The Coevolution of Virulence: Tolerance in Perspective (original) (raw)
Figure 1
The importance of intercepts: point versus range tolerance.
To understand differing interpretations of the evolutionary consequences of tolerance, it is necessary to consider when point and range tolerance will disagree. Below is one scenario where they will agree, and two where they may not. (A) With fitness in the absence of infection identical, at whichever parasite density measured, the fitness of the genotype with the flatter slope will be higher; here, genotype a1 is more tolerant than b1 regardless of how it is assessed. Both point and range tolerance measures therefore agree over the more tolerant genotype. (B) Here genotypes differ for their intercept, and the genotype with the higher point tolerance differs depending on whether parasite density is measured at d1 (where b2>a2) or d2 (where a2>b2). The fitness at d1 is strongly influenced by fitness in the absence of infection, while fitness at d2 is more strongly influenced by how fitness declines with increasing I. Under range tolerance, however, a2 is more tolerant, despite the fact that it is less (point) tolerant at low densities. (C) Here the point tolerance is always higher for a3, but the range tolerance depends upon the range of I considered; if tolerance is measured across the range depicted by d1, genotype b3 would be considered less tolerant, but it would be considered more tolerant if the range measured was d2. Genotype b3 is always less fit, however.