The beginning of this chapter asked why biological sequences are similar to one another. Let's answer that question now. You've seen that biological sequences like proteins may have important functions necessary for the survival of an organism. You've also seen that DNA sequence can mutate randomly, and this may change how a sequence functions. Over time, both functional constraints and random processes impact the course of sequence evolution. The degree to which a sequence follows a functional or random path depends on natural selection and neutral evolution. So the reason why sequences are similar to one another is because they start out similar to one another and follow different paths. When you read a BLAST report, you will find that your knowledge of molecular and evolutionary biology will help you interpret the similarities and differences you see.