One of the most controversial yet enduring hypotheses about what genetic algorithms (GAs) are good for concerns the idea that GAs process building-blocks. More specifically, it has been suggested that crossover in GAs can assemble short low-order schemata of above average fitness (building blocks) to create higher-order higher-fitness schemata. However, there has been considerable difficulty in demonstrating this rigorously and intuitively. Here we provide a simple building-block function that a GA with twopoint crossover can solve on average in polynomial time, whereas an asexual population or mutation hill-climber cannot.
Categories and Subject Descriptors
General TermsAlgorithms, Performance, Theory.
KeywordsMutation, crossover, modularity, building block hypothesis, genetic algorithms theory, royal roads. , incorporates a wide fitness valley that must be overcome by a crossover event and all other fitness improvements are provided by mutation. These functions seem quite contrived: There is no intuitive explanation for why it should be the case that the positions of the global optimum and local optima should be just as they are (such that the global optimum is located at a genotype produced by crossover of the locally optimal genotypes). Although such a function does satisfy the essential property of distinguishing polynomial versus exponential expected time complexities of crossover and mutation-based algorithms respectively, their contrived structure makes it difficult to see what they have to offer with respect to our understanding of what GAs are good for in general.
WHAT A GA IS GOOD FORSome of the other functions that show a principled distinction in the ability of crossover [26][27] do use a building-block structure but are too complex to facilitate rigorous proofs for their time complexities [13]. In particular, the HIFF function [27] illustrates building-block structure directly inspired by the BBH but the ability of a GA to outperform mutation-based algorithms on this function is dependent on its multi-level building-block structure, and indeed the requirement for a true GA rather than a 'crossover hill-climber ' [4][27] requires additional modifications to the function (specifically, the use of non-complementary global optima [27]). As yet, no simple single-level building-block function has been provided where crossover is provably essential.In this paper we return to a simple separable building-block function to see if we can rescue some intuition and yet maintain rigorous proofs that distinguish polynomial and non-polynomial time complexities for a GA with and without crossover respectively. In this paper we address only the original conception of a building-block that includes the assumption of tight linkage [6][8][10] [11][17] and ordinary one-or two-point crossover that could potentially exploit this structure as described in the BBH. Note that a large body of work on linkage learning [9] and modelbuilding methods, e.g. [19], has developed in the GA community that addresses the exploi...