Immigrants do all the work

Genetic Agorithms with Memory- and Elitism-based Immigrants in Dynamic Environments was the paper selected for presentation and discussion in last Friday’s seminar. The article, authored by Shengxiang Yang, was published in the last Fall edition of Evolutionary Computation and addresses evolutionary optimization in dynamic environments.

Yang proposes two Genetic Algorithms (GAs) for dynamic optimization based on Grefenstette’s classical Random Immigrants GA (RIGA). RIGA tackles (or tries to) changing optima by inserting, in each and every generation, a certain number of randomly generated individuals that replace the worst individuals in the population (or randomly selected individuals, in another version). This way, genetic novelty is constantly being introduced and the population is expected to have enough diversity to react to changes in the environments. However, RIGA suffers from a major “weakness”: the raw building-blocks introduced by the randomly generated individuals are quickly removed from the population because their fitness is usually below average. RIGA is frequently chosen as a peer-algorithm for comparison purposes in evolutionary dynamic optimization studies, but, due to this “weak spot”, it may be questioned if a Standard GA is not better suited to assess the efficiency of a new method (in fact, studies currently being developed in our lab reinforce this hypothesis). In order to improve RIGA’s performance, several alternative RIGA-based methods have been proposed in the past few years.

The two GAs described in Yang’s paper try to overcome the problem with random immigrants by inserting in the population mutated copies of the elite — Elitism-based Immigrants Genetic Algorithm (EIGA) — or mutated copies of the chromosomes kept in a memory — Memory-based Immigrants Genetic Algorithm (MIGA). Memory-based approaches for dynamic optimization use a memory to store good solutions, which are retrieved later, periodically, or when the environments changes. Memory GAs are known to improve traditional GAs performance when the dynamics of changes are cyclic, that is, the fitness function returns to previous “shapes” from time to time; on the other hand, memory schemes are not that effective when the changes are not periodic. Therefore, and as expected, MIGA outperforms other GAs when the changes are cyclic. EIGA is better when the changes in the environment are not severe. This behaviour is explained by the fact that introducing mutated copies of the best individual in the population provides the GA with means to tackle small changes because the algorithm is maintaining a kind of sub-population around the optimal solution, and small shifts in the environment are easily traceable by those near-optimal individuals.

Summarizing, the study shows that MIGA and EIGA are able to outperform other GAs under the conditions of the test set. However, there is one question that remains unanswered: what happens when changing the parameter values? For instance, diversity maintenance schemes for dynamic optimization deal with non-stationary environments by maintaining the diversity at a higher level. This means that maybe the optimal mutation probability of these algorithms is different from those of Standard GAs. Shouldn’t a proper comparison between the algorithms consider a range of possible mutation probabilities (Yang’s studies used the traditional pm = 1/l, where l is the chromosome length)? And what about population size? Isn’t population size the major bottleneck for GAs’ performance in stationary environments? Is it possible that a variation in the population size of the GA for dynamic optimization conduces the research to different conclusions? Studies currently under way will try to answer to some of these questions.

This entry was posted in Dynamic Optimization Problems, EA, Friday_Paper_Seminar by cfernandes81. Bookmark the permalink.

About cfernandes81

Carlos M. Fernandes was born in Luanda in 1973 and lives in between Lisbon, Portugal, and Granada, Spain. He graduated (Technical University of Lisbon, 1998) in Electrotechnics Engineering and owns a master degree in the same field since 2002 (Technical University of Lisbon). He is currently pursuing a Ph.d. on Bio-inspired Computing. From 2001 to 2005 he was an assistant at Instituto Politécnico de Setúbal. (He is also a photographer and photography teacher.) Bio-inspired Computing is his major field of research: Genetic Algorithms, Estimation of Distribution Algorithms, Ant Colony Optimization, Particle Swarm Optimization and other metaheuristics. He is particularly interested in the hybridization of Bio-inspired Computing techniques with Self-Organization, Self-Organized Criticality Models and diversity maintenance strategies. In the present, Dynamic Optimization Problems are his mains target for applying such techniques. website: www.carlosmfernandes.com email: c.m.fernandes.photo@gmail.com

2 thoughts on “Immigrants do all the work

  1. I don’t think mutation rates bigger than 1/l would help. After all, it’s something you can achieve with several steps, without the benefit of checking if the intermediate solutions are right. Maybe it would be better to increase the number of new solutions generated by mutation, instead of the mutation rate.
    And good luck with the studies under way

  2. Mutation rates do not necessarily need to be bigger than 1/l. Sometimes less mutation is better, especially when there is already another mechanism that guarantees genetic diversity (like random immigrants).
    According to my experience, 1/l is usually the optimal mutation probability for Standard GAs for dynamic optimization. But other methods may require more or less mutation in order to attain an optimal performance (but never more than 2/l).

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s