The theory of evolution by natural selection, first formulated in Darwin's book "On the Origin of Species" in 1859, is the process by which organisms change over time as a result of changes in heritable physical or behavioural traits. In biology, evolution is the change in the characteristics of a species over several generations and relies on the process of natural selection. Evolution depends on there being genetic variation? in a population which affects the physical characteristics (phenotype) of an organism. The study of the history of life forms on planet earth is covered under Evolutionary Biology. Study of fossils, comparative anatomy, and related biochemistry present evidence for evolution. The chapter summarizes the concept of the origin of life. It briefs about the theoretical aspects of the evolution of life forms. Evolution is a process that results in changes in the genetic material of a population over time. Evolution reflects the adaptations of organisms to their changing environments and can result in altered genes, novel traits, and new species. An example of macroevolution is the evolution of a new species. It presents students with adaptive radiation and biological evolution and the different concepts covered.