Humans have evolved for millions of years. This happened due to mutations in the genes. Charts have shown that in Roman times Britians have evolved to be taller and the effect cigarettes have on humans. Once every new mutation occurs it must spread through the population.
Whales were originally land animals. They had long skulls and carnivorus teeth. They still have carnivourus teeth. Whales were on land about 47 million years ago. Whales evolved into underwater animals. They moved to water because there was not sufficiant food on land for the whales. Their legs and arms got smaller as their tails got longer.
Giraffe's aren't as different as any other mammals. Giraffe's stretched their neck to make it as long as they are today. They stretched their necks by reaching for leaves in the trees. Later , their offspring would inherit their long nevks and stretch it even more to make theirs long and it goes on for generations which made giraffe's neck the length they are now.