Concept for the role of modality in systems with two predators. The upper section describes the three basic types of modality differences with their subgroups (a) sensu strictu, b) and c) sensu latu). To visualize modality (that is, the qualitative aspect of selection pressure caused by predation) two points are needed. The basic phenotype (that is, the phenotype in an environment without any predation pressure) serves as the initial point C, lying on the origin. The ‘immunity point’ I
represents the terminal point, after which natural selection caused by predator x stops (that is, the phenotype is completely defended or ‘immune’). Its coordinates are defined by the modality of the predators given in the first and second row (‘Modality pred. 1’, colored black,’ Modality pred. 2’ colored gray) with k being a positive coefficient and A/B as variables. Between C and I
a vector can be formed, representing the direction and length of selection. In the case of predator 1, this vector always lies on the x-axis; therefore, the protection of a phenotype against predator 1 can be read off its x-coordinate. The same is true for predator 2 in type I and II systems, but not for type III. For each type, a description and a theoretical example are given. Additionally for type IIIb, optimal responses in environments with a single (left) or both (right) predators as well as the costs for a mismatching phenotype (defended against the wrong or only one predator) are described in the bottom boxes.