[MacDorcha]

That question is the basis for several people I know (and I don't dispute it) who argue that this is evidence for ID.

The "mechanism" that prevents macro-evolution is "upper limits"

(ie, you can't cross breed 600lb gorillas to get a larger breed to the point of getting a 3000 lb gorilla.)

[Junior]

But, if the environment favors it, you do get larger and larger creatures. For instance, the first dinosaurs from about 225 million years ago were fairly small, ranging in size from chickens to horses. Less than 100 million years later you have the big sauropods topping the scales at, in some cases, 100+ tonnes. In these cases the upper limits were set by engineering, not biology.

A more recent example would be whales, who massed maybe 50 kg when they were still land dwellers, but now in many cases mass 30+ tonnes.

As for your gorilla example, given enough time and the right pressures, a 1500 kg ape might not be out of the question.