Immunology as a Metaphor for Computational ... - Napier University

Chapter 3. Immune Systems for Scheduling 76¢ It is dependent on a uniform distribution of peaks in the search spaceIt requires a comparison of every population member to every other population¢member in each generation, i.e. N 2 comparisons for a population of size N,therefore is time-consuming.For these reasons, the approach did not seem suitable to the task of discoveringschedule building blocks; we do not know how many blocks (i.e. niches) are required,and they are unlikely to be evenly distributed across the search space. Therefore thisapproach was rejected for inclusion in SCHED2 IS.3.10.1.2 ‘Pitt Approach’In this approach to classifier systems, described in [DeJong, 9898], a population ofrules is concatenated into a single individual which is then manipulated by a GA.This allows diversity to be maintained within each rule set but is inherently inefficientas it manipulates whole rule-sets, rather than populations of rules. Furthermore,this method also requires that the number of rules (or building blocks in the case ofSCHED2 IS) is pre-judged in order to form an individual chromosome, although areasonable ’guess’ will suffice. Therefore, it was also rejected as the engine for discoveringbuilding-blocks in SCHED2 IS.3.10.1.3 An Immune System Model[Smith et al., 1993] proposed a theoretical model of an immune system which can beused to evolve a set of antibodies that recognise a range of diverse, binary antigenstrings. This work (verified experimentally in [Forrest et al., 1993]) showed that an immunesystem model could both detect common patterns (schemas in the binary case) ina noisy environment and also maintain diversity in that many types of antibody evolvedin niches, each niche responsible for recognising a particular antigen. Its success lies inthe novel fitness scheme introduced, referred to as the diversity algorithm. (This workhas previously been described in detail in chapter 2.) This model has three appealingfeatures as far as our scheduling system is concerned.