s against herbivores, and suggest that the processes of herbivore avoi dance learning and diet selection are probable causes of positive bias . We propose that mammalian herbivores exposed to a given level of tox icity will show stronger feeding aversion to higher toxicity, resultin g in positively biased synergistic selection of plant defence traits. Positive bias produced by avoidance learning may, in a similar way, al so select for defence signals.