4.6 Evolution, epigenesis, metabolism.
When Bryan Goodwin (1959, 1963) studied the temporal organisation of cells, he described three time-windows through which we can look at activities within and between cells:
the evolutionary scale of the genetic system that serves long term adaptation and transmission of hereditary features.
the developmental scale of the epigenetic process: the differentiation of tissues or morphogenesis
the biochemical scale of metabolic changes, that serve maintenance functions of the cell
Map 4.4.3 Time windows through which to look at the human world
Every system has a reaction- (or response-) time that determines it's position on the time-scale. It is the time required for a system to respond to a stimulus or change in the environment The response times are different for the three systems. If we look at single cells, the values for the metabolic responses are between 0,1 sec and 100 sec. Adjoining values are seen in the epigenetic system (ontogenic development): 100 sec. to 3 hrs. Genetic fluctuations in a population of microorganisms in response to a stimulus (change in the environment) take days or weeks before they stabilize.
In a cell's metabolism oscillations can arise when e.g. two enzymes are coupled by a reciprocal feedback inhibition (the increase of the one inhibits the other). These are non-linear oscillations in the window of 0.1 to 100 seconds, and can show complicated types of interaction. One consequence of their interaction is that it gives rise to sub-harmonic phenomena, with periods of 5-30 min. or longer. Since this is well within the range of epigenetic processes, a metabolite which oscillates at such a low frequency can be a significant element in developmental processes. In this way rapid and slow systems are coupled to each other for a weak or strong interaction.
If a stimulus acting upon the cell is strong and prolonged there will be an initial rapid response by the metabolic system, followed by a slower response due to changes in macro molecular concentrations (macromolecules are proteins with a specific configuration at their surface that carry important "prescriptions" for development). This is a second level response which indicates that the parameters of the metabolic system are changing. Thus it must be regarded as a response by the epigenetic system of the cell. The epigenetic variables are the controlling parameters of the metabolic system, and actually define the steady state of the metabolic system. A higher order (epigenetic) process, of low oscillation frequency, sets the parameters for the lower order process, that has a higher frequency of oscillation.
In this way the system with the rapid response time will carry out the directions as prescribed by the slower moving system.
It is a two-way hierarchy:
historical information is provided by the genome and passed on from the core (top, left in the Map) to the peripheral structures (bottom, right in the Map)
fresh information, vital for adjustment to the environment and for survival, is passed on from the periphery (bottom) upwards (Map 4.4.3).
The two streams of information interact whenever they meet each other. If they represent conflicting tendencies, the result of their transaction is reconciliation. Oscillations in living systems will not be suppressed, because they establish coupling or reconciliation between higher- and lower order control systems. We have seen the example of coupling between the metabolic and epigenetic systems in a developing organism. As a coupling device, oscillation has a vital role to fulfill.
Chemical oscillation can be generated in various ways. Two or more metabolic structures may be in competition for precursor substances or for substrates that they have in common. This is analogous to a predator-prey relationship (see below), which causes the population densities of both predator and prey animals to oscillate with a phase difference.
As Goodwin points out, we benefit from the directness of system theory. Since variables of a system of lower order (with a shorter response time) can be eliminated from the dynamic equation of higher order systems, the description of higher order systems need not be more complex than that of lower systems. The complexity of nature is less confounding in a systems approach. This is also true for the complexity of human behaviour, as will be explained in Chapters 7 and 8.
The oscillatory cycles in living systems vary from milliseconds to months or longer. Networks of coupled oscillators are the skeletal frame of life.
The transmission of information in a network of oscillating systems displays a curious biological counterpart to the wave/particle duality in physics (is a light-beam a series of corpuscles or an EM wave? ). A chemical signal propagated in a biological matrix, can be considered from a material viewpoint: a shift in the density of a population of cells (or molecules), or from a communication viewpoint as oscillations in a dynamic network. Both views are equally valid, although the one will be more suitable than the other depending on the specific purpose.