Understanding of Mutation
Genes are responsible for cellular differences as a result of random DNA mutations, not always a part of the evolutionary process. This means that mutations can be beneficial, neutral, or harmful to the organism. Some mutations such as cells differentiating into blood cells and other types were most likely caused by mutations which were favored by evolution. Such that the first cells in organisms may have become differentiated to serve different tasks by accident and these changes in DNA allowed the organism to survive over organisms without the mutation. As such, future generations would incorporate differentiated or specialized cells and the process of evolution would continue in this way into more complex organisms. Mutation is an intrinsic part of evolution but its exact mechanisms are unknown. For example, it is known that some mutations are damaging because they are caused by dangerous external forces such as radiation. Then there are mutations which may occur for unknown reasons.
Simon, E. J., Reece, J. B., & Dickey, J. L. (2010). Essential biology with physiology. (3rd ed.). San Francisco, CA: Pearson/Benjamin Cummings.