Correspondence between a fitness landscape model and a dose-response model. On the left panel, a fitness landscape model is illustrated as in Figure 1–4, in a two-traits phenotypic space. Four environments are represented with increasing concentration of a drug (with optima 0, 1, 2, 3). Environment with optimum 0 (in green) represents the environment without drug, while environment 1, 2, 3 represent increasing concentrations of the drug. In this model, fitness depends on the Euclidean distance to the optimum, and a mapping function (see inset of Fig. 1). It is possible to set a threshold value for absolute fitness below which a phenotype cannot persist / grow. This threshold is indicated by circles (the colour of the circle corresponds to the different environments). In many cases, resistant mutants can have a positive growth rate in absence of drug (while the reverse is not true: susceptible phenotype do not grow in presence of the drug). Hence the threshold contours will be often nested (but it is possible to imagine cases where this is not the case). In this representation, it is easy to see that an absolute fitness criterion (= being within the threshold contour) is not synonymous with adaptation (= being close to the optimum). The dot S represents the position of a susceptible phenotype, while R1 and R2 represent two resistance mutants. The right panel illustrates the ‘dose response’ curves relating, dose to absolute fitness, for each phenotype (S, R1, R2). For instance, R1 is within the threshold contour of dose 1, but not of dose 2 and 3. Hence, its dose-response is zero for doses 2 and above. Note also that R1 is further apart from optimum 0, compared to S. Hence, its absolute fitness is lower than that of S at dose 0. This correspondence shows that it is entirely possible (and straightforward) to relate fitness modes to more traditional one-dimensional dose-response models. Furthermore, since LD50, IC50, MIC or other ecotoxicological measures can be defined using dose response, they can also be defined in the fitness landscape model. Note however that these measures have been rightly criticized as being only partial fitness summaries (Regoes et al. 2004, Sampah et al. 2011, Wen et al. 2016). They are also often obtained in absence of competition, or often concern only a particular life stage. Note also that absolute measures of fitness are often more appropriate (than relative fitness) when dealing with the demography of the treated species (Day et al. 2015). However, relative fitness is in general more relevant to study environment specialization, where the “cost of resistance” matters.