Abstract
Groups exhibit properties that either are not perceived to exist, or perhaps cannot exist, at the individual level. Such ‘emergent’ properties depend on how individuals interact, both among themselves and with their surroundings. The world of everyday objects consists of material entities. These are, ultimately, groups of elementary particles that organize themselves into atoms and molecules, occupy space, and so on. It turns out that an explanation of even the most commonplace features of this world requires relativistic quantum field theory and the fact that Planck’s constant is discrete, not zero. Groups of molecules in solution, in particular polymers (‘sols’), can form viscous clusters that behave like elastic solids (‘gels’). Sol-gel transitions are examples of cooperative phenomena. Their occurrence is explained by modelling the statistics of inter-unit interactions: the likelihood of either state varies sharply as a critical parameter crosses a threshold value. Group behaviour among cells or organisms is often heritable and therefore can evolve. This permits an additional, typically biological, explanation for it in terms of reproductive advantage, whether of the individual or of the group. There is no general agreement on the appropriate explanatory framework for understanding group-level phenomena in biology.
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Notes
‘Rule out geometric, then physical and then chemical explanations before invoking biologically specific ones’ (The Life of a Leaf by Steven Vogel; University of Chicago Press, 303 pp, 2012).
This sequence of dealing with the three areas is traditional and reflects the often-quoted assertion that ‘all chemistry is physics and all biology is chemistry’.
That we are able to understand the world at all, that by and large things happen as we expect them to, is because we – meaning all living creatures – are products of evolution. See http://www.iep.utm.edu/evo-epis/ for a short introduction to evolutionary epistemology.
It can also be that the number of parts is so large that summing up their properties is no longer a feasible exercise (as with the molecules of gas in a room). Then one settles for a statistical description of the whole. What we mean here is something different: even in principle it is impossible to look at the whole as a number of non-interacting parts.
Meaning number of constituent units.
Again, meaning one brought about by an increase in the number of subunits or components.
The sol-gel transition is an example of a percolation transition and critical behaviour.
It has been conjectured that systems close to equilibrium follow the rule of minimum rate of entropy production. We know of no thermodynamic principle that can act as a guide to predicting the state of a system far from equilibrium.
There are also individuals consisting of one cell; in such situations ‘tissue’ lacks meaning.
In the case of a cell ‘reproduction’ implies the production of a second cell (‘daughter’ or ‘offspring’) identical – in a sense to be discussed later – to the first (‘parent’). In a different form of reproduction, known as sexual reproduction, the offspring can be derived from two dissimilar individuals. In that case it resembles its parents but is distinct from both of them.
Biological form and structure may also be explicable on a physical basis, at least with regard to its origins, and to that extent may not demand an explanation based on natural selection (Newman and Comper 1990).
The relative abundances of the chemical elements are explained by invoking a particular history of the universe; on a smaller time scale, the precise course of annealing can cause the same alloy to end up with drastically different properties.
For example, the facts that we all have one nose and two eyes, occupy a certain volume of space, and share many basic concepts – number, common names and verbs.
Real or virtual photons. ‘In physics, a virtual particle is a transient fluctuation that exhibits many of the characteristics of an ordinary particle, but that exists for a limited time’ ( http://en.wikipedia.org/wiki/Virtual_particle ).
i.e. one cannot assign an absolute sense to the concept of rest or no movement; one can only say that a body is at rest (or is moving) relative to another.
In ‘first quantized’ quantum mechanics, position and momenta of particles become non-commuting operators. Their wave functions are representations of the Poincaré group, ‘irreducible unitary’ representations to be precise.
Quantum field theory ‘second quantized’ quantum mechanics. The analogue of the classical field, e.g. the familiar electromagnetic field, is a set of non-commuting operators called quantum fields. The quantum field is a continuous function of space and time just as the classical one.
They are both created by the same electron quantum field that embodies Poincaré symmetry. We might say that all we see around us, including ourselves, are approximate and highly reducible product representations of the Poincaré group.
A fit that is absent in biology (Nanjundiah 2005).
The mathematical demonstration of this is one of the million dollar Clay Institute problems.
The photon chemical potential is zero at all temperatures. Therefore at any temperature such a beam can be thought of as a Bose–Einstein condensate of identical photons of indefinite number.
With antisymmetric wavefunctions dictated by the Pauli Exclusion Principle (PEP).
Because the Pauli Exclusion Principle forbids the resulting proton from going into one of the levels already occupied by other nuclear protons.
If the mass is above the Chandrasekhar limit, the star becomes a black hole.
Or, as we will see, when the temperature drops below a critical value; there are other routes to gel formation as well.
Experimental techniques used for monitoring sol-gel transition must be very sensitive to structural changes and should not disturb the system mechanically. Fluorescence techniques are particularly useful for elucidation of detailed structural aspects of the gels. The technique is based on the interpretation of the change in anisotropy, emission intensity and viewing the lifetimes of injected aromatic molecules to monitor the change in their microenvironment (Birks 1965). These techniques have been successfully used to perform experiments on polymerization and chemical gel formation (Pekcan et al. 1994). Later studies, using pyrene as an extrinsic fluoroprobe, showed that the glass transition both for the linear bulk polymer (Pekcan et al. 1997) and gels (Yilmaz et al. 2002) could be described by a percolation model. In these studies, the fluoroprobe monitors the change occurring in the rigidity of the medium near the glass transition. Photon transmission was used to study the sol-gel and gel-sol transitions of κ- and ι-carrageenan in pure water and in cationic solution (Kara et al. 2003; Pekcan and Kara 2005).This technique was also used to monitor the gelation of acrylamide (AAm) (Kara and Pekcan 2000) and N-isopropylacrylamide (NIPA) (Kara et al. 2002).
In percolation language these are the average cluster size and the strength of the infinite network.
Although the molecular structure and the formation of physical gels are rather different from those of chemical gels, the basic properties of the gel state probed by rheological and viscoelastic measurements show close similarities. It is possible to use the theoretical approaches originally derived only for chemical gels in describing the properties of physical gel networks, with some success (Yilmaz et al. 2002; Kara et al. 2003; Pekcan and Kara 2005).
As explained earlier (note xii), not every biological phenomenon need be a product of evolution. However, evolutionary (=historical) explanations play a role in biology that they do not in standard physics or chemistry.
Organisms, cells, proteins and amino acids belong to the category of ‘things that contain other things’. The others have more to do with properties that are common to the members of a category and are useful for distinguishing between categories.
Gene mutations constitute one source of variation. As far as we know mutations take place ‘at random’, meaning that the probability of occurrence of a mutation is independent of its consequence. The physics and chemistry behind the generation of form can be responsible for variations that occur within the same genetic background and the same external environment. Variations can also be induced by the environment. If morphological variations fall within predictable categories, and if variations induced by the environment subsequently find a genetic, and thereby heritable, correlate, patterns of variation may be inherent to the organism’s biology and not random; see Müller and Newman (2003).
The nature of the link between genotype and phenotype falls outside evolutionary theory. In spite of being intensively studied it remains a major problem in biology. The chief reason for this state of affairs is that a large number of factors can influence the phenotype. A partial list would include (a) single (‘Mendelian’) genes of major effect; (b) many genes; (c) the physical environment; (d) the biological environment; (e) gene–environment correlations; and (f) stochastic effects.
Normally DNA and chromosomes reproduce only when housed in cells. On the other hand cells and multicellular organisms do so autonomously. But that too is possible only in an appropriate environment (which however does not reproduce).
In the case of cells and multicellular organisms the point is obvious. The phenomena of meiotic drive and segregation distortion provide examples of a DNA sequence or chromosome being more likely to be transmitted through meiosis than another (usually the homologue; see Taylor and Ingvarsson 2003).
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We are grateful to Stuart Newman for several helpful comments on a pre-submission draft.
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[Saçlıoğlu C, Pekcan Ö and Nanjundiah V 2014 Group behaviour in physical, chemical and biological systems. J. Biosci. 39 1–13] DOI 10.1007/s12038-013-9398-4
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Saçlıoğlu, C., Pekcan, Ö. & Nanjundiah, V. Group behaviour in physical, chemical and biological systems. J Biosci 39, 177–189 (2014). https://doi.org/10.1007/s12038-013-9398-4
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DOI: https://doi.org/10.1007/s12038-013-9398-4