Monday, February 1, 2010

Evolution does not optimise everything

When discussing biological molecules performing some function I often read statements by physicists and chemists along the lines, "evolution over billions of years has optimised this physical property so the molecule can perform its function."

My understanding of evolutionary biology is limited. However, I think these statements present a mis-understanding.
Evolutionary pressures lead to changes that will increase the chances of survival, or perhaps more correctly over time the biomolecular systems which can survive best in their environment survive....
But the crucial point is that the "fitness function" of a biomolecule depends on many variables. The optimum fitness function will not optimise all variables.

Consider the specific example of light-harvesting complexes for photosynthesis. The fittest will not necessarily be those which transport energy the fastest. There are many other variables such as robustness to UV light, sensitivity to changes in pH, matching the solar spectrum, .....

A similar issue arises with photovoltaic cells. Everyone is trying to achieve a greater efficiency and there are often statements such as "if we can make organic cells operating at 12 per cent (or some other number) we will save the planet". But there are many other key variables: cost, durability, stability, scarcity of materials, .....
If someone found a way to make 2 per cent solar cells that were made of naturally occuring materials, cost 1 cent/square metre to manufacture, and were completely reliable for 30 years, ..... well I think we would be done....

Please correct my mis-understandings....


  1. Dennett explains the 'mindless nature' of evolutionary process quite nicely in this article. Instead of molecules (and their functions, which appear to be main concern in your research), he uses a subsystem-level entity (the human eye) as his example:

    Here's the relevant excerpt:

    "Take the development of the eye, which has been one of the favorite challenges of creationists. How on earth, they ask, could that engineering marvel be produced by a series of small, unplanned steps? Only an intelligent designer could have created such a brilliant arrangement of a shape-shifting lens, an aperture-adjusting iris, a light-sensitive image surface of exquisite sensitivity, all housed in a sphere that can shift its aim in a hundredth of a second and send megabytes of information to the visual cortex every second for years on end. [...]

    "We can't yet say what all the details of this process were, but real eyes representative of all the intermediate stages can be found, dotted around the animal kingdom, and we have detailed computer models to demonstrate that the creative process works just as the theory says.[...]

    "Brilliant as the design of the eye is, it betrays its origin with a tell-tale flaw: the retina is inside out. The nerve fibers that carry the signals from the eye's rods and cones (which sense light and color) lie on top of them, and have to plunge through a large hole in the retina to get to the brain, creating the blind spot. No intelligent designer would put such a clumsy arrangement in a camcorder, and this is just one of hundreds of accidents frozen in evolutionary history that confirm the mindlessness of the historical process.

  2. Ross, you're absolutely right. Evolution by natural selection favors fitness (the ability to contribute more copies of yourself to the next generation than everyone else), rather than optimality.

    Other constraints on optimality that that come to mind are physical constraints (evolving traits that break the laws of physics, e.g. the speed of diffusion across a membrane prevents the evolution of enormous amoebae), and phylogenetic constraints (evolving new traits that conflict with all your existing traits).

  3. Just to play devil's advocate...
    while it's certainly true that overall fitness is selected for, there are aspects of fitness we are able to identify. In the example of the light harvesting complex, the fact that it is so supremely efficient at transferring excitation energy is surely relevant to its overall fitness. Certainly all the other features you mention are relevant as well, but when we see something with 99% efficiency in an essential organismal process, it is not unreasonable to assert that this efficiency (and its associated transport speed) has been selected for.

  4. You guys are missing his point (the last paragraph).

    He's saying: why the 12% number? Why not 11 or 10 or 5, if it makes us free ponies at the same time? Why is this threshold cited? It's a question someone familiar with organic photovoltaics should be able to answer.

  5. Regarding evolution, there has recently been an article on the Red Queen idea. The Red Queen highlights the importance that the fitness landscape itself is always changing (the Red Queen in L. Carroll's books says you have to run as fast as you can, just to stay in place). Moreover, the fitness landscape depends on the fitness itself!

    I agree re: photovoltaics. It seems that the cells you can make with fruit skins are actually not that bad, but that's not counting the cost of electrodes, TiO2, etc. Of course, this does not take into account the REAL driving force of science, which is that invention is the mother of necessity - not the other way around.

  6. I was not responding to the comment about OPV, but it's true that I frequently hear the 10% number bandied about. It's possible that some people who state that "10%=saved world" really mean it, but I think it's really just a shorthand that, of course, makes assumptions about many of the other features in the original post.

    I think people just don't feel like saying "We work on OPV assuming that the cost of it will be much less per square meter than the competing solid semiconductor technologies. Assuming that the cost per square meter stays as low as we believe is possible for a roll-to-roll printable technology, and assuming that OPV can be shown to have 20+ year lifetimes (with reasonable performance degradation) in the field, then reaching a 10% efficiency of the product would be revolutionary to the PV markets and to the world energy system."

    I would also comment that even if the solar cell were free (100% free), it would still cost a lot of money (often estimated at 3-5USD/Watt for systems that have ~10% efficiency, which is currently about half of the installed cost of PV) to install the modules (the so-called "balance of systems" costs). Since balance of systems costs mostly scale with area rather than power, having a 1-2% efficient solar cell that was actually free might not be worth putting on racks in the field. But BOS costs are coming down just like cell costs, so clearly there can be a tradeoff on efficiency and cost, at some point.