Panic grass (Dichanthelium lanuginosum) can live in geothermally heated soils only when the fungus Curvularia protuberata is present. In order for Curvularia protuberata to colonize the soil, the Curvularia thermal tolerance virus (CThTV) must also be present. This web of coevolutionary interactions between multiple species is referred to as .

Explanation:

The term ‘diffuse co-evolution’ was given by Janzen in 1980 to describe the idea that selection on traits often reflects the actions of many community members, as opposed to pairwise interactions between species

The idea was further clarified by Gould in 1988 by focusing on a variety of ecological and genetic mechanisms that might lead to diffuse co-evolution in response to selection from multiple species

Diffuse co-evolution as a whole can be defined as when selection imposed reciprocally by one species on another is dependent on the presence or absence of other species

In the given example panic grass can live only when the fungus protuberata is present and for Curvularia protuberata to colonize Curvularia thermal tolerance virus must be present so there is a web of multiple species where one species is dependent on other species and that other species in turn is dependent on different species hence referred to as diffuse co-evolution

substitution is the answer in this case