24

3

Regulation and Control

range of variation; in other words, there may be directive correlations of directive

correlations carried on through many levels.

As the range of directive correlation increases, more and more causal connexions

are required. This is particularly apparent when considering coordinated activities.

An action such as running requires the coordination of many muscles; each one must

take account of the others, and all have a common goal. nn muscles may therefore

require as many as n squared plus nn² + n physical interconnexions. If the muscles are physically

distant from each other, the construction and maintenance of these interconnexions

may represent a considerable burden; but if they are concentrated within a nervous

centre, only nn afferent and nn efferent connexions are required, together withnn more

leading to the goal itself; physical economy in the total length of the connexions

provides a natural explanation for the existence of nerve centres (cf. Chap. 24).

Clearly, directive correlation is practically synonymous with organic integration,

bringing into connexion (through the objective property of directive correlation) what

would otherwise be independent, disconnected entities.

A great advantage of the concept of directive correlation is that it eliminates the

need for teleology and provides a mathematical model for purposive activity.

3.6

Timescales of Adaptation

One can identify three timescales: proximate (short term, often associated with

behaviour)—such as immediate response to sudden danger (e.g., fleeing from a

fire); ontogenetic, or the abilities that accumulate over the lifetime of an individ-

ual (medium term, often associated with learning, or a pattern of behaviour); and

phylogenetic, or the inheritable changed capacities associated with changes in the

genome, which constitute evolution of a species (long term). Proximate adaptation

may take place through the medium of reception of information (e.g., a toxin binding

to a cell surface receptor) followed by appropriate gene expression (cf. Sect. 3.2),

but in many animal responses there is no time even for this, but simply for muscu-

lar action. The mechanisms for phylogenetic adaptation, involving DNA mutations,

are now similarly well established. It is only in recent years, however, that a con-

siderable repertoire of molecular mechanisms for ontogenetic adaptation has been

discovered, including the establishment of gene methylation patterns that more or

less permanently (unless there is a drastic change in circumstance) fix which genes

are potentially expressible in a given cell. The vast accumulation of nongenic (“non-

coding”) DNA in most eukaryotes is no doubt of great value here, permitting the

synthesis of small interfering RNAs that gradually build up a repertoire for modu-

lating gene expression according to the particular circumstances of the individual

cell.

This rather clear-cut structure of adaptive timescales is not readily applicable

to prokaryotes. First, their genomes are extremely plastic and can acquire genetic

material from the environment throughout the lifetime of the organism. Second, the

meaning of “lifetime of an individual” is not so clear: When a bacterium divides,