Sunday, 20 September 2015

My Favourite Metaphor For Evolution

Scientists have just published an incredible 'Tree of life' for 2.3 million species - a grand achievement in itself (see link at the bottom of the page). The tree of life is one of the very apt metaphors for evolution of life on this planet over several billion years - the other one is the web of life. Both are good in their own way of metaphorically illustrating patterns of evolution from the origin of life right through to the rich variety of life seen today - all of which share the same common ancestry.

My favourite metaphor, though - one which I don't think anyone else has used in relation to biological evolution - is what I call the 'cloud metaphor' for evolution. Scientifically the water cycle is an easily conceivable pattern. Our sun heats the earth’s water; water evaporates into vapour into the air; clouds are formed after the air currents that take the vapour up into the cooler atmosphere condense; those clouds precipitate after growth and collisions in the higher atmosphere, producing a water cycle for the earth. The cloud metaphor based on that cycle is already a very popular one in common parlance. We have distilled terms like ‘casting a cloud’ over a situation, to mean a negative occurrence, or ‘clouded’ to become troubled, or ‘cloudy’ blurred, or ‘clouded’ in suspicion. Nowadays we even have cloud-computing and web data stored in the 'clouds'. How can we apply that to biological evolution?

In the material realm of physics, chemistry and biology, metaphors are there to provide illustration and visualisation to mathematical models. In evolution we are used to the tree of life or web of life metaphors, because the phylogenetic 'tree of life' is good for studies of taxa and the many ancestral lineages (although in some cases, with cladistic ‘hierarchies’ it may be difficult to practically place species in their correct topological relations, even though the theoretics are robust).  But as for evolution as a whole, in what they call 'morphospaces' - a term coined by Matt Ridley - one can zoom in further on the mathematical details, whereupon we find my cloud metaphor comes into its own. I find it to be a beautiful metaphor, and I suspect the only reason we don’t use it much is because its complexity is difficult to conceive. 

Trees of life provide easy branching effects, whereas clouds of probability are much more like the studies of equilibrium and disequilibrium in thermodynamics, where small pockets of order cause slowly populated genetic algorithms that facilitate evolution. This is like a ‘cloud’ of probability searching the space and finding the states of survival systems.  Clouds, of course, are dynamic objects - they do not have a singular position or formation, they have no single coordinate or single velocity - they have a smeared formation over a complex volume of activity and internal movement, and they are continually in states of probability, which makes the metaphor for evolution at a mathematical level particularly compelling because it aptly describes how the laws of physics direct the evolutionary process, as random walk statistics are factored into the dynamics of the probability cloud. Because the laws of physics are driving evolution in morphospace, it means that the particular mathematical weighting on the construction paths is providing those stable pathways, due to the search space being severely skewed by the constraints placed on the system by the laws of physics.

So where in the tree of life metaphor, addition, subtraction and division would be modelled on natural selection, mutations and the consequent genetic variation – in the cloud metaphor it is more closely related to computations that program genetic algorithms, and the laws of physics acting as the programmer.  With this one mass of vapour that we observe in nature’s water system we can constructed a very rich and diverse range of metaphorical ideas from what we observe in nature.

  * Here is the full 'Tree of life' article