Figure 2.

Illustration of how facultative parasites could be used to address questions related to the costs of plasticity by studying clades that contain free-living species (FL), facultative parasites (FP) and obligate parasites (OP). FL exhibit the free-living phenotype and OP exhibit the parasitic phenotype regardless of environmental conditions. However, FP can be induced to exhibit either a free-living phenotype (typically under benign environmental conditions) or a parasitic phenotype (typically under stressful conditions). (a) No costs to plasticity: the FP exhibiting a free-living phenotype achieve equal fitness to FL under benign environmental conditions, and FP exhibiting the parasitic phenotype achieve the same fitness as OP under stressful environmental conditions. (b) Costs to plasticity: FP always achieve lower fitness than the non-plastic species exhibiting the same phenotype.