Complexity_ A Guided Tour - Melanie Mitchell [86]
For example, at any given time, the immune system must determine which regions of the huge space of possible pathogen shapes should be explored by lymphocytes. Each of the trillions of lymphocytes in the body at any given time can be seen as a particular mini-exploration of a range of shapes. The shape ranges that are most successful (i.e., bind strongly to antigens) are given more exploration resources, in the form of mutated offspring lymphocytes, than the shape ranges that do not pan out (i.e., lymphocytes that do not bind strongly). However, while exploiting the information that has been obtained, the immune system continues at all times to generate new lymphocytes that explore completely novel shape ranges. Thus the system is able to focus on the most promising possibilities seen so far, while never neglecting to explore new possibilities.
Similarly, ant foraging uses a parallel-terraced-scan strategy: many ants initially explore random directions for food. If food is discovered in any of these directions, more of the system’s resources (ants) are allocated, via the feedback mechanisms described above, to explore those directions further. At all times, different paths are dynamically allocated exploration resources in proportion to their relative promise (the amount and quality of the food that has been discovered at those locations). However, due to the large number of ants and their intrinsic random elements, unpromising paths continue to be explored as well, though with many fewer resources. After all, who knows—a better source of food might be discovered.
In cellular metabolism such fine-grained explorations are carried out by metabolic pathways, each focused on carrying out a particular task. A pathway can be speeded up or slowed down via feedback from its own results or from other pathways. The feedback itself is in the form of time-varying concentrations of molecules, so the relative speeds of different pathways can continually adapt to the moment-to-moment needs of the cell.
Note that the fine-grained nature of the system not only allows many different paths to be explored, but it also allows the system to continually change its exploration paths, since only relatively simple micro-actions are taken at any time. Employing more coarse-grained actions would involve committing time to a particular exploration that might turn out not to be warranted. In this way, the fine-grained nature of exploration allows the system to fluidly and continuously adapt its exploration as a result of the information it obtains. Moreover, the redundancy inherent in fine-grained systems allows the system to work well even when the individual components are not perfectly reliable and the information available is only statistical in nature. Redundancy allows many independent samples of information to be made, and allows fine-grained actions to be consequential only when taken by large numbers of components.
Interplay of Unfocused and Focused Processes
In all three example systems there is a continual interplay of unfocused, random explorations and focused actions driven by the system’s perceived needs.
In the immune system, unfocused explorations are carried out by a continually changing population of lymphocytes with different receptors, collectively prepared to approximately match any antigen. Focused explorations consist of the creation of offspring that are variations of successful lymphocytes, which allow these explorations to zero in on a particular antigen shape.
Likewise, ant foraging consists of unfocused explorations by ants moving at random, looking for food in any direction, and focused explorations in which ants follow existing pheromone trails.
In cellular metabolism, unfocused processes of random exploration by molecules are combined with focused activation or inhibition driven by chemical concentrations and genetic regulation.
As in all adaptive systems, maintaining a correct balance between these two modes of exploring is essential. Indeed, the optimal balance shifts