Evolution

Like all living things, animals undergo changes as each new generation succeeds the one before. These changes are usually so slight that they are very difficult to see, but over thousands or millions of years, they can completely alter the way animals look and also the way they behave. This process of change is called evolution. It allows animals to exploit new opportunities and to adapt to changes that take place in the world around them. Evolution works by modifying existing characteristics, usually through a series of extremely small mutations. The result of this is that every animals is a living store of evolutionary history- one that helps to show how sufferer species are related 

From: Animals