# Genetic algorithms, principles and perspectives: a guide to GA theory

Genetic Algorithms - Principles and Perspectives - A Guide to GA Theory

enter A comparison of selection methods for nine sample data sets. The fitness measured relative the average performance on that data set is plotted against the generation the fitness was achieved again relative to the average. The large symbols are the centroids of the nine data sets for each method. It is natural to think that tournaments that are larger still would yield even better results. This however can lead to problems because if the selection pressure is too high then only the fittest individuals can survive so, the diversity of the population is wiped out and the gene pool stagnates.

When this happens, we get premature convergence of the GA and since our main interest is in accuracy rather than speed we choose tournaments of 10 individuals for selection. Another key parameter in any GA is the mutation rate probability, p m. This controls the ability of a population to generate new solutions. If p m is too low then the performance of the GA will depend too strongly on the original population because new solutions will be harder to come by. This will often result premature convergence as a result of gene pool stagnation. On the other hand, if p m is too high then we lose a lot of the power of the evolutionary approach.

Mutation will destroy good features as well as bad so it will be more difficult for offspring to inherit the good traits of their parents. Both of these extremes can be seen in Fig. In our case, since the size of the search space varies enormously with the number of terms, the optimum p m will vary slightly depending on the size of the polynomial. We also see that lower p m performs better on the larger search spaces of polynomials with more terms. This is expected and is the motivation for using a GA to explore this space rather than simply using a random search.

Fitness achieved after generations relative to worst fitness achieve and averaged over runs of the GA for different values of the mutation probability, p m. The band represents one standard error from the averaging and the results are shown for polynomials with 5, 10, 15, and 20 terms. Yet another tunable parameter of a GA is the recombination probability, p r. This is the probability that two parents, once selected, will breed to produce offspring that proceed to the next generation. If they do not breed a clone of the parent is produced with the possibility of mutation to carry forward to the next generation.

The result of varying p r is shown in Fig. We can see that at least a small amount of breeding is important but that there is not much benefit in having very large p r. It is in fact less desirable to have p r too large because this generates many more distinct polynomials that must be fit to the data which is very time consuming so it is more sensible to choose a lower value of p r and run the GA longer.

Fitness achieved after generations for different values of the recombination probability, p m colour scheme same as Fig. Finally, the size of the population needs to be chosen. As one would expect, the result will improve with a larger population but very large populations take more time to evolve and do not make efficient use of the GA's strength which is to evolve a starting population towards better solutions. If the population is too small however then the performance will be limited by the genes of initial population.

For the final result, we run for many more generations than in testing and run the GA several times in order to remove dependence on the initial population. The remaining objects are randomly split into two sets of each for cross-validation and testing we will explain these terms below. Cross-validation is an imperative step in any machine learning algorithm that helps fight against overfitting. If an algorithm is allowed to fit very complicated functions to the training data then it is common that the learned model will not generalize well to data outside the training set.

In order to get a handle on this, we use a separate data set for cross-validation. The parameters of the model are not trained on the cross-validation data so the performance of the model on the this set gives an indication how well the model generalises. The model can then be adjusted in order to yield the best performance on the cross-validation set see Fig.

Note however that by selecting models based on the cross-validation set, we have learned from the data so we need yet another data set as a final test that can check the performance of the final model with optimized parameters. Schematic demonstration of cross-validation procedure.

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As the model complexity is increased beyond the optimum it begins to fit noise in the training set and performs worse on cross-validation step. The model is chosen in order to minimize validation error.

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We can also use this technique to choose the degree of the polynomial, N. In this instance however the dimension of the search increases dramatically with increasing N so it can be harder to find good solutions and the training error will not necessarily decrease unless the GA is evolved for many more generations. We are therefore searching for the optimal choice of 20 terms from a possible terms. The error is minimized in the redshift range where most of the training data lies.

The error grows for larger redshifts and this is reflected in the larger rms error for galaxies in this region. Photometric redshift prediction and error bars for a representative subsample of galaxies. The error bars are due to errors in the photometric data and so depend on the particular model chosen for z phot. Distribution of photometric error obtained by repeatedly selecting training and test sets at random.

Results shown for repetitions and the best fit Gaussian distribution is included from comparison. In order to further test the performance of the method, we have applied the GA with all hyperparameters fixed to other data sets with different redshift ranges. The result is shown in Fig. This is synthetic data based on simulations to which some noise is added. We therefore train a GA with 11 variables to reproduce the redshift. The hand added noise in this data set is much lower than that of real data so it is possible to include many more terms in the polynomial before overfitting.

The result of a fit with terms is shown in Fig. In both cases, the GA has performed well demonstrating that the method generalizes well to new data sets with larger redshift ranges. In this section, we briefly mention the performance of the code and how it scales with training set size. In Fig. The solid lines represent linear scaling with training set size.

If even faster performance is required then it would be possible to make use of the readily parallelizable nature of GAs. Performance analysis for runs of GA with generations on the DR11 training set with different numbers of training examples and different numbers of terms. The solid lines shows linear scaling with training set size. In this paper, we have demonstrated the use of a GA to explore the space of polynomial models for photometric redshift estimation.

This result compares favourably with current methods in the literature. The success of the model is likely due to the small number of degrees of freedom in the final model so overfitting can be avoided while still maintaining the flexibility to adapt well to the training set using the GA. The resulting best-fitting polynomial has been presented and can be easily used to generate photometric redshift catalogues without the need to run any additional code.

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GAz: a genetic algorithm for photometric redshift estimation Robert Hogan. Oxford Academic. Google Scholar. Malcolm Fairbairn. Navin Seeburn. Cite Citation. For instance, in the knapsack problem one wants to maximize the total value of objects that can be put in a knapsack of some fixed capacity. A representation of a solution might be an array of bits, where each bit represents a different object, and the value of the bit 0 or 1 represents whether or not the object is in the knapsack. Not every such representation is valid, as the size of objects may exceed the capacity of the knapsack.

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The fitness of the solution is the sum of values of all objects in the knapsack if the representation is valid, or 0 otherwise. In some problems, it is hard or even impossible to define the fitness expression; in these cases, a simulation may be used to determine the fitness function value of a phenotype e.

The next step is to generate a second generation population of solutions from those selected through a combination of genetic operators : crossover also called recombination , and mutation. For each new solution to be produced, a pair of "parent" solutions is selected for breeding from the pool selected previously. By producing a "child" solution using the above methods of crossover and mutation, a new solution is created which typically shares many of the characteristics of its "parents".

New parents are selected for each new child, and the process continues until a new population of solutions of appropriate size is generated. Although reproduction methods that are based on the use of two parents are more "biology inspired", some research [4] [5] suggests that more than two "parents" generate higher quality chromosomes. These processes ultimately result in the next generation population of chromosomes that is different from the initial generation.

Generally the average fitness will have increased by this procedure for the population, since only the best organisms from the first generation are selected for breeding, along with a small proportion of less fit solutions. These less fit solutions ensure genetic diversity within the genetic pool of the parents and therefore ensure the genetic diversity of the subsequent generation of children.

Opinion is divided over the importance of crossover versus mutation. There are many references in Fogel that support the importance of mutation-based search. Although crossover and mutation are known as the main genetic operators, it is possible to use other operators such as regrouping, colonization-extinction, or migration in genetic algorithms. It is worth tuning parameters such as the mutation probability, crossover probability and population size to find reasonable settings for the problem class being worked on. A very small mutation rate may lead to genetic drift which is non- ergodic in nature.

A recombination rate that is too high may lead to premature convergence of the genetic algorithm. A mutation rate that is too high may lead to loss of good solutions, unless elitist selection is employed.

## Genetic Algorithms: Principles and Perspectives - A Guide to GA Theory | Colin Reeves | Springer

In addition to the main operators above, other heuristics may be employed to make the calculation faster or more robust. The speciation heuristic penalizes crossover between candidate solutions that are too similar; this encourages population diversity and helps prevent premature convergence to a less optimal solution. This generational process is repeated until a termination condition has been reached. Common terminating conditions are:.

Genetic algorithms are simple to implement, but their behavior is difficult to understand. In particular it is difficult to understand why these algorithms frequently succeed at generating solutions of high fitness when applied to practical problems. The building block hypothesis BBH consists of:. Despite the lack of consensus regarding the validity of the building-block hypothesis, it has been consistently evaluated and used as reference throughout the years.

Many estimation of distribution algorithms , for example, have been proposed in an attempt to provide an environment in which the hypothesis would hold. Indeed, there is a reasonable amount of work that attempts to understand its limitations from the perspective of estimation of distribution algorithms. There are limitations of the use of a genetic algorithm compared to alternative optimization algorithms:.

The simplest algorithm represents each chromosome as a bit string. Typically, numeric parameters can be represented by integers , though it is possible to use floating point representations. The floating point representation is natural to evolution strategies and evolutionary programming. The notion of real-valued genetic algorithms has been offered but is really a misnomer because it does not really represent the building block theory that was proposed by John Henry Holland in the s.

This theory is not without support though, based on theoretical and experimental results see below. The basic algorithm performs crossover and mutation at the bit level. Other variants treat the chromosome as a list of numbers which are indexes into an instruction table, nodes in a linked list , hashes , objects , or any other imaginable data structure. Crossover and mutation are performed so as to respect data element boundaries. For most data types, specific variation operators can be designed. Different chromosomal data types seem to work better or worse for different specific problem domains.

When bit-string representations of integers are used, Gray coding is often employed. In this way, small changes in the integer can be readily affected through mutations or crossovers. This has been found to help prevent premature convergence at so-called Hamming walls , in which too many simultaneous mutations or crossover events must occur in order to change the chromosome to a better solution.

Other approaches involve using arrays of real-valued numbers instead of bit strings to represent chromosomes. Results from the theory of schemata suggest that in general the smaller the alphabet, the better the performance, but it was initially surprising to researchers that good results were obtained from using real-valued chromosomes. This was explained as the set of real values in a finite population of chromosomes as forming a virtual alphabet when selection and recombination are dominant with a much lower cardinality than would be expected from a floating point representation.

An expansion of the Genetic Algorithm accessible problem domain can be obtained through more complex encoding of the solution pools by concatenating several types of heterogenously encoded genes into one chromosome. For instance, in problems of cascaded controller tuning, the internal loop controller structure can belong to a conventional regulator of three parameters, whereas the external loop could implement a linguistic controller such as a fuzzy system which has an inherently different description.

This particular form of encoding requires a specialized crossover mechanism that recombines the chromosome by section, and it is a useful tool for the modelling and simulation of complex adaptive systems, especially evolution processes. A practical variant of the general process of constructing a new population is to allow the best organism s from the current generation to carry over to the next, unaltered.

This strategy is known as elitist selection and guarantees that the solution quality obtained by the GA will not decrease from one generation to the next. Parallel implementations of genetic algorithms come in two flavors. Coarse-grained parallel genetic algorithms assume a population on each of the computer nodes and migration of individuals among the nodes.

Fine-grained parallel genetic algorithms assume an individual on each processor node which acts with neighboring individuals for selection and reproduction. Other variants, like genetic algorithms for online optimization problems, introduce time-dependence or noise in the fitness function. Genetic algorithms with adaptive parameters adaptive genetic algorithms, AGAs is another significant and promising variant of genetic algorithms. The probabilities of crossover pc and mutation pm greatly determine the degree of solution accuracy and the convergence speed that genetic algorithms can obtain.

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Instead of using fixed values of pc and pm , AGAs utilize the population information in each generation and adaptively adjust the pc and pm in order to maintain the population diversity as well as to sustain the convergence capacity. In AGA adaptive genetic algorithm , [21] the adjustment of pc and pm depends on the fitness values of the solutions. In CAGA clustering-based adaptive genetic algorithm , [22] through the use of clustering analysis to judge the optimization states of the population, the adjustment of pc and pm depends on these optimization states.

It can be quite effective to combine GA with other optimization methods. GA tends to be quite good at finding generally good global solutions, but quite inefficient at finding the last few mutations to find the absolute optimum. Other techniques such as simple hill climbing are quite efficient at finding absolute optimum in a limited region. Alternating GA and hill climbing can improve the efficiency of GA [ citation needed ] while overcoming the lack of robustness of hill climbing. This means that the rules of genetic variation may have a different meaning in the natural case.

For instance — provided that steps are stored in consecutive order — crossing over may sum a number of steps from maternal DNA adding a number of steps from paternal DNA and so on. This is like adding vectors that more probably may follow a ridge in the phenotypic landscape. Thus, the efficiency of the process may be increased by many orders of magnitude. Moreover, the inversion operator has the opportunity to place steps in consecutive order or any other suitable order in favour of survival or efficiency.

A variation, where the population as a whole is evolved rather than its individual members, is known as gene pool recombination. A number of variations have been developed to attempt to improve performance of GAs on problems with a high degree of fitness epistasis, i. Such algorithms aim to learn before exploiting these beneficial phenotypic interactions.

As such, they are aligned with the Building Block Hypothesis in adaptively reducing disruptive recombination. Problems which appear to be particularly appropriate for solution by genetic algorithms include timetabling and scheduling problems, and many scheduling software packages are based on GAs [ citation needed ]. GAs have also been applied to engineering. As a general rule of thumb genetic algorithms might be useful in problem domains that have a complex fitness landscape as mixing, i.

Observe that commonly used crossover operators cannot change any uniform population. Mutation alone can provide ergodicity of the overall genetic algorithm process seen as a Markov chain. Examples of problems solved by genetic algorithms include: mirrors designed to funnel sunlight to a solar collector, [28] antennae designed to pick up radio signals in space, [29] walking methods for computer figures, [30] optimal design of aerodynamic bodies in complex flowfields [31].

In his Algorithm Design Manual , Skiena advises against genetic algorithms for any task:.

### A Guide to GA Theory

The pseudobiology adds another level of complexity between you and your problem. Second, genetic algorithms take a very long time on nontrivial problems. I have never encountered any problem where genetic algorithms seemed to me the right way to attack it. Further, I have never seen any computational results reported using genetic algorithms that have favorably impressed me.

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Genetic Algorithms: Principles and Perspectives: A Guide to GA Theory is a survey of some important theoretical contributions, many of which have been. Genetic Algorithms: Principles and Perspectives: A Guide to Ga Theory YamiPred: A Novel Evolutionary Method for Predicting Pre-miRNAs and Selecting.

Stick to simulated annealing for your heuristic search voodoo needs. In , Alan Turing proposed a "learning machine" which would parallel the principles of evolution. Starting in , [36] the Australian quantitative geneticist Alex Fraser published a series of papers on simulation of artificial selection of organisms with multiple loci controlling a measurable trait.

From these beginnings, computer simulation of evolution by biologists became more common in the early s, and the methods were described in books by Fraser and Burnell [37] and Crosby In addition, Hans-Joachim Bremermann published a series of papers in the s that also adopted a population of solution to optimization problems, undergoing recombination, mutation, and selection. Bremermann's research also included the elements of modern genetic algorithms.

Many early papers are reprinted by Fogel Although Barricelli, in work he reported in , had simulated the evolution of ability to play a simple game, [41] artificial evolution became a widely recognized optimization method as a result of the work of Ingo Rechenberg and Hans-Paul Schwefel in the s and early s — Rechenberg's group was able to solve complex engineering problems through evolution strategies. Fogel , which was proposed for generating artificial intelligence. Evolutionary programming originally used finite state machines for predicting environments, and used variation and selection to optimize the predictive logics.

Genetic algorithms in particular became popular through the work of John Holland in the early s, and particularly his book Adaptation in Natural and Artificial Systems His work originated with studies of cellular automata , conducted by Holland and his students at the University of Michigan. Holland introduced a formalized framework for predicting the quality of the next generation, known as Holland's Schema Theorem.

In the late s, General Electric started selling the world's first genetic algorithm product, a mainframe-based toolkit designed for industrial processes. The New York Times technology writer John Markoff wrote [47] about Evolver in , and it remained the only interactive commercial genetic algorithm until Evolutionary algorithms is a sub-field of evolutionary computing.

Swarm intelligence is a sub-field of evolutionary computing. Evolutionary computation is a sub-field of the metaheuristic methods. Metaheuristic methods broadly fall within stochastic optimisation methods.

## Genetic Algorithms - Quick Guide

From Wikipedia, the free encyclopedia. Part of a series on the Evolutionary algorithm Artificial development Cellular evolutionary algorithm Cultural algorithm Differential evolution Effective fitness Evolution strategy Gaussian adaptation Evolutionary multimodal optimization Grammatical evolution Particle swarm optimization Memetic algorithm Natural evolution strategy Promoter based genetic algorithm Spiral optimization algorithm Genetic algorithm Chromosome Clonal selection algorithm Crossover Mutation Genetic memory Genetic fuzzy systems Selection Fly algorithm Genetic programming Cartesian genetic programming Linear genetic programming Multi expression programming Schema Eurisko Parity benchmark v t e.

Main article: Selection genetic algorithm.

Introduction To Genetic Algorithm Explained in Hindi

Main articles: Crossover genetic algorithm and Mutation genetic algorithm. Main article: genetic representation. See also: List of genetic algorithm applications. This section needs additional citations for verification.