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Natural Selection is a natural example of Maxwell's Demon
Full details can be read at Evolution and the Second Law of Thermodynamics, but the following is a summary of the analysis:
- The DNA of a more evolved (i.e. fitter) organism has lower entropy than that of a less evolved organism.
- This lowering of entropy occurs as a result of mutation, which is (or at least can be in principle) an irreversible process.
- Therefore the Second Law of Thermodynamics is broken slightly everytime an organism evolves into a fitter organism.
This seems to prove that evolution does contradict the Second Law of Thermodynamics. But all is not lost. In fact the proof is very analogous to the proof that Maxwell's Demon can break the Second Law.
Consider the particular case of the demon who is letting fast molecules go one way through a door in a wall and slow molecules go the other way:
- Each time a molecule is allowed through, the entropy of the overall system decreases a tiny bit.
- The process of travelling through the door is reversible.
- Therefore the Second Law is being broken by the demon.
The catch to this argument is that something has to be irreversible in order for the reductions in entropy to accumulate into a macroscopic reduction. When the demon shuts the door, the door has to stay shut, and not open again. And the only way this can happen is if the process of shutting the door results in an increase in entropy. In particular the kinetic energy of the door has to be dissipated in some form which is not likely to reassemble back into the original motion of the door.
In the case of evolution by natural selection, it is the natural selection which has to be irreversible if accumulation of entropy decreases is to occur. And, in particular, the processes of birth and death must be irreversible. This suggests the interesting thought experiment consisting of an imagined world where birth and death are thermodynamically reversible, and organisms get born and then unborn again, and they also die and then un-die. In such a world evolution would not occur, because fitter organisms that reproduced more successfully would also un-reproduce more successfully, and they would gain no long term advantage over the less fit organisms.
I believe that this is the most thorough analysis of the relationship between evolution by natural selection and the Second Law of Thermodynamics. In summary:
- Evolution from less fit to more fit via mutation does involve a total decrease in the entropy of the system,
- but, accumulation of these "lucky" decreases in entropy can only occur if the processes of birth, growth and death are thermodynamically irreversible.