Often the desirable level of any particular characteristic is something that sits in the middle between the extremes of excess and deficiency. However, the optimal survival of an individual could be slightly skewed towards the excess extreme reports modelling work in Nature Communications. This finding could explain how autoimmune diseases evolve and are retained. An example of this is seen in individuals with autoimmune disorders where they are effectively able to defend themselves against enemies, but at the cost of also attacking healthy tissue. This is also the case in anaphylactic shock, where comparatively harmless allergens provoke potentially fatal systemic immune reactions.
Mark Urban and collaborators present a theoretical framework, where slightly biased optimal survival is enough to promote the development of extraordinary defences through eco-evolutionary feedbacks.
Excessive investments in defence behaviours suggest a reaction to situations and substances that the body should normally ignore. The new model suggests that an individual’s survival is exponentially related to the investment in defence, with a threshold after which the victim is invulnerable. This, combined with a linear decrease in fertility with increasing investment, should lead to the highest possible level of defence that compromises the least in fecundity. However, the authors note that the population will be always composed of well adapted individuals and maladapted ones and they suggest that those maladapted individuals will promote enemy reproduction, pushing the population in general to invest even greater resources in defence.