But this causes a real problem for evolutionary biologists. Just because two organisms have a similar structure, like a wing, does not necessarily mean that they are related to one another. We have to be very careful not to let these analogies confuse us when we puzzle out which animals are related to one another. One final line of evidence for evolution, completely lacking in Darwin's day, is molecular evolution. Molecules themselves change over time, because the genes that code for them are changing or mutating.
Mutations are an alteration in the genetic instructions that shape the molecules of living systems.
Changes at the molecular level occur slowly. If these building blocks are altered too radically, they might lose their ability to work at all. So the pace of molecular evolution is often very slow. The greater the similarities between the biochemistry of two organisms, the more likely it is that they are related. For example, consider the reaction between antibodies and the invading antigen.