EE4047_Part7 - Part 7: Problems and Difficulties City...

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City University of Hong Kong Part 7: Problems and Difficulties Problem 1: Premature Convergence and Genetic Drift ± Stochastic errors in sampling caused by small population sizes ± Genetic Drift : Population converges on a single peak without differential advantage ± Premature Convergence : Loss of population diversity before optimal value has been found ± Reason: Loss of niche and species
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Solution: Niche Maintenance ± Preselection : offspring replaces the inferior parent if its fitness is better than that of the inferior parent ± Diversity is maintained because strings tended to replace strings similar to themselves ± Crowding : individual is compared to a random subpopulation of crowding factor members. The individual with the highest similarity (bit-by-bit similarity) is replaced by the new string Niche Maintenance ± Fitness Sharing : a function defined to determine the neighborhood and degree of sharing for each string in the population. ± Triangular sharing function ± ||•|| can be hamming difference or Euclidean distance. ± An individual’s derated fitness is computed as: = else 0 if 1 ) ( share ij share ij ij d d d s σ where j i ij x x d = = = N j ij i i s d s x f x f 1 ) ( ) ( ) ( where N is population size 1 ij d share ) ( ij d s
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Niche Maintenance ± Mating restriction : Close individuals are mated with one another as long as the family average fitness continued to rise (works with preselection); If not, crossbreeding between different families was tried. (Encourage speciation and reduce the production of lethals [a child of parents from two different niches].)
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EE4047_Part7 - Part 7: Problems and Difficulties City...

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