You can observe the curvature of space/time? What is the mechanism that causes gravity? Gravity is simply a name given to the observable phenomenon that two masses will attract each other. Speciation is the term given to the observable phenomenon that a subpopulation of an organism can change to the point where it can no longer breed with the original population. This has in fact been observed. Much like the case with gravity, it is the mechanisms that aren't fully understood.
You mean the worms and fruitflys that are cited on theTalk Origins website? That's pretty thin evidence, especially since the examples seem to be drawn mostly (all?) from laboratory experimentation.
Moreover, the theory of evolution by gradual change flatly contradicts the fossil record. Any theory that purports to explain observable phenomenon should not dismiss contradictory evidence with hand-waving.