Organisms' adaptations to their environments are the products of natural selection. Adaptations are the result of the editing process of natural selection. When exposed to specific environmental pressures, certain inheritable variations favor the reproductive success of some individuals over others. Evolution is the one unifying biological theme . Life evolves .
Species change over time and their history can be described as a branching tree of life.
Species that are very similar share a common ancestor at a recent branch point on the phylogenetic tree. Less closely related organisms share a more ancient common ancestor. All life is connected and can be traced back to primeval prokaryotes that existed more than three billion years ago. Descent with modification is the source of all biological diversity. The cell is life's basic unit of structure and function.