In the universe, a large amount of liberated energy (that is, energy not tied up as matter), exists as heat. Most energy eventually filters down to heat, after being transformed through all its various states. For example, if I use the chemical energy stored in my body to push a car, I give it kinetic energy. This kinetic energy is then itself transformed into heat energy through friction with the air and with the ground, and in the car’s internal mechanisms. This is one way to show some of the most fundamental laws of classical physics the laws of Thermodynamics, which explain how heat works and is governed.
However, one of these laws, specifically the second law, can be stated as “the disorder of the system will increase over time”. But how do these physical laws relate to biology, and evolution in particular? To explain this, we must introduce the concept of Entropy. Entropy is defined as a quantitative measure of disorder in a system. To rephrase the second law of thermodynamics in these terms, it tells us that the entropy of a closed system will either stay the same or increase over time. This presents a problem when we try to explain evolution. In evolution, the system gets more and more ordered over time, so the entropy in fact decreases over time. This seems to violate the second law of thermodynamics, a supposedly inviolable law of the universe. So how can this be? The second law has been shown to be true in a variety of other circumstances, so we cannot assume simply that the law has been broken.
Firstly, there are two problems with this approach in applying the second law to this scenario. Most importantly, the second law in this sense applies only to closed systems, where energy and entropy cannot be exchanged with the rest of the universe. Life, and indeed earth itself, is not a closed system, so it still falls in line with the second law. Secondly to this, entropy of the universe as a whole, a true closed system (as far as we know) does indeed increase.
As life metabolises, it absorbs energy and grows, which causes a decrease in entropy. However, due to another law of thermodynamics, this transformation is not 100% efficient some energy is always wasted, which is given off as heat. Thus, the entropy of the closed system (the universe) does increase, even though on a local scale, the entropy appears to be decreasing. As evolution can be seen as a massive chain of the growth and reproduction of constantly mutating and changing organisms, then we see that it does not violate the laws of thermodynamics, though some would try to tell you otherwise.
