Estimates of total diversity range from 5 million to over 30 million species. To make this diversity more comprehensible, biologists classify species into categories. A six-kingdom system recognizes two prokaryotic groups and divides the Monera into the Archaebacteria and Eubacteria. There is unity in the diversity of life forms at the lower levels of organization. Evolution is the core theme of biology Evolution is the one unifying biological theme.
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. A mechanism of evolutionary change is natural selection . Populations have the potential to produce more offspring than will survive or than the environment can support.