A common argument against biological evolution is that the theory contradicts the second law of thermodynamics. The second law says that disorder, or entropy, always increases or stays the same over time. How then can evolution produce more complex life forms over time? The answer is that the second law is only valid in closed systems with no external sources of energy. Since the Earth receives continual energy from the Sun, the second law does not apply.
A common argument against evolution is that the theory contradicts the Second Law of Thermodynamics that claims disorder, or entropy, always increases or stays the same over time. This law has plenty of everyday examples. Buildings break down over time, and food spoils if not eaten soon enough. In both cases, the amount of disorder increases with time, but the opposite is never true. Buildings don’t strengthen themselves, and no amount of waiting will cause rotten food to become edible again. But because evolution results in an increase in the order and complexity of species — which is a decrease in entropy — some critics claim evolution violates the Second Law of Thermodynamics.
Defining the System
However, this objection is grounded in a misunderstanding of the second law, which states any isolated system will increase its total entropy over time. An isolated system is defined as one without any outside energy input. Because the universe is an isolated system, the total disorder of the universe is always increasing.
With biological evolution however, the system being considered is not the universe, but the Earth. And the Earth is not an isolated system. This means that an increase in order can occur on Earth as long as there is an energy input — most notably the light of the sun. Therefore, energy input from the sun could give rise to the increase in order on Earth including complex molecules and organisms. At the same time, the sun becomes increasingly disordered as it emits energy to the Earth. Even though order may be increasing on Earth, the total order of the solar system and universe is still decreasing, and the second law is not violated.?
Misapplication of The Second Law
To claim that evolution violates the Second Law of Thermodynamics is also grounded in a misunderstanding of where the law applies. Nobody has ever figured out how to apply the second law to living creatures. There is no meaning to the entropy of a frog. The kinds of systems that can be analyzed with the second law are much simpler.
A living organism is not so much a unified whole as it is a collection of subsystems. In the development of life, for example, a major leap occurred when cells mutated in such a way that they clumped together so that multicellular life was possible. A simple mutation allowing one cell to stick to other cells enabled a larger and more complex life form. However, such a transformation does not violate the Second Law of Thermodynamics any more than superglue violates the law when it sticks your fingers to the kitchen counter.
There are many examples of order arising from disorder in nature. Research conducted by Ilya Prigogine1 and others on systems far from equilibrium has shown that order can spontaneously arise in systems that are driven in the right way. It turns out that living systems are characterized as being far from equilibrium.
The Second Law of Thermodynamics also has interesting implications for cosmology, as it requires that universe began in a highly ordered state.