Evolution in Four Dimensions: Genetic, Epigenetic, Behavioral, and Symbolic Variation in the History of Life

Between the Acts: An Interim Summary

Between the Acts: An Interim Summary In part III we shall look at the interactions of the four systems of information transmission that we have described in the previous six chapters. Since we have covered a lot of ground, a summary and comparison of the salient properties of the four systems may be helpful. To avoid excessive repetition, we have done this in the form of two tables. The first table describes the way in which information is reproduced and varies for each of our four dimensions of heredity and evolution. It shows (1) whether the organization of information is modular (the units can be changed one by one) or holistic (the components can’t be changed without destroying the whole); (2) whether or not there is a system dedicated to copying that particular type of information; (3) whether or not information can remain latent—unused but nevertheless transmitted; (4) whether information is passed solely to offspring (vertically) or to neighbors as well (horizontally); (5) whether variation is unlimited and capable of indefinite variation, or limited, in that only a few distinct differences can be transmitted. Tables such as this are always something of an approximation, because in biology things rarely fall into discrete categories. We have said before that the different epigenetic systems overlap, and also that putting transmitted substances that affect the development of an animal’s form with epigenetic inheritance systems, while putting those that affect its behavior in a different category, is somewhat arbitrary. Tables also have to make do with words such as “sometimes” and “mostly,” rather than spelling out the details. For instance, we say that the direction of transmission for the genetic system is “mostly vertical,” which is shorthand for “genetic transmission is vertical in eukaryotes, except on the probably rare occasions when DNA is transferred from individual to individual by various vectors, or directly through ingestion; in bacteria and other prokaryotes horizontal transfer may be quite common and evolutionarily important, although we do not have enough data to know how frequent it is.” 230 Between the Acts Table 1 The re-production of information Inheritance system Organization of the information Dedicated copying system? Transmits latent (nonexpressed ) information? Direction of transmission Range of variation Genetic Modular Yes Yes Mostly vertical Unlimited Epigenetic Self-sustaining loops Holistic No No Mostly vertical Limited at the loop level, unlimited at the cell level Structural templating Holistic No No Mostly vertical Limited at the structure level, unlimited at the cell level RNA silencing Holistic Yes Sometimes Vertical and sometimes horizontal Limited at the single transcript level, unlimited at the cell level Chromatin marks Modular and holistic Yes (for methylation) Sometimes Vertical Unlimited Organism-level developmental legacies Holistic No No Mostly vertical Limited Behavioral Behavioraffecting substances Holistic No No Both vertical and horizontal Limited at the single behavior level, unlimited for lifestyles Nonimitative social learning Holistic No No Both vertical and horizontal Limited at the single behavior level, unlimited for lifestyles Imitation Modular Probably No Both vertical and horizontal Unlimited Symbolic Modular and holistic Yes, several Yes Both vertical and horizontal Unlimited An Interim Summary 231 Whatever the shortcomings of tables, they do help to highlight patterns of similarities and differences. When viewed in this way, it is obvious how alike the genetic and the symbolic inheritance systems are. In both, variation is modular; both can and often do transmit latent information; and in both, variation is practically unlimited. These properties give the two transmission systems enormous evolutionary potential by providing vast amounts of heritable information that can be sifted and organized by natural selection and other processes. The first table also shows that whereas with the genetic and epigenetic systems the direction of information transmission is mainly vertical, from parents to progeny, with other systems there is a significant amount of horizontal transmission to peers or neighbors . In fact there is a kind of jump in the direction in which information flows, with horizontal transmission becoming much more common as we move to the behavioral systems and socially mediated learning. This introduces a bias that can alter the effects of selection in significant ways. It also means that a new set of considerations (about why, how, and when horizontal transmission is occurring) have to be included in evolutionary thinking about changes based on information transmitted by behavioral or symbolic means. Whereas the first table focuses on the nature and reproduction of information , the second one...