Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
24-03-2014 Multimodal optimization: speciation E. Eiben and J. E. Smith, Introduction to Evolutionary Computing, Springer, 2003 – chapter 5 Campbell, et al. 2008. Biology, 8th ed. Pearson Benjamin Cummings. Motivating examples 2 1 24-03-2014 Classification Organisms are classified into a hierarchical grouping of closely relates organisms and that progressively includes more and more organisms. Species The species is the basic biological unit around which classifications are based. However, what constitutes a species can be difficult to define and there are multiple definitions of species in use today. 2 24-03-2014 Early definition attempts (John Ray, 1627-1705) A group of organisms that can interbreed and produce viable, fertile offspring Criticisms of the Biological Species Concept 1. It applies only to sexually reproducing species. 2. Distinguishing between species on the basis of reproductive separation is problematic because it can be difficult to determine how much reproductive separation is needed to distinguish between species. 3. The definition refers only to current populations and ignores the species status of ancestral populations. 3 24-03-2014 Criticisms of the Biological Species Concept Can a Great Dane interbreed with a Chihuahua? Criticisms Some species look and behave differently Can interbreed and have viable, fertile offspring Example: coyotes, wolves, dogs 4 24-03-2014 Criticisms Normally, 25% genetic diversity implies different species However, human and chimp genomes differ by only 1.2% Evolutionary Species Concept (G. Simpson, 1940) “A single lineage of ancestor-descendant populations that maintains its identity from other such lineages and that has its own evolutionary tendencies and historical fate.” 5 24-03-2014 Phylogenetic species concept “an irreducible (basal) grouping of organisms diagnosably distinct from other such groupings and within which there is a parental pattern of ancestry and descent.” Species, adopted definition A group of interbreeding individuals that share a gene pool and are reproductively isolated from other species. 6 24-03-2014 Speciation Species: basic classification unit Continuous lineage - information passed through genes Speciation: the mechanism by which a new species is formed. Two types of speciation are: 1. 2. Allopatric speciation Sympatric speciation Allopatric speciation (extrinsic mechanism) Allopatric (allo = other, patric = place) Occurs when two populations are geographically isolated from one another. They are separated by distance or an impassable barrier. 7 24-03-2014 Speciation (extrinsic and intrinsic) mechanism Sympatric speciation (intrinsic mechanism) Sympatric (sym = same, patric = place) New species develops when members of a population develop a genetic difference that prevents them from reproducing with members of the original species. 8 24-03-2014 Speciation, summing up As new species evolve, population become reproductively isolated i.e., member of 2 populations cannot interbreed and produce fertile offspring This tend to lead to phenotypic homogeneity within species, and differences between species Speciation can occur at widely different rates Speciation in EAs Niching does not prevent individuals in one niche (maximum) to mate individuals in another niche (maximum). Speciation inspired EAs allow individuals to mate only with similar ones Similarity can be based either on distance between representations, or on matching between extra bits added to individual representation 18 9 24-03-2014 Niching and Speciation Fernando Lobo University of Algarve Speciation methods • Deb and Goldberg proposed two mating restricting schemes based on phenotypic and genotypic distances between mating individuals. • Idea: if the distance is closer than a parameter , they participate in the crossover operation. Otherwise another individual is chosen at random as a candidate mate. – This process continues until a proper mate is found. If no such member exists, then a random individual from the population is chosen as mate. 10 24-03-2014 Speciation methods • Niching with operator. is implemented in the selection • Mating restriction with crossover operator. is implemented in the • Deb and Goldberg used • Results on F1 and F2 were improved. F1: equal peaks F1(x) = 11 24-03-2014 F1 results F1 with and without mating restriction 12 24-03-2014 Other speciation method • Using tag bits (Spears, 1994) • Tag bits are appended to every individual. • Each species correspond to a particular configuration of those bits. • Mating is restricted to individuals containing the same set of tag bits. • Spears allowed the tag bits to be mutated. Restricted Tournament Selection (RTS) • RTS is a crowding-like method proposed by Harik (1995). • RTS is a modification of tournament selection that restricts an individual from competing with others that are very different from it. 13 24-03-2014 Recalling Crowding Attempts to distribute individuals evenly amongst niches Deterministic Crowding: randomly select parents p1, p2, and produce offspring o1, and o2 Evaluate the fitness value of offspring, f (o1) and f (o2), and calculate their distances to parents, i.e., d(p1, o1), d(p1,o2), d(p2, o1), and d(p2, c2). If [d (p1, o1)+d (p2,o2)]≤ [d (p1, o2)+d (p2, o1)], the competition is between p1 vs. o1 and p2 vs o2. Otherwise, the competition is between p1 vs o2 and p2 vs o1. Determine the winner. Individuals with higher fitness values win the competition and will stay in the population; Losers are discarded. 27 RTS works as follows • Select two solutions at random, A and B, from the population and perform crossover and mutation, resulting in two new solutions, A´ and B´. • For each new solution (A´ and B´), scan w individuals (randomly chosen) from the population and pick the one that is most similar to it. Call them A´´ and B´´. • A´ competes with A´´. If A´ is better, then it replaces A´´ in the population. (Do the same for B´and B´´) – The algorithm is steady-state (solutions are introduced in the population incrementally). 14 24-03-2014 RTS • RTS performs much better than the standard crowding mechanism. • Also much simpler to use than fitness sharing. • Harik used w as 4 times the number of desired peaks to be found, and obtained good results. Summary on niche EAs 15