Biology Reference
In-Depth Information
behaving as they would in the homologous location in a mouse, they induced the host chick
beak to make teeth,
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structures that have been absent from birds for the last 80 million years.
Migration is not the only behaviour that underlines the way that cells respond to environ-
mental cues rather than just working blindly from internal information. The molecular inter-
actions that regulate the cytoskeleton of an epithelial cell near a wound can 'know' nothing
about the size and shape of wound inflicted by a predator or an accident, yet the effect of
these interactions, all following their own simple rules in all of the cells involved, is to close
the wound perfectly (Chapter 17). The simple, ignorant interactions involved in all of these
systems can therefore generate an embryo that behaves
as if
its components had 'knowledge'
of larger scale plans and structures.
The concept of emergence is still ill-defined, at least by the usual standards of scientific
ideas, but it already fulfils a critical role in linking two very different views of biology.
One view, epitomized by biochemistry and molecular biology, is essentially reductionist
and focuses on the smallest components of living systems, the molecules themselves. The
other view, the name of which changes with fashion but currently seems to be 'systems
biology', is holistic and focuses on the behaviour of whole organisms or physiological
systems. These two views, both valid and both useful, have long been difficult to reconcile
and the dialogue between their proponents has often been strained.
5
e
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Emergence is
a concept that can unite these views naturally, because it focuses on the way in which inter-
action of 'dumb' agents studied by reductionists can give rise to 'smart' phenomena studied
by holists. Emergence is what frees researchers from looking for traces of high-level concepts
in low-level components. That a leading edge can navigate does not mean that any of its
interacting proteins have the quality of navigation somehow built into their structure. By
analogy, the fact that humans show love does not have to mean (as the evolutionary biologist
and theologian Teilhard de Chardin maintained) that love has to be a property even of the
atoms from which humans and the rest of the world are made.
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The Multilayered Organization of Morphogenetic Processes
A key feature of the morphogenetic mechanisms described in this topic is that they involve
many different layers of organization, and that in the progression upwards from the molec-
ular level, features emerge at each layer that were absent from the layer below (
Figure 28.1
).
Furthermore, different morphogenetic processes can share the same bottom layers, or at least
substantially overlapping sets of bottom layers, yet achieve quite different final outcomes
because they use different upper layers of control.
The lowest layer of most morphogenetic processes is the simple self-assembly of macromol-
ecular complexes.
)
Enzyme subunits come together to form a functional enzyme, actin mono-
mers form complexes with nucleating proteins and with each other, adhesion molecules and
junctional proteins form cross-linked adhesive complexes, etc. Given the right mix of compon-
ents, self-assembly of these macromolecular complexes is automatic and, to a first approxima-
tion (whichwill be discussed inmore detail later) they can be taken for granted by higher layers.
)
Self-assembly itself rests on layers such as chemistry, which determines the shapes and binding potentials
of protein subunits, and chemistry rests on atomic physics, etc: these are outside the scope of this topic,
which is why self-assembly is regarded here as the lowest layer relevant to the discussion.
