Then, they would be limited by starvation. They would have found a new niche. They would have adapt to new ways of life, so called adaptive radiation . On the mainland, they could not as easily undergo adaptive radiation, because all of the other niches were already filled by other creatures. On Galapagos, the niche was vacant, and there was no competition for the finches in any niche.
Every new generation has some randomness.
We know this, because we can take a specific characteristic of an animal and breed that to accentuate it. Dogs, horses, cows are all examples of artificial selection . Consider an Alaskan Malamute vs. a Mexican Chihuahua. The changes that occur are called natural selection. Likewise, a giraffe would occur because there would be some giraffes with slightly longer necks by chance.