Biology Reference
In-Depth Information
populations of cells. Selective constraint is propagated in the
proto-organism and determines the microenvironments to which
the cells stochastically adapt. By using the same mechanism as
that producing ontogenesis, the proto-organism evolves to adapt
when the environmental conditions change. Its ontogenesis and
phylogenesis are produced at the same time by a single process of
ontophylogenesis.
We took the example of a colony growing on a solid substrate,
but the same metabolic constraint applies for a spherical colony
growing in a liquid medium. In this case, diffusion occurs from the
exterior towards the centre of the colony, and the gradients follow
this axis. The concentration of nutrients decreases as they pass from
the exterior towards the centre while conversely, the metabolite
concentration increases. The region poor in nutrient where cell
growth ceases is therefore the centre of the sphere. The embryos of
many organisms in the first stages of embryogenesis form a ball of
cells like this and precisely in their centre a cavity forms, called the
blastocoel.
The general idea that cells differentiate depending on their posi-
tion in the organism, as in our example of the heap of cells, is not
new. It is the basis of Lewis Wolpert's theory of positional infor-
mation and has been expressed by other authors in various forms
(Wolpert, 1969, 1989, 1991). As regards the role of metabolism,
Charles Manning Child (1869-1954) undertook considerable exper-
imental and theoretical work to demonstrate that the existence
of metabolic gradients in the embryo is the causal factor of its
development (Child, 1941). The role of ontogenetic or phylogenetic
constraints is likewise well-known (Maynard Smith
et al
., 1985;
Arthur, 1988; Williams, 1992). All these theories are nevertheless
deterministic. Ontophylogenesis is distinct from them by virtue of
its probabilistic, selective and unified conceptual context. The ques-
tion of the level at which selection operates (whether at the mole-
cule, the cell or the organism) also recurs in evolutionary biology
(Dawkins, 1976; Brandon and Burian, 1984; Williams, 1992). In
this respect, Leo Buss (1987) also suggested that natural selection
is applied to cells and that multicellularity arises from competition
