Biodiversity refers to the multitude of species that exist in nature. Species must be able to adapt to ever-changing environments on Earth or face extinction. Competition for resources such as food, space, water and other important factors necessary for survival ensures that the best adapted individuals survive and the others die off. Variations among living things may give certain individuals a survival advantage in a particular environment. If those individuals survive, reproduce and pass those traits on to their progeny, they are deemed to be "fit". Over the years, as these variations which are selected for by the environment accumulate, eventually speciation may occur. Since the Earth is not a static place and is ever-changing, natural selection is constantly selecting the best adapted individuals to survive. Consider the introduction of antibiotics in the 20th century. Bacteria are not all genetically alike. Many individuals die in the presence of these drugs, however, some have a resistance gene which is a variation. The individuals with a resistance gene in the presence of antibiotics would be able to survive, reproduce and pass the gene to their offspring. This results in a resistant strain of bacteria. Although this is not an example of the formation of a new species, it shows in a short period of time how a selecting agent in the environment can cause certain individuals to die off, while others survive successfully.