Nothing can occur in nature without an instruction set, any more than a cat wandering around on a keyboard (as mine does sometimes) will produce an app. God placed genetic codes into His creation that determines what that creature will be. My understanding is that a vast array of codes were placed in the initial creatures. So, for example, Adam incorporated every race in his genetic makeup.
If the genetics are close enough, a certain degree of mix and match can occur, such as "ligers". Mules are a result of cross breeding donkeys and horses. However, few of the hybrids are fertile and cross breeding is still the best way to produce mules.
No one even thinks of crossing a whale with a penguin. Obviously they are not sufficiently alike. Speciation occurs when a particular variation is better suited to its environment. The pepper moth was once touted as evidence for evolution. It seemed to change from a light to a dark colour. However, when its environment changed, the light version of the moth became predominant once more. As the background became lighter once more, so the dark moth became more visible to predators.
Hard to know with giants. Goliath almost certainly had a thyroid problem, which is the usual cause of giantism. In reality, David was in little danger from Goliath, as long as he did not get too close. A sling and stone was the ancient equivalent of a rifle, up against a spear and a sword. David had 5 stones, just in case. He only needed one.