Monday, July 16, 2007

Spontaneous order, fungal networks, and circuses

You wait ages to feel moved to post, and then three things come along at once. In no particular order…

Biological modularity can emerge as a spontaneous, self-organized process, say Ricard Solé and Sergi Valverde in J. R. Soc. Interface. By modularity they mean particular sets of interactions in the great web of biochemistry (rather than, say, organs):

Modularity is particularly obvious in cellular networks, where it can be detected at the topological level. These networks include the webs of interactions among proteins, genes, enzymes and metabolites or signalling molecules.

Such a trend seems to call for an adaptive explanation. In particular, what seems to demand explanation are ‘motifs’ in these networks

patterns of interconnections occurring in complex networks at numbers that are significantly higher than those in randomized networks.

Solé and Valverde cite a couple of adaptive explanations “based on a genetic programming approach”. But they don’t agree with them.

Instead, they say such modularity is the inevitable consequence of evolution by gene (or genome) duplication (“the driving force behind the evolution of complex organisms”, they say) followed by divergence, as evolution tinkers with the newly enlarged network of interactions (letting natural selection back in, it looks like)

I’ve clearly, as is my wont, become aware of this debate somewhere in the middle — the paper cites several is-modularity-evolution-or-is-it-self-organization? papers from the past few years. But, as far as I can tell, Solé and Valverde are saying that this is the first explanation for such modularity based on “fundamental, dynamical rules”.

In other network news, Proc R Soc B has just published a paper by Daniel Bebber et al. looking at fungal networks.

they have evolved to explore and exploit a patchy environment rather than ramify through a three-dimensional organism. Unlike all the other biological transport systems studied, the fungal network is not part of the organism, it is the organism.

The challenges such a fungal mycelium faces are to explore its environment, exploit resources, transport them around the rest of the organism, and to be sufficiently robust to resist physical damage and the many fungus-eaters in the soil.

To investigate how fungi meet such challenges, they grew Phanerochaete velutina in the lab, growing it out from an inoculated lump of wood towards other lumps of wood. They found that the fungus began in exploratory mode, sending out lots of narrow filaments. But as it completed searching its environment it consolidated its resources into fewer, bigger bundles of hyphae, growing between maor resource patches, with more exploratory filaments in new territory, and crosslinks to provide some redundancy in the case of network damage. “Netowrk development”, say Bebber et al.
involves over-production of links and nodes in the exploratory phase, followed by selection and positive reinforcement of some links and recycling of the remainder during the consolidation phase … A
similar sequence of events is apparent in the development of other biological transport networks including those formed by acellular slime moulds [and] foraging ant trails … and may well represent a universal feature of self-organized biological networks. However, in each of these systems, the final network structure is likely to represent a context-specific balance between the need for efficient transpor t, cost and robustness.

In other words, they’re stressing the diversity of different structures produced by flexibility and responsiveness to environmental variation, as much as whatever general laws such networks might have.

Philip Loring (who I believe blogs here) approaches questions of resilience, connectedness and persistence from a different angle in Ecology and Society. He proposes the circus as an icon of resilience, in that it has retained its identity while changing its contents — i.e. getting rid of freak shows and (mostly) animal acts.
Through the many forms they have taken over the last 150 yr, circuses have changed significantly while sustaining a singular identity. As a successful and enduring social system, their intriguing history exposes the nuances of sustainability theory, from resilience to pathologies, and illustrates that sustainability requires a complex dynamic between identity, tradition, and change.

The article, to my mind, contains a bit too much on the history of the circus, without exploring the implications of the circus-as-sustainability metaphor. Also, while ‘circus’ has survived for 200 years, most individual circuses haven’t. This is fine for institutions and systems, but not so much for species and ecosystems. But it’s a nice comparison.

1 comment:

冰封王子 said...

So intriguing... I feel shined, as if I am lucky enough to have a glance at a beam of light, shining around CREATOR'S finger.

Now I understand why one of my friend think resiliance is more interesting than neutral theory in community ecology. :)