The great diversity of organisms is the result of more than 3.5 billion years of evolution that has filled every available niche with life forms. Natural selection and its evolutionary consequences provide a scientific explanation for the fossil record of ancient life forms, as well as for the striking molecular similarities observed among the diverse species of living organisms. The millions of different species of plants, animals, and microorganisms that live on earth today are related by descent from common ancestors. Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities which reflect their evolutionary relationships.
Species is the most fundamental unit of classification.
Though the national standards and other guidelines present biodiversity in the traditional way as an important topic in the life sciences, they do not directly address the importance of biodiversity from either an ecological point of view or quality of life perspective. The concept of biodiversity takes on increased importance and meaning when we think of the many reasons to be concerned about the current threats to biodiversity. It is also important to protect the diversity of life because it helps maintain important ecological functions, such as oxygen production, pollination, and flood control, which in turn help support all life on Earth. Biodiversity, then, is more than a biology or science topic. It is a concept that cuts across disciplinary boundaries, and it is an environmental issue with board ramifications for the quality of human life.