Biology Reference
In-Depth Information
composition and physical properties of the medium, such as its
ionic concentration and temperature. In addition, each cell struc-
ture in a state of differentiation requires an optimal metabolic sup-
ply whether the supply is present or not in the environment.
3) Cells have a past on which they depend
A cell is the result at a given moment of its previous evolution.
The molecular combination possibilities at the following instant
are limited by its structure which promotes certain molecular
interactions and prevents others, as seen in cellular compartmen-
talisation (see chapter 4, §4.3.2). There, the structure of the
cell restricts the molecular combination possibilities and affects
the future differentiation potential of the cell. When the struc-
ture changes through natural selection, under pressure from a
change in the environment, its differentiation potential likewise
changes.
Ontophylogenesis breaks with genetic determinism by including
two factors absent from evolutionary synthesis — the stochastic
character of interactions between molecules and the direct action of
natural selection on ontogenesis. It does not however involve any
emergence of irreducible properties. It is not a question of sponta-
neous self-organisation of the organism's components. Natural
selection is a global constraint exerted on the organism, the origin
of which is external to it.
6.1.1
The model of the heap of cells and the origin
of multicellularity
How does the general principle of ontophylogenesis that we have
just set out apply to multicellular organisms with differentiated
cells? If it is the fundamental principle, it should shed light on how
they function and the conditions in which they have appeared during
evolution.
The simplest forms of multicellular organisms depend on the
environmental and metabolic conditions in which the organisms
live. For example, if placed in a medium poor in nutrients, the
