Biology Reference
In-Depth Information
To connect two nodes of a network a link is needed with a ratio of
0.5. From the moment this value is exceeded, i.e. when the compo-
nents of the network can be connected by multiple links, the prob-
ability of forming a single network connecting all the components is
greatly increased until this event becomes inevitable. Kauffman
assumes that this is how life occurred. It was not therefore an acci-
dent but the result of this spontaneous tendency to self-organize
shown by high connectivity Boolean automata. This first result
seems compatible with the structure of real protein networks the
nodes of which are indeed highly interconnected (chapter 4, §4.1.5).
However, this experiment does not take into account the fact that
biochemical networks also have to be functional. It does not indi-
cate whether the networks formed are ordered or chaotic. Kauffman
investigated this question in another study dealing with cell differ-
entiation (HU pp. 71-112).
A differentiated cell results from the stable expression of a sub-set
of genes of all those forming a genome. The question is therefore
whether Boolean automaton networks generate ordered states in
which the same nodes (genes) are constantly (or cyclically) acti-
vated. Kauffman's results show that this is possible on condition
that each node at the most only depends on two other nodes of the
network. If the connectivity of the network is greater, the latter
very rapidly becomes chaotic. This is a problem in itself in regard
to experimental reality, since the connectivity of actual networks is
very great. However, a high connectivity network could still pro-
duce ordered states if it were biased so as to direct its behaviour
towards stable states. Gérard Weissbuch, who perfected the model,
calls this bias the
p
parameter (HU pp. 84 and 103; Weissbuch, 1999).
Put simply, it means that, as in the case of Bénard's instability, a con-
straint needs to be exerted on the Boolean automaton network for it
to organise itself. Although it does not seem to have occurred to the
adherents of self-organisation, this ruins what Kauffman wants to
demonstrate, since it is not spontaneous organisation.
Do these models help resolve the contradiction in genetic
determinism caused by the non-specificity of molecules? When
Kauffman carried out his work, no actual networks of proteins or
