2. Firn RD and Jones CG (1996) In "Phytochemical Diversity and Redundancy in Ecological Interactions". ed Romeo et al., Plenum Press, NY. pp. 295-312.
The Screening Hypothesis is a model for the evolution of secondary product diversity that is based on extensive evidence, accumulated over several decades, which has shown that specific, high-potency biological activity is a rare property for a molecule to possess. Evidence to support this observation comes from studies conducted at the organism, organ, tissue, cell and molecular levels of organisation.
The proportion of naturally occurring molecules showing any biological activity will depend on the concentration at which they are assayed. Consequently it is argued that the meaningful comparative discussion of biological activity of any collection of endogenous compounds can only be made with reference to the concentration that will exist at the site of action of the compounds. When biological activity is only shown by a substance when assayed at concentrations above those that will occur in natural situations, it may be of pharmacological interest but such activity should not be assumed to be meaningful in terms of the discussing the possible role(s) that the substance might play in the organism that produces it.
The Screening Hypothesis is based on concepts which place very severe constraints on the evolutionary framework governing the generation, retention and use of secondary products. It is suggested that these constraints operate at a high level and that mechanisms to generate and retain chemical diversity must have evolved at an early stage in the evolution of life. Indeed it seems likely that secondary products may have been produced as a consequence of "biochemical inventiveness"which must have been inevitable when primary metabolism was evolving. "Secondary metabolism" might well have evolved concurrently with primary metabolism and primitive organisms may have evolved their "secondary metabolism" to exploit the inevitability of the generation of some chemical diversity.
Finally we return to the theme of the book, redundancy. It is apparent that a debate as to whether any secondary product is "redundant" becomes a semantic argument. If one defines redundant as no longer playing a role (i.e. the biological activity of a compound, which was once of value to the producer, is no longer exploited), the Screening Hypothesis would suggest that the many secondary products never had a role which resulted from their inherent biological activity hence cannot be considered redundant in this sense of the word. Likewise if one defines redundant as being superfluous or over-copious (Geddie W, 1959), the Screening Hypothesis would provide an arguments against the view that the majority of secondary products evolved as a result of any superfluous production hence most secondary products cannot be considered redundant in this sense. The Screening Hypothesis suggests that the term "redundant" would really only be appropriate in a very few circumstances. The Hypothesis suggests that the synthesis of many compounds which bring no short term benefit to the producer is a necessary part of the overall mechanisms employed by plants and microbes to produce the occassional chemical which possesses useful biological activity. Most secondary products are only "redundant" in the way that most antibodies are "redundant". The production of the majority of these substances results in no short-term benefit but short-term costs are compensated for by the longer term benefits that result when the rare biologically active compound is made. The majority of secondary products and antibodies are not redundant in any commonly accepted use of the word but are a necessary consequence of the need to generate chemical diversity.