 Sleek, lithe, and with acceleration speeds that can outstrip a Ferrari, cheetahs are the world's fastest land mammal. But why is it so important for cheetahs to be able to run so fast? Well, one of the cheetah's natural habitats are the savannas of Africa where you can find lions and hyenas who both compete with cheetahs for prey and sometimes even try to kill them for food. Cheetahs need to be able to outstrip their prey and their predators. To do so, they run very fast. A flexible spine and long legs give the cheetah the perfect shape and build for speed. Yet even within the species, some cheetahs can run faster than others. Which one of these cheetahs do you think will win the race for food? In nature, all organisms compete with each other for resources in order to survive. Competition between members of the same species is what makes organisms evolve. This theory of competition and survival of the fittest is called natural selection and was originally proposed by Charles Darwin in his book On the Origin of Species in 1859. Different individuals show a wide degree of variation. He proposed that individuals with characteristics most suited to their environments are more likely to survive, reproduce and pass on their characteristics. Meanwhile, individuals that are less well adapted to their environment might not survive and reproduce. Their less well adapted characteristics will eventually become removed from the population. This process causes species to change gradually and enables new species to eventually emerge. After it was published, On the Origin of Species caused a storm of controversy. And Charles Darwin was ridiculed. But his theory of natural selection is now widely accepted. During the 20th century, we discovered that the characteristics of an organism that are passed from one generation to the next are controlled by genes. Scientists were able to explain Darwin's theory of natural selection in terms of the selection of favorable genes. Natural selection can directly impact humans when it comes to treating diseases. The discovery of the antibiotic penicillin in 1928 was lauded as a great advancement in therapeutic medicine and it soon became widely used. However, within just a few years, some bacteria could withstand the effects of penicillin. They had adapted a resistance to the antibiotic. These fitter bacteria survived and in the late 20th century, bacteria populations began to emerge that were resistant to penicillin. Nowadays, antibiotics are used more sparingly and scientists are attempting to find other ways to treat infections. But how do you suppose they can do this? So from this video, you should know that variation within a population means some individuals have an increased chance of survival and passing on characteristics onto their offspring. You have also learned that inherited antibiotic resistance in bacteria poses problems for modern medicine and is an example of adaptation and natural selection in action.