Biology Reference
In-Depth Information
6.4.5
The role of morphogenetic gradients
in the Darwinian model
Morphogen molecules form gradients in the embryo (Gurdon and
Bourillot, 2001; Tabata and Takei, 2004). In deterministic theories,
it is acknowledged that cells differentiate specifically according
to their position in these gradients, i.e. according to the concentra-
tion of morphogens in their immediate environment (Wolpert, 1989).
In order to analyse the role of gradients in the Darwinian model
we observed how they were formed at the start of the simulations,
when the bilayer was being constituted. Their formation is simulta-
neous with that of the bilayer: they are the result of the same
dynamics, as the gradients and the cell bilayer gradually and mutually
reinforce each other. The bilayer forms owing to stochastic changes
in the cell types, but this process can only occur because it is sta-
bilised by the parallel formation of the molecular gradients. At the
same time, the cells differentiate according to their position in the
gradients. Cells of the RED type are found where there are most
red molecules and in the same way, cells of the GREEN type coin-
cide with the peak of the green molecule concentration. There is
therefore no contradiction between the Darwinian model and the
existence of morphogenetic gradients.
6.4.6
Does the Darwinian model lead to the
emergence of new properties?
The relevance of the concept of emergence that we have already
questioned (chapter 5) can be re-assessed by simulating the
Darwinian model. Can we consider cessation of the multiplication
of cells in the bilayer, which is a spontaneous non-programmed phe-
nomenon, as an example of emergence? Growth arrest would seem to
be a property at the tissue level of the system, emerging from inter-
actions between cells. To answer this question we must remember
that the concept of emergence presupposes a fundamental plan
explaining the origin of order: the interactions between the compo-
nents of a system brought into each other's presence induce new non-
predictable properties which appear suddenly and spontaneously
