Well all I can point out is that evolution is still occuring, notibly in species of Insects, Arachnids and Reptiles. The reason it's noticable is due to their life expetancy and gestation periods, basically you can have many tens, hundreds or thousands of generations of a specie within your own life time.
As for evolution as "A tree of life", you have to note that you might be able to look at a normal tree and thing that everything branches off neatly, however evolution contain alot of chaos.
For instance Horse and a Donkey mate, what do they give birth to?
An 'Ass' or 'Mule', now what happens when two Ass/Mules mate?
Well it's stated it will give birth to either a Horse or a Donkey, however if this continued as a trend over time you would end up with something thats not pure horse, not pure donkey and no longer even Ass/Mule.
As for how the creation of complex organisms was derived, well suggested is at first there could have been many mutated forms of lesser organisms, they would have been potentially very weak alone but would have found strength in numbers.
(Notibly Coral colonies are a good example of how so many small organisms could cluster together to create shelter as a whole)
The other aspect of being close together is the ability to multiply and reproduce, even mitosis. I suggest as theory that mitosis needs accompanying cells to be able to cause it's reaction, without them the reaction wouldn't occur. (My theory is if a flake of skin falls from you to the ground, it's not like within a week it's turned into the preverbial horror star "The stuff" and consuming all in sight)
I could write far more, however I think I've probably written enough to conclude that Darwinism is old in the tooth for a explaination, and technology does move use to more precise answers, however you can't rule it out as being the forfather to Evolution theory.
Just remember that Darwinism didn't just cover the substance of a creature but the life it leads too, as organisms evolve they can also become more community driven which in turn leads to alterations in what evolutionary paths they would take (Like mutants being singled out)