Monday, November 06, 2006

Is natural selection a law of physics?

More on applying the physics of statistical mechanics and thermodynamics to ecology and evolution. This paper by Guy Hoelzer and colleagues appeared recently in the Journal of Evolutionary Biology:

On the logical relationship between natural selection and self-organization

…In this study, we attempt to describe the logical framework that relates the general process of self-organization to the specific process of natural selection. We describe natural selection as a mechanism that coordinates the coevolution of species in an ecosystem to effectively capture, process and dissipate solar energy into the earth’s shadow. Finally, we conclude that natural selection is an emergent process founded on the same thermodynamic imperatives that are thought to underlie all self-organization. This perspective suggests … the possibility that there may be a physical basis for understanding the origin of the process of natural selection. Rather than being merely a fluke of nature, the origin of natural selection that may be driven by energy flows across gradients.

As an example of the kind of links between biology, thermodynamics, and self-organization that they are pursuing, Hoelzer et al. give photosynthesis. They point out how good it is at levelling out the gradient of solar energy, and suggesting either that "photosynthesizing life is a statistically favoured component of the biosphere, or that a high-flux channel for light transduction is a favoured endpoint, towards which perturbed ecosystems recover. Physical principles expressed in terms of stable end states imply a natural teleology, which we have suggested (somewhat imprecisely) is the reduction of the spectral and spatial energy gradient imposed by the situation of earth in a solar radiation bath."

They also invoke metabolic scaling theory, which explains living things' energy use in terms of the geometry of transport networks (something similar seems to apply to the geometry of lightning strikes and river basins), and suggest that a taking a self-organizational view is a good way to try and understand why, the more energy an organism uses (whether metabolically, or, for humans, in society), the fewer offspring it has. (More about all this here and in ITBOAH

Other striking quotes:

We offer the conjecture that the truly ultimate explanation for any dynamical event, and the qualities of any emergent dissipative structure, including organisms and ecosystems, is thermodynamic.

Natural selection is to self-organization as language is to communication. Language is not synonymous with communication, yet it is a quantum leap forward in communicative effectiveness. It is also not merely a more complicated for of simpler modes of communication. … Language has emerged from the drive to communicate just as natural selection has emerged from the drive to break down gradients.

Much of this is not particularly new. Dynamic, complex physical systems can take on orderly, structured states, and physicists such as Stuart Kaufmann have long suggested that the same principle applies to living things — that self-organization, as well as, or instead of, natural selection, can explain the complexity and structure of life.

Likewise, it's not a new idea that thermodynamics can explain the structure of life, and that this structure evolves to flatten out energy gradients as much as possible, and degrade energy/produce entropy as quickly as possible.

And biologists have usually resisted these concepts. Natural selection is so successful that bolt-ons from physics seem superfluous. There's no reason to believe that natural selection should maximize anything thermodynamic, and many biologists take issue with the assumptions and predictions of this approach. Someone once pointed out that the reason horses had evolved was to make more horses, not to make horse manure. John Maynard Smith thought harder about these things than most biologists, and (I think) remained sceptical. (Try here or here.)

So either all the current upsurge of this stuff is just a cyclic blip, or we're really inching towards some kind of new insight. I'm not sure which.

No comments: