Biology Reference
In-Depth Information
FIGURE 2.5
A generalized
view of a morphogenetic mech-
anism typical of an embryo. As
explained in the text, feedback
usually operates on many layers
and the picture is not a simple as
this diagram implies. This topic
concentrates mainly on events
below the dotted line; events
above the line are covered very
well by other texts.
'Genetic programme'
Control systems that
bais probability of
assembly/disassembly
Measurement of
appropriateness
of size/shape of
structure
Feedback
Self-assembly of
molecular subunits
a structure) is used to control the processes that produce that output (for example, self-
assembly and enzymatic destruction). It is the addition of this type of feedback to processes
of physical self-assembly that results in a truly 'biological' system and, when reduced to the
most basic level of description, many morphogenetic processes have the form of
Figure 2.5
.
This figure will be discussed in more detail in Chapter 25.
Of course,
Figure 2.5
is oversimplified, and multiple layers of negative feedback are
common. For example, the innervation of a particular muscle by a particular neuron may
depend at the finest level on the self-assembly, in the neuron, of cytoskeletal structures char-
acteristic of axon extension (Chapter 8). Fast, highly localized (10 nm scale) feedback will
modulate the assembly and disassembly of these structures so that they are optimized for
the advancement of the plasma membrane at the leading edge of the extending axon. Feed-
back on the 100
e
1000 nm scale will further modulate the self-assembly and disassembly of
motile cytoskeleton to ensure that the axon extends along a trail of some guidance molecule
rather than in a random direction (Chapter 11). A cell-scale feedback system may then be
used to switch off the axon extension mechanism completely once the cell has reached its
target, or it may be used to kill the cell if it has reached the wrong target (Chapter 24).
And, in any biological system, there is always the slow feedback of natural selection that
will favour the reproduction of those organisms whose genomes encode morphogenetic
mechanisms that build organisms most able to thrive and reproduce.
The addition of feedback to a morphogenetic system gives it a property far more
powerful than mere self-assembly. In this topic, that property is referred to as 'adaptive
self-organization'.
)
Systems that show adaptive self-organization can arrange their struc-
tures in ways not simply dictated by the properties of the structures' subunits, but also
)
Adaptive self-organization is also known by the terms 'swarm intelligence', 'hive intelligence', 'distributed
optimization', 'adaptive routing' and by similar phrases, depending on context.
