posted 06. January 2004 13:35                   

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Neutral genotype-phenotype mappings can be observed in natural evolution and are often used in evolutionary computation. In this article, important aspects of such encodings are analyzed.

First, it is shown that in the absence of external control neutrality allows a variation of the search distribution independent of phenotypic changes. In particular, neutrality is necessary for self-adaptation, which is used in a variety of algorithms from all main paradigms of evolutionary computation to increase efficiency.

Second, the average number of fitness evaluations needed to find a desirable (e.g., optimally adapted) genotype depending on the number of desirable genotypes and the cardinality of the genotype space is derived. It turns out that this number increases only marginally when neutrality is added to an encoding presuming that the fraction of desirable genotypes stays constant and that the number of these genotypes is not too small.

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Abstract: Self-adaptation is used in all main paradigms of evolutionary computation to increase efficiency. We claim that the basis of self-adaptation is the use of neutrality. In the absence of external control neutrality allows a variation of the search distribution without the risk of fitness loss

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posted 10. January 2004 18:07                   

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Historical constraints

The evolutionary history of an organism limits the developmental pathways that it can evolve either because of its current form (McKitrik,’93; Resnik ’95; but see Schwenk,’95), or because of the patterning mechanisms available to it (Holder,’83; Gould,’89; Hall,’96).


developmental buffering


Genetic changes can take place without phenotypic effect, because of feedback loops and buffering in developmental pathways (Kauffman,’83; Duboule and Wilkins,’98).


genetic constraints


Bias in the appearance of certain genotypes because of the nature of the genetic
system, probability of types of mutation, or recombination.

physical constraints


The physical properties of the egg or the embryo, the extra-embryonic membranes
and environment, limit or bias the evolutionary changes that can take place during
development (Elinson,’87).

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Horizontal constraints


Some developmental pathways influence many di¡erent processes in development. Changes in any pathway are likely to have a negative selective value because of pleiotropic effects (Raff,’96).


Vertical constraints

Development is sometimes viewed as a temporally hierarchical process (the inverted cone model-Arthur,’97). Changes in early stages of development may have multiple effects on later stages, and are therefore constrained (Raff,’96).


Structural constraints


Constraints that are the results of the basic laws of physics. Organisms are limited in the form they can have because exceeding certain physical boundaries will be selected against. Often referred to as ‘universal constraints’ (Maynard Smith et al., 85; Resnik,’95).


Functional constraints


Functional integration of different systems is an important aspect of organismal form.The limit this places on morphological changes constitutes a constraint (Galis,’ 93; Schwenk,’95).

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The physical characteristics of animals evolve because their genes change over successive generations. It is not always clear, though, which genes are involved1,2. The genes that regulate embryonic or larval development are likely candidates, because they control how the animal develops its characteristic form and features. It is possible that natural selection might produce evolutionary change after the adjustment of just a few such switches on the genetic control panel of development.
Writing in this issue, Sucena et al.3 and Gompel and Carroll4 provide evidence that this can indeed happen. They show that modifications at just a few developmental hotspots underlie ‘parallel’ evolutionary changes that occurred independently in different species.

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The attainment of a constrained transformation
may open a new adaptive space, and allow an adaptive radiation that was not possible before see Wagner ’88; Muller and Wagner, ’91 for the
relation between constraint and innovation). This
may indeed be the case with cetaceans. A further
example is provided by the transformations leading from a reptilian scale to feathers (Prum, ’99), which allowed the rapid adaptive radiation of birds.

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