1/10/2019

Evolution and entropy

Filed under: — Bravus @ 8:05 am

The claim is sometimes made that entropy, or the laws of thermodynamics, prohibit the possibility of evolution.

The laws of thermodynamics can be stated in a number of ways in words, and more precisely in equations, but here is one way of stating them:

  1. Energy cannot be created or destroyed in an isolated system.
  2. The net entropy of an isolated system always increases.
  3. The entropy of a system approaches a constant value as the temperature approaches absolute zero (-273.15o C).

A more amusing but less accurate version I have seen is:

  1. You can never win, you can only break even.
  2. You can only break even at absolute zero.
  3. You can never reach absolute zero.

Returning to the first set of laws, the First Law obviously needs to be slightly modified in the light of General Relativity and Einstein’s famous equation E = mc2 to ‘matter-energy cannot be created or destroyed’, but the bottom line remains the same.

Entropy has a technical definition, or rather a number of different technical definitions, expressed in equations, but it is often understood as the ‘disorder’ of a system. So an increase in entropy is a decrease in order, and so on. Essentially, energy tends to change from more useful forms, that can do work, to less useful forms, over time.

The claim I referenced in the first sentence – that entropy forbids evolution – relies on the notion that evolving from a single-celled organism to something like a human being (with a human brain, perhaps the most complex matter we know of) requires a considerable increase in order, and therefore a net decrease in entropy.

The answer is right there in the Second Law, though: the words ‘in an isolated system‘. A single-celled organism does not evolve into a human being if it is placed in a sealed chamber and isolated from incoming energy – in the forms of heat, light, food and air – from the environment.

Perhaps the people who make this claim want to regard the whole of Earth as an isolated system, and argue that the evolution of all lifeforms from simpler (less-ordered) lifeforms is prohibited by the Second Law of Thermodynamics?

But the answer to that involves stepping outside and looking up, even on a cloudy day. Earth is not an isolated system, because it receives vast amounts of energy from that big nuclear fusion generator in the sky, the Sun.

Local increases in order – decreases in entropy – are certainly not prohibited: a human brain is much more ordered than all the food that goes into making it.

In practical terms, no system in the universe is closed and isolated. Even the Solar System emits solar energy to the space around it. But certainly, in considering Earth, the energy coming in from the Sun is a huge part of the overall energy picture.

(As a side note, high energy, short wavelength visible light arrives with the ability to do work, including the work of photosynthesis. It passes through various processes, and then is emitted as low energy, long wavelength infrared radiation, which radiates off into space. Unless intercepted by greenhouse gases, in which case it hangs around a bit longer, warming the globe…)

And it turns out that the nuclear fusion process of hydrogen combining to form helium that produces the Sun’s energy involves a net increase in entropy – a net decrease in order. And, given that Earth receives only a tiny fraction of the energy the Sun puts out, this increase in entropy occurring in the Sun completely dwarfs the local decrease in entropy involved in evolution. So, in the local system of our Solar System, net entropy increases, in agreement with the Second Law

No rules of thermodynamics are contravened by the processes of evolution.


For ease of navigation I will include links to each of the other posts in this series at the bottom of each post.

Why I think it’s important to understand evolution
Cosmogenesis, abiogenesis and evolution
Facts, Theories and Laws
Radiocarbon dating
Radiometric dating and deep time
Four Forces of the Universe
Probability and evolution
Species and ‘baramin’, macro- and micro-evolution
Mitochondrial Eve and Y-chromosomal Adam
Transitional fossils
Complexity – irreducible and otherwise