Ever since the rst biological reactions and organisms became established on Earth, evolution and the processes that drive evolution have been continuous (Figure 2.1 and Figure 2.2). Evolution is a never-ending process. Even when it appears to slow down, it continues. The processes of mutation and selection are everywhere, including in the cells of your body and the billions of organisms that live in and on you. Mutations may cause disease or may be virtually undetectable, but they occur unabated throughout every cell cycle. The organisms that live in your body mutate and change continuously, and natural selection kills those that can no longer survive, while allowing others to thrive, including those that help digest the food that you eat. When you are sick, your immune system attacks the pathogenic organisms, but leaves other cells alone, which is necessary because, if it attacked all foreign cells, you would die. When you take an antibiotic, this kills many other types of cells, including the pathogens as well as the benecial microbes. This is a type of selection also, and eventually many of the microbes may become resistant to the antibiotics due to evolutionary events, including mutation and acquisition of resistance by transfer of genes from other microbes. Evolution occurs around the world constantly. It was once thought that evolution was a slow process. However, we now know that evolution, and even speciation, can occur rapidly. Some of these processes can be observed in short-term as well as long-term experiments and observations. Fossils are only one tiny example of the myriad organisms that have existed on Earth. Even so-called living fossils, such as crocodiles, sharks, and cycads, are changing and are not identical to the same organisms that were living millions of years ago. Evolution is a dynamic and constant process.