Distinction between quantitative (a, b) and qualitative differences (c, d) of inducible defenses. C (white) represents a non-induced morph, P1 (light gray) represents a morph defended against the predator 1 and P2 (dark gray) represents a morph defended against predator 2. The triangles, the square and the circle depict the phenotype. In the case of quantitative differences, the changes can be put in order in terms of an increase or decrease (represented by the different sizes of the triangles). This is true for both a) gradual responses (C <P1 <P2) and b) antagonistic responses (P1 <C <P2) In contrast, qualitative differences cannot be put in order in terms of an increase or decrease (represented by the different shapes of the triangles), as changes in different traits would lead to differently shaped phenotypes. This can either be the case, because a) independent changes occur (here: P1 gets higher than C and P2 gets wider than C, so for one trait (for example, width) it is C = P1 <P2 for the other trait (for example, height) it is C = P2 <P1), or b) because the changes to the traits occur to a different extent (here: P1 is higher than P2, but P2 is wider than P1, so for one trait (for example, width) it is C <P1 <P2 for the other trait (for example, height) it is C <P2 <P1).
Herzog and Laforsch BMC Biology 2013 11:113 doi:10.1186/1741-7007-11-113