Chromosome (evolutionary algorithm): Difference between revisions

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{{Evolutionary algorithms}}

In [[genetic algorithm]]s (GA), or more general, [[evolutionary algorithm]]s (EA), a '''chromosome''' (also sometimes called a '''[[genotype]]''') is a set of parameters which define a proposed solution of the problem that the evolutionary algorithm is trying to solve. The set of all solutions, also called ''individuals'' according to the biological model, is known as the ''[[Population model (evolutionary algorithm)|population]]''.<ref name=ga-description>{{cite web|title=Introduction to genetic algorithms: IV. Genetic Algorithm|url=http://www.obitko.com/tutorials/genetic-algorithms/ga-basic-description.php|accessdate=12 August 2015}}</ref><ref name=":0">{{Cite book |last1=Eiben |first1=A.E. |url=https://link.springer.com/10.1007/978-3-662-44874-8 |title=Introduction to Evolutionary Computing |last2=Smith |first2=J.E. |date=2015 |publisher=Springer |isbn=978-3-662-44873-1 |series=Natural Computing Series |___location=Berlin, Heidelberg |pages=28–34 |language=en |chapter=Components of Evolutionary Algorithms |doi=10.1007/978-3-662-44874-8}}</ref> The genome of an individual consists of one, more rarely of several,<ref>{{Citation |last=Baine |first=Nicholas |title=A simple multi-chromosome genetic algorithm optimization of a Proportional-plus-Derivative Fuzzy Logic Controller |date=2008 |url=https://ieeexplore.ieee.org/document/4531273 |work=NAFIPS 2008 - 2008 Annual Meeting of the North American Fuzzy Information Processing Society |pages=1–5 |publisher=IEEE |doi=10.1109/NAFIPS.2008.4531273 |isbn=978-1-4244-2351-4 }}</ref><ref>{{Citation |last1=Peng |first1=Jin |last2=Chu |first2=Zhang Shu |title=A Hybrid Multi-chromosome Genetic Algorithm for the Cutting Stock Problem |date=2010 |url=https://ieeexplore.ieee.org/document/5694457 |work=3rd International Conference on Information Management, Innovation Management and Industrial Engineering |pages=508–511 |publisher=IEEE |doi=10.1109/ICIII.2010.128 |isbn=978-1-4244-8829-2 }}</ref> chromosomes and corresponds to the [[genetic representation]] of the task to be solved. A chromosome is composed of a set of genes, where a gene consists of one or more semantically connected [[Parameter|parameters]], which are often also called ''decision variables''. They determine one or more [[Phenotype|phenotypic]] characteristics of the individual or at least have an influence on them.<ref name=":0" /> In the basic form of genetic algorithms, the chromosome is represented as a binary [[string (computer science)|string]],<ref>{{Cite book |last=Holland |first=John H. |url= |title=Adaptation in natural and artificial systems |date=1992 |publisher=MIT Press |isbn=0-585-03844-9 |edition= |___location=Cambridge, Mass. |language=en |oclc=42854623}}</ref> while in later variants<ref>{{Citation |last1=Janikow |first1=C.Z. |title=An Experimental Comparison of Binary and Floating Point Representations in Genetic Algorithms |date=1991 |url=http://www.cs.umsl.edu/~janikow/publications/1991/GAbin/text.pdf |work=Proceedings of the Fourth International Conference on Genetic Algorithms |pages=31–36 |editor-last=Belew |editor-first=Richard K. |editor2-last=Booker |editor2-first=Lashon B. |place=San Francisco, CA |publisher=Morgan Kaufmann Publishers |isbn=1-55860-208-9 |last2=Michalewicz |first2=Z.}}</ref><ref name=ga-tutorial>{{cite journal |last1=Whitley |first1=Darrell |title=A genetic algorithm tutorial |journal=Statistics and Computing |date=June 1994 |volume=4 |issue=2 |doi=10.1007/BF00175354 |citeseerx=10.1.1.184.3999| s2cid=3447126}}<!--|accessdate=12 August 2015--></ref> and in EAs in general, a wide variety of other [[data structure]]s are used.<ref name=":1">{{Cite journal |last=Whitley |first=Darrell |date=2001 |title=An overview of evolutionary algorithms: practical issues and common pitfalls |url=https://linkinghub.elsevier.com/retrieve/pii/S0950584901001884 |journal=Information and Software Technology |language=en |volume=43 |issue=14 |pages=817–831 |doi=10.1016/S0950-5849(01)00188-4}}</ref><ref>{{Citation |last1=Bäck |first1=Thomas |last2=Hoffmeister |first2=Frank |last3=Schwefel |first3=Hans-Paul |title=A Survey of Evolution Strategies |date=1991 |url=https://www.academia.edu/27025389 |work=Proceedings of the Fourth International Conference on Genetic Algorithms |pages=2–9 |editor-last=Belew |editor-first=Richard K. |editor2-last=Booker |editor2-first=Lashon B. |place=San Francisco, CA |publisher=Morgan Kaufmann Publishers |isbn=1-55860-208-9 }}</ref><ref name=":5">{{Cite book |last=Koza |first=John R. |url=https://www.worldcat.org/oclc/26263956 |title=Genetic programming : on the programming of computers by means of natural selection |date=1992 |publisher=MIT Press |isbn=0-262-11170-5 |___location=Cambridge, Mass. |oclc=26263956}}</ref>

==Chromosome design==

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=== Chromosomes for complex representations ===

The chromosomes presented above are well suited for processing tasks of continuous, mixed-integer, pure-integer or combinatorial optimization. For a combination of these optimization areas, on the other hand, it becomes increasingly difficult to map them to simple strings of values, depending on the task. The following extension of the gene concept is proposed by the EA GLEAM (General Learning Evolutionary Algorithm and Method) for this purpose:<ref name=":4">{{Citation |last1=Blume |first1=Christian |last2=Jakob |first2=Wilfried |title=GLEAM - An Evolutionary Algorithm for Planning and Control Based on Evolution Strategy |date=2002 |url=https://publikationen.bibliothek.kit.edu/170053025/3814288 |work=Conf. Proc. of Genetic and Evolutionary Computation Conference (GECCO 2002) |volume=Late Breaking Papers |pages=31–38 |access-date=2023-01-01 }}</ref> A gene is considered to be the description of an element or elementary trait of the phenotype, which may have multiple parameters. For this purpose, gene types are defined that contain as many parameters of the appropriate data type as are required to describe the particular element of the phenotype. A chromosome now consists of genes as data objects of the gene types, whereby, depending on the application, each gene type occurs exactly once as a gene or can be contained in the chromosome any number of times. The latter leads to chromosomes of dynamic length, as they are required for some problems.<ref>{{Cite journal |last1=Pawar |first1=Sunil Nilkanth |last2=Bichkar |first2=Rajankumar Sadashivrao |date=June 2015 |title=Genetic algorithm with variable length chromosomes for network intrusion detection |url=http://link.springer.com/10.1007/s11633-014-0870-x |journal=International Journal of Automation and Computing |language=en |volume=12 |issue=3 |pages=337–342 |doi=10.1007/s11633-014-0870-x |issn=1476-8186}}</ref><ref>{{Citation |last=Blume |first=Christian |title=Optimized Collision Free Robot Move Statement Generation by the Evolutionary Software GLEAM |date=2000 |url=http://link.springer.com/10.1007/3-540-45561-2_32 |work=Real-World Applications of Evolutionary Computing |volume=1803 |pages=330–341 |editor-last=Cagnoni |editor-first=Stefano |access-date=2023-06-25 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/3-540-45561-2_32 |isbn=978-3-540-67353-8}}</ref> The gene type definitions also contain information on the permissible value ranges of the gene parameters, which are observed during chromosome generation and by corresponding mutations, so they cannot lead to lethal mutations. For tasks with a combinatorial part, there are suitable [[Genetic operator|genetic operators]] that can move or reposition genes as a whole, i.e. with their parameters.

[[File:Genmodell Chromosombeispiel.png|thumb|212x212px|Three exemplary genes matching the adjacent gene type definitions in a chromosome organized as a list]]

[[File:Gene model gene types.png|left|thumb|224x224px|Three exemplary genes matching the adjacent gene type definitions in a chromosome organized as a list]]