Evolution can be defined as the process of changes in the pattern of inheritance of certain groups of organisms in a certain population for several generations. In those certain populations, there are organisms that may have been in the same genus but different species. They may have evolved from a similar root and have evolved differently as the time goes by. With this, natural selection comes along. Natural selection is a process defined by Charles Darwin in which the organism(s) that can best adapt in a certain ecology or population tend to survive than those who can't. Different species in a certain population may increase or decrease in a certain time frame which is affected by different factors.
Evolution and natural selection are used together because using these concepts can best determine the origin of the present species in this world. In biology, the study of life is the center of attention. We can therefore explore life if we can be able to track the roots of every species currently thriving on earth.