The ships aren't so much evolved as they are added on to. Look at US Navy fighters today, or the US fighter planes of World War 2. Better yet, look at Boeing's original 707 and compare it to a modern 777.
Design elements that work well are kept; those that do not are changed. This gives rise to a series of designs called a 'ship line'. Starlancer showed this off very well, in the progression from the Predator on to the Patriot and the Phoenix, and to a lesser extent, the Shroud (I've missed one in there somewhere).
I know in my case, as a 3d modeller, shiplines arise from commonalities of elements. Being able to reuse the same bits over and over again, in different ways, makes it easier to build things. Take a look, for example, at my Waterdancer, Braavosi, Misato, Kierra, Molly, Jaycy, Mavra and (to a lesser degree) Nathaniel. All of them have share things in common. The engines on the Waterdancer are actually reused on the Kierra. The Kierra, Jaycy, Misato and Molly all have the same cockpit and central fuselage. That central fuselage was taken from the original Nathaniel. The Nathaniel's engines are what gave rise to the original engines on the Waterdancer.
I'm sure that a similar process was followed by the Volition team working on Shivan ships.
