Emergence - A System's elusive property
Systems
Theory of Biology is a relatively new approach to how scientists once viewed
having to explain phenomena. With a purely reductionist methodology, a given
system was broken down into individual components and the complete knowledge of
these components was additively thought to explain the behavior of the
"whole". It was evident however that this was not the case. Use of
computational tools and mathematical models to simulate biological systems in
silico based on experimental data further reinforced this purview. Thus Systems
Biology was born. Unlike the traditional reductionism, systems biology is an
interdisciplinary approach that employs holism by considering not only
components of a system but other attributes such as the complexity of
interactions between components, environmental perturbations, subsystems,
boundaries, etc. One of the more interesting of its considerations is a
property called Emergence.
What exactly is
Emergence?
For
an understanding, emergent properties were thought to be irreducible, non-
functionalizable fundamental
properties exhibited by systems/sub-systems that could not be deduced from the
properties of the systems/sub-systems components. In other words, the systems
showed novel behavior that could not be explained by the properties of its
components.
Fig1: A simplified look at emergent
properties of a system (eg. Formation of water)
Emergentist
theorists, systems philosophers and biologists tread carefully even today to
explain emergent properties in its entirety. This is primarily because although
emergence is observationally intuitive, for it to be useful in scientific
application its definition must encompass all its aspects some of which are
still under debate. In most current applications, its presence is either taken
for granted or adjusted for numerically.
Defining
what counts as an emergent property has gone through a variety of views. Early
20th century saw Emergence as a vital force which meant it was treated as a non-physical
property. Other philosophers conjectured that emergence would disappear as the
tools to study systems improved. One of the earliest comprehensive views on
emergence came from C.D. Broad. He went a step further to say that
"complete knowledge" about components both in isolation and in "other
wholes" were insufficient to predict the system's behavior. Although he
was vague about what "other wholes" meant, a simple example of a
chemical A (component) in 2 different environments XA and YA illustrates
that properties exhibited by A in both environments will be different.
Therefore we may infer that information about A from YA (other
whole) and A in isolation are both insufficient to help predict A's properties
in System X.
Down
the line more clarity on the topic led to classifying emergence based on 2
notions. Strong emergence is when system level phenomena are dependent on properties of its
components but not determined by it.
It remains non deducible from lower level rules. Weak emergence on the other hand
is when higher level phenomena is deducible from lower level rules but the
phenomena is unexpected. Although
both are important to biological systems, strong emergence is of greater
interest at a fundamental level. This is because of the nature of its
definition. Strongly emergent properties cannot be deduced by complete
knowledge of lower level phenomena which means the fundamental principles of
lower level domains need to be expanded upon to provide a mechanistic
explanation of the higher level phenomena in question. This explains why
science is more interested in strong compared to weak emergence.
Emergent properties in
Biological systems
At
the level of tissues and cells, many system level properties such as
homeostasis, regulation, plasticity appear to be emergent. One of the simpler
cases where emergence is evident is in the dogma of molecular biology
(DNA->RNA->Protein). While it is true that genes determine the amino acid
sequence of a protein, these gene level rules are not sufficient to describe
the properties of proteins as there are many other processes involved such as
mRNA regulation and protein folding which appear only at those levels.
Fig2: Biochemical network with nodes
representing metabolites and connections representing enzyme catalyzed
interconversions.
In
more recent years, study of dynamic properties of biochemical and genetic
networks with respect to internal/external conditions and time has unearthed
plenty about the intrinsic behavior of sub systems, their energetics and
interactions with their parent system. Let us take up the example of the cell (System)
and its organelles (subsystems). It was observed under specific cases of
modeling that the properties
of the cell was in part a function of its organelle as a "whole"
which meant that, studying the components of the organelles themselves could
not explain the cell's behavior. This was a key point presented by the case
studies of F.G. Bruggeman and team. Another avenue in which emergent properties
play a significant role is in genetic networks and gene expression shifts.
Precisely speaking, properties such as critical dynamics of transitions in gene
expression. To maintain genetic stability and aid evolvability, it is important
for genetic networks to have an optimal robustness (resistance to external
perturbations) and flexibility (degree of freedom). This optimal state is an
emergent property called "Criticality" and it operates in and around
phase transitions. There are many other emergent levels of regulation based on
other contexts such as transcriptional or protein regulation such as mRNA
degradation, protein folding and ubiquitination.
In
conclusion, while we have begun to account for the effects of emergence, we
still continue to grapple with what it is and the extent of its role in systems
biology. To quote anti-reductionist Karl Popper,
" life
itself may be viewed as an emergent property of matter"
Sanjay Narayanaswamy
References:
1. Boogerd
FC et al., 2005. 'Emergence and its place in nature: A case study of
Biochemical networks' Synthese 145:
131-164.
2. Christian
TS et al., 2012. 'Criticality Is an Emergent Property of Genetic Networks that
Exhibit Evolvability' PLoS Computational Biology 9: e1002669.
3. Pepper S.
1926. 'Emergence' Journal of Philosophy 23:
241-245.
4. Srdjan K.
2015. 'Systems biology, Emergence and antireductionism' Saudi Journal of Biological Sciences 23: 584-591.
5. Schoen
EL. 2007. 'The Re-emergence of Emergence: The Emergentist hypothesis from
Science to religion' International
Journal for Philosophy of Religion 62: 119.
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