Lib: the way I see it, its very simple.
That which is fittest always survives, by definition. Whatever survives, that is obviously what was the strongest/fastest/smartest or whatever. So believing in evolution is no more than believing that the fastest runner wins the race.
Its a natural process, so yes, over millions of years you are going to see drastic changes. You have to account for the cumulative effect here, becuase if you just put a fish and a human next to each other, you're going to say "no way, they're not related" . But if you factor in millions of years, gradual changes stack up. Sort of like if you see a person all the time, you don't notice ther slight ageing, but if you see them once, and then again 15 years later, you're going to notice a big difference.,