C3.7 Multi-Parent Recombination
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C Multiparent Recombination AE Eiben Leiden University Abstract In this section we survey recombination op erators that can utilize more than two parents to create ospring Some multiparent recombination op erators are dened for a xed number of parents eg havearity three while in some op erators the number of parents is a random number that might b e greater than two and in yet other op erators the arity is a parameter that can b e set to an arbitrary integer We pay sp ecial attention to this latter typ e of op erators and summarize results on the eect of op erator arityonevolutionary algorithm p erformance C Intro duction To make the coming survey unambiguous we have to start with setting some conventions on terminology The term population will b e used for a multiset of individuals that undergo es selection and repro duction This terminology is maintained in genetic algorithms ev olutionary programming and genetic programming but in evolution strategies all individuals in a or strategy are called parents We however use the term parents only for those individuals that are selected to undergo recombination In other words parents are those individuals that are actually used as inputs for a recombination op erator the arity of a recombination operator is the numb er of parents it uses The next notion is that of a donor b eing a parent that actually contributes to at least one of the alleles of the children created by the recombination op erator This contribution can be for instance the delivery of an allele as in uniform crossover in canonical GAs or the participation in an averaging op eration as in intermediate recombination in ES As an illustration consider a steadystate GA where individuals form the p opulation and two of them are chosen as parents to undergo uniform crossover to create one single ospring If bypurechance the ospring only inherits alleles from parent then parent is a donor and parent is not C Miscellaneous op erators We b egin this survey with pap ers where the multiparent asp ect has an incidental character By an incidental character we mean that the op erator is dened and used in a sp ecic application and has for instance a certain xed arityoreven if the denition is general and would allow comparison between dierentnumb er of parents this asp ect is not given attention The recombination mechanism of Kaufman is applied for evolving mo dels for agiven pro cess where a mo del is an arrayofa numberofblocks and mo dels may dier in the numbers of blo cks they contain Recombination of four mo dels to create one new mo del is dened as follows The size of the child the numb er of blo cks equals the size of eachofitsparents with probability The ith blo ck of the child is chosen with equal probability from those parents that have there is an exception of this latter rule of cho osing one of the at least i blo cks Let us note that parents blo cks but that exception has a very problemsp ecic reason therefore we rather present the general idea here In an extensive study on bit vector function optimization sto chastic iterated genetic hill climbing SIGH is studied and compared with other techniques such as GAs iterated hillclimbing and simulated annealing Ackley SIGH applies a sophisticated probabilistic voting mechanism c IOP Publishing Ltd and Oxford UniversityPress Handbook of Evolutionary Computation C with timedep endent probability distributions co oling where m voters m b eing the size of the p opulation determine the values of a new bitstring SIGH is shown to b e b etter than a GA with p oint and uniform crossover on four out of the six test functions and the overall conclusion is that it is comp etitive in sp eed with a variety of existing algorithms In the intro ductory pap er on the parallel genetic algorithm ASPARAGOS Muhlen b ein psexual voting recombination is applied for the quadratic assignment problem Let us remark that the name psexual is somewhat misleading as there are no dierent genders and no restriction on having one representative of each gender for recombination The voting recombination pro duces one child of p parents based on a threshold value v It determines the ith allele of the child by comparing the ith alleles of the selected parent individuals If the same allele is found more often than the threshold v this allele is included in the child other bits are lled in randomly In the exp eriments the values p and v are used and it worked surprisingly well but comparison between this scheme and usual twoparent recombination was not p erformed An interesting attempt to combine genetic algorithms with the Simplex Metho d resulted in the ternary simplex crossover Bersini and Seront If x x x are the three parents sorted in decreasing order of tness then the simplex crossover generates one child x by the following two rules i If x x then x x i i i i ii if x x then x x with probability p and x x with probability p i i i i i i Using the value p the simplex GA p erformed b etter than the standard GA on the DeJong functions The authors remark that applying a mo died crossover on more than three parents is worth to try The problem of placing actuators on space structures is addressed byFuruya and Haftka The authors compare dierent crossovers among others they use uniform crossover with twoaswell as with three parents in a GA using integer representation Based on the exp erimental results they conclude that the use of three parents did not improve the p erformance This might b e related to another conclusion indicating that for this problem mutation is an ecient op erator and crossover might not be imp ortant Uniform crossover with an arbitrary number of parents is also used by Aizawa as part of a sp ecial schema sampling pro cedure in a GA but the multiparent feature is only a sideeect and is not investigated y Pal for a multimo dal spin A socalled triadic crossover is intro duced and tested b lattice problem The triadic crossover is dened in terms of two parents and one extra individual chosen randomly from the p opulation The op erator creates one child it takes the bits in p ositions where the rst parent and the third individual have identical bits from this parent and the rest of the bits from the other parent Clearly the result is identical to the outcome of a voting crossover on these three individuals as parents Although the pap er is primarily concerned with dierent selection schemes a comparison between triadic p oint and uniform crossover is made where triadic crossover turned out to deliver the b est results C Op erators with undened arity In the intro duction to this section we dened the arity of a recombination op erator as the number drawings the parents it uses In some cases this number dep ends on the outcomes of random op erator is called without knowing in advance how manyparents would b e applied In this section we treat three mechanisms of this kind Global recombination in evolution strategies allows the use of more than tworecombinants Back Schwefel In ES there are two basic typ es of recombination intermediate and discrete recombination b oth having a standard twoparentvariant and a global variant Given a p opulation of individuals global recombination creates one ospring x by the following mechanism S T i i x or x global discrete recombination i i x i S T S i i i combination x x x global intermediate re i i i i S T i i where the two parents x x S T fg are redrawn for each i anew and so is the i i contraction factor The ab ove denition applies to the ob ject variables as well as the strategy i parameter part ie for the mutation stepsizes s and the rotation angles s Observe that c C Handbook of Evolutionary Computation IOP Publishing Ltd and Oxford University Press the multiparent character of global recombination is the consequence of redrawing the parents S T i i x x for each co ordinate i Therefore probably more than two individuals contribute to the ospring x but their number is not dened in advance It is clear that investigations on the eects of dierent numb ers of parents on algorithm p erformance could not be p erformed in the traditional ES framework The option of using multiple parents can be turned on or o that is global recombination can be used or not but the arity of the recombination op erator is not tunable Exp erimental studies on global versus twoparent recombination are p ossible but so far there are almost no exp erimental results available on this sub ject In Schwefel it is noted that appreciable acceleration is obtained by changing to bisexual from asexual scheme ie adding recombination using two parents to the mutationonly algorithm but only slight further increase is obtained when changing from bisexual to multisexual recombination ie using global recombination instead of the twoparentvariant Recall the remark on the name psexual voting The terms bisexual and multisexual are not appropriate either for the same reason individuals have no gender or sex and recombination can be applied to any combination of individuals Genepool recombination GPR was intro duced byMuhlenbein and Voigt as a multi parent recombination mechanism for discrete domains It is dened as a generalization of twoparent recombination TPR Applying GPR is preceded by selecting a genep o ol consisting of wouldb e parents Applying GPR the twoparent alleles of an ospring are randomly chosen for each lo cus with replacement from the genep o ol and the ospring allele is computed using any of the standard recombination schemes for TPR Theoretical