It may not be easy for one species to change into another. Imagine fish evolving to become dry land species. It is easy to see how problems arise as fins become legs and lungs are needed. Somehow, the evolutionary process got there. How? Almost certainly, the features of modularity and redundancy were critical. It is possible to slot pre-existing gene-protein networks into new control networks without upsetting them too much. That is modularity.
Suppose mutations occur in some mechanisms, and eventually they are selected for functions quite different from those they originally supported. How, then, to maintain the function that they originally served? What were back-up mechanisms now become primary. That is the value of redundancy. This is the basic explanation for how nature can modify its "aircraft design" while still ensuring that the aircraft continues to fly. By Denis Noble: The music of life - Biology beyond genes.