In the Light of Evolution Volume I: Adaptation and Complex Design (2007) / Chapter Skim
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Pages 45-64
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From page 45... ...
These properties reduce the number of regulatory changes needed to generate viable selectable phenotypic varia tion, increase the variety of regulatory targets, reduce the lethality of genetic change, and increase the amount of genetic variation retained by a population. By such reductions and increases, the conserved core processes facilitate the generation of phenotypic *
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From page 46... ...
research of the modern era has revealed that heritable phenotypic variation requires genetic change, that is, DnA sequence change. Changes
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From page 47... ...
, it identifies regulatory changes as ones particularly important for animal evolution, but unlike others it also emphasizes the targets of regulation. We include four steps from genetic variation to viable phenotypic variation of anatomy and physiology, and we wish to show at which steps the facilitation of variation occurs, and how it occurs.
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From page 48... ...
We will address three points of the proposals. What are the conserved core components and processes, what are their special properties that facilitate the generation of phenotypic variation by regulatory change, and what, in turn, are the regulatory innovations that have facilitated the use of core processes?
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From page 49... ...
, inductions, complex cell competence, additional specialized cell types, formation of the body plan's map of selector gene compartments (both transcription factors and signaling proteins) , various regulatory processes
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From page 50... ...
would be developmental examples. These complex processes are, we argue, combinations of different conserved core processes linked in new regulatory configurations, conserved in their entirety.
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From page 51... ...
WEAK REGULATORY LINKAGE linkage, which denotes the connecting of processes to each other or to particular conditions, is central to our theory because different core processes must become linked, by regulatory means, in different combinations, and operated in different amounts, states, times, and places for the generation of new anatomical and physiological traits. regulatory linkage pervades development and physiology.
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From page 52... ...
regulatory linkages can evolve with little constraint. neuronal transmission is a more complex two-state example, a physiological process comprising several core processes.
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From page 53... ...
The ease with which simple signals can entrain complex processes reflects the capacity of core processes to engage in weak regulatory linkage. Finally, the action of enhancer binding proteins in eliciting or repressing transcription (a complex specific output)
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From page 54... ...
neither provides much information about the outcome but just acts on the conserved switch. To summarize, the relevant point of these examples is that regulatory change is easily effected when conserved core processes have an inherent capacity for weak regulatory linkage, that is, when switch-like behavior and alternative states of function are already built into them.
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From page 55... ...
robustness and adaptability are essential to the kind of evolution we have described, wherein core processes are used in different combinations, amounts, and states to produce new traits. They strongly reduce the requirements for regulatory change, and hence genetic change, and increase the frequency of viable phenotypic variations.
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From page 56... ...
The adaptability and robustness of normal muscle, nerve, and vascular development have significant implications for evolu tion, for these processes accommodate to evolutionary change as well. in the case of the evolving wing, if bones undergo regulatory change (driven by genetic change)
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From page 57... ...
. he called this enormous range of phenotypes, which are achievable without genetic change, the animal's ‘‘norm of reaction'' to the environment.
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From page 58... ...
Their phenotypes, too, could then be stabilized by genetic change under selective conditions, imposed by the experimentalist. A major implication about phenotypic variation from these studies and ideas is that when novelty of some kinds is achieved in the course of variation and selection, rather little is really new; most components and regulatory linkages of the trait were already there.
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From page 59... ...
The latter class would not be evoked by the environment and then stabilized, but stabilized directly by regulatory change driven by genetic variation. COMPARTMENTATION Thus far we have discussed how conserved core processes facilitate regulatory change, but we should also discuss how various regulatory processes, evolved in pre-Cambrian animals, have facilitated the use of core processes in different combinations, amounts, and states, while decreasing their chances of interference (pleiotropy)
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From page 60... ...
that select stage-specific target genes, and in sexual dimorphism, target genes are selectively expressed in each sex. EXPERIMENTAL EVIDENCE FOR FACILITATED VARIATION To summarize, we argue that robustness, adaptability, modularity, capacity for weak regulatory linkage, exploratory behavior, and state selection of the conserved core processes, as well as the regulatory com partmentation of the conserved core processes, are key properties of the animal's phenotype that facilitate the generation of anatomical and physi ological variation by regulatory change, which ultimately requires genetic change to be heritable.
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From page 61... ...
in both cases, genetic change results in regulatory change, which modifies the use of the conserved core processes. The theory predicts that developmental biologists will continue to find (i)
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From page 62... ...
laws of variation begin to emerge, such as regulatory change as the main target of genetic change, the means to minimize the number and complexity of regulatory changes, and the regulatory redeployment
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From page 63... ...
These views are not at all lamarckian, nor does facilitated phenotypic variation require selection for future good. such facilitation arose, we think, as a by-product of the evolution of the special properties of the core processes, namely, of their robustness, adaptability, modularity, exploratory behavior, and capacity for weak regulatory linkage.
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