Genetic Algorithm

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This is an explanation of what is genetic algorithm and the use of it.

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  • Two parentsAttract matesOther hunting foodChild both characteristicsSuperfit
  • There should not be too many local maximasThere shouldn’t be a single global maximumTherefore better chromosomes must be closely related.Problem: Preparing a time table.Invalid chromosomes must guide towards valid chromosomes, using fitness valueBetter to invest sub-goals rewarded going for the ultimate goalPremature convergence: Set of highly fit individuals come to dominate the population, making a local maxima. Which in turn make it difficult to converge to more effective solutionSlow finishing: Population might have converged, but might not have found a global maxima
  • Passing the genes without disruption of crossoverProbability 0.001
  • Better individuals -> more of their genes to next generationGood schemata - Likelihood of better solution increase
  • Power of GA – Finding good building blocksInteraction – In a chromosome, contribution of a gene to the fitness value depends on values of other genesIssues: Might not allow closely related genes to be placed togetherInteraction between genes can become high (specially in multi-dimensional data)
  • AssumptionsPopulation is infiniteFitness function accurately shows the effectiveness of a solutionGenes in a chromosome don’t interact significantlyGenetic drift, mutation rate
  • Genetic Algorithm

    1. 1. An overview of Genetic AlgorithmBy David Beasley, David R. Bull and Ralph R. Martin 090070T – T.P.K. Dahanayakage 090150N – K.M.T.V. Ganegedara
    2. 2. Introduction Population Evolves  Natural Selection  Survival of the fittest Applications?  Computer, Bridges, Garment, etc.
    3. 3. Analogy Think about successive generations
    4. 4. Analogy (ctd) Super-fit Evolve according to environment
    5. 5. Basic Concept Set of solutions for a problem  Each solution – fitness score Reproduce a new set of solutions by “Cross-breeding”  Most-fit: get selected  Least-fit: not selected – die out Result?  Offsprings with characteristics from most- fit
    6. 6. What just happened? Good characteristics of a generation was spread in a successive generation Most promising areas of solution space are searched
    7. 7. AlgorithmBEGIN Generate population Calculate fitness for each individual WHILE NOT CONVERGED DO BEGIN FOR population_size/2 DO BEGIN Select 2 parents for mating Combine and produce an offspring Calculate the fitness for the new individual Insert the offspring to the new generation END ENDEND
    8. 8. Lesson on Biology Chromosome  Organized collection of coiled DNA DNA
    9. 9. Fitness function Must represent the “fitness to the environment” or “ability” of a chromosome’s Issues of fitness range  Premature convergence  Slow finishing
    10. 10. Reproduction Selection of parents  Random  Favors the fittest Crossover  Single point crossover  Cut 2 chromosomes at a random point  Swap over tails to create 2 new chromosomes
    11. 11. Reproduction (ctd) Crossover is not the only case!  0.6 - 1.0 chance  Otherwise replicate the parent Mutation  Alter the genes of crossover-ed with a small probability
    12. 12. Example 0101001100 1011001001 0101001001 1011001100 Before mutation: 0 1 0 1 0 0 1 0 0 1 After mutation: 0101101001
    13. 13. Convergence Fitness of the BEST and AVERAGE moves to a global optimum Gene is said to have converged  95% of the population has converged Population is said to have converged  All the genes have converged
    14. 14. Other techniques
    15. 15. “Schemata” and “Scheme” Definition of Schema  Pattern of gene  String comprise {0,1,#} Ex: Chromosome 0110 contains following “Schemata”  #110, #1#0, 01##, etc. A chromosome is said to contain a schema if it matches a particular schemata
    16. 16.  Order of schema – Number of non-# symbols Length of schema – Distance between outer most non-# symbols. Ex: #1#0
    17. 17. Schema Theorem Individuals in a population are given reproductive trials Number of trials α Fitness of an individual Higher fitness value -> Good schemata Good Schemata receives exponentially increasing number of trials in successive generations!
    18. 18. Building Block Hypothesis Definition  Schemata short in length and tend to improve performance when incorporated to an individual Properties of a successful coding scheme  Related genes close together  Little interaction between genes
    19. 19. Exploration and Exploitation Exploration  Exploring unknown areas Exploitation  Utilizing already-learnt to find better solutions Tradeoff  Ex: Random search and Hill climbing GA combines both in an optimal way!
    20. 20. Practical Aspects of GA
    21. 21. Parent selection Individuals are copied to a “mating pool”  Highly fit – more copies  Less fit – lesser copies How to determine number of copies?  Explicit fitness remapping  Implicit fitness remapping
    22. 22. Explicit fitness remapping Individual’s fitness Average fitness of population Issue: Number of copies should be an integer Solution:  Fitness scaling  Fitness windowing
    23. 23. Implicit fitness remapping Tournament selection  2 random individuals  Copy the one with higher fitness value to the mating pool  Continue until the pool is full
    24. 24. Generation gaps andsteady-state replacement Generation gap  Proportion of individuals in a population replaced in each generation Steady-state replacement  Only few individuals are replaced in a generation  Considerations:  Parent selection – Random, Fitness  Replacement – Random, Inverse fitness
    25. 25. Thank you Q & A Session

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