By analogy with an ecological ‘niche’ (the set of environments in which an organism can survive and reproduce), I shall also use the term niche for the set of all possible environments which a given replicator would cause to make copies of it.
Idea-niche fit; the best niche is the one that promotes spontaneous or catalyzed replication of the idea by the environment and is hard the vary such that changing the idea even slightly kills its replication in that niche
