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 consequence 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.
New heritable characteristics can result from new combinations of existing genes or from mutations of genes in reproductive cells. Life on earth is thought to have begun as simple, one-celled organisms about 4 billion years ago. Misinterpreting the nature of evolutionary change in populations, believing that all individuals change slowly over time. They propose the use of real examples of natural selection in action, and they have described activities using the familiar dandelion. Those holding such views seem particularly vulnerable to the influence of various groups wishing to reduce attention to evolution in science classes.