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could take place in the absence of isolation (in contrast to the usual view, cf. Mayr 1963). |
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Once an adaptive system discovers that given combinations of genes (or their alleles) offer a persistent advantage, new modes of advance become possible. If the given combinations can be handled as units they can serve as components ("super genes") for higher order units. In effect the system can ignore the details underlying the advantage conferred by a combination, and operate simply in terms of the advantage conferred. By so doing the system can explore regions of a, i.e., combinations of the new units, which would otherwise be tried with a much lower probability. (For example, consider two combinations of 10 alleles each under the steady state of section 7.2. If each of the alleles involved occurs with a frequency of 0.8, the overall combination of 20 alleles will occur with a frequency (0.8)20@ 0.01. On the other hand, if each of the two 10-allele combinations is maintained at a frequency of only 0.5, then the 20-allele combination will occur with frequency (0.5)2 = 0.25. I.e., the expected time to occurrence will be reduced by a factor of 25.) Since combinations of advantageous units often offer an advantage beyond that of the individual unitsas when the units' effects are additive (linear independence) or cooperativethey are good candidates for early testing. (The cooperative case where one unit effects an enrichment which can be exploited by another is particularly common; cf. cooperating cell assemblies or stages of a complex production activity such as illustrated in Figure 3.) |
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We have already discussed (section 6.3) the way in which inversion can favor association between genes. However, by controlling representation, the adaptive system can bring about changes which go much further, producing a hierarchy of units. The basic mechanism stems from the introduction of arbitrary punctuation marks to control operators (see usage (4) in section 8.3 and the discussion on pages 152-53). The adaptive system introduces a distinct punctuation mark (specific symbol string) to mark off the combinations which are to be treated as units at a given level of the hierarchy. Then the operators for that level are restricted to act only at that punctuation. (E.g., crossover takes place only at the positions marked by the given punctuation.) By introducing another punctuation mark to treat combinations of these units, in turn, as new units, and so on, the hierarchy can be extended to any number of levels. The resulting structure offers the possibility of quickly pinpointing responsibility for good or bad performance. (E.g., a hierarchy of 5 levels in which each unit is composed of 10 lower level units allows any one of 105 components to be selected by a sequence of 5 tests.) In the hierarchy, the units at each level are subject to the same "stability" considerations |
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