Though the exact evolutionary rationale behind the development of immunological tolerance is not completely known, it is thought to allow organisms to adapt to antigenic stimuli that will consistently be present instead of expending considerable resources fighting it off repeatedly. Tolerance in general can be thought of as an alternative defense strategy that focuses on minimizing impact of an invader on host fitness, instead of on destroying and eliminating the invader. Such efforts may have a prohibitive cost on host fitness. In plants, where the concept was originally used, tolerance is defined as a reaction norm of host fitness over a range of parasite burdens, and can be measured from the slope of the line fitting these data. Immune tolerance may constitute one aspect of this defense strategy, though other types of tissue tolerance have been described.