The tree of life illustrates evolutionary relationships among all organisms as they are currently understood.
Life is incredibly diverse, from the tiniest bacteria to the tallest trees. Biodiversity is the number and variety of species in a given area. However, biodiversity can also refer to genetic diversity within a species. Evolutionary theory explains both the similarities and differences among organisms. All life shares an ancient common ancestor. In the same way that a family tree can be drawn to show all the descendants of a single individual, a great tree of life can be built to illustrate the relationships among all organisms as descendants of the first common ancestor. The area of study for naming and categorizing life, taxonomy, is always changing. As scientists have collectively learned more from continuing research and rapidly improving technology, classification schemes have experienced their own evolution. Before DNA technology, relationships among organisms were inferred by similarities in structure, physiology, and lifestyle. Today, scientists use a combination of these traits, as well as DNA, RNA, and protein similarities to make hypotheses regarding the evolutionary history of groups of organisms, known as a phylogeny. A phylogenetic tree can be built to show relatedness among any number of species or other taxonomic groups. These diagrams are constructed using shared characteristics among the groups. Trees built based on different sets of traits can yield different results. It can be challenging to differentiate similarities between organisms, anatomical structures, or DNA, based on common ancestry (homologous) and similarities between organisms based on similar evolutionary pressures (analogous). DNA evidence helps distinguish homology from analogy, but it has its limitations as well. Since there are holes in the fossil record, many organisms that serve as intermediate species have not yet been discovered. Using a method called cladistics, organisms are sorted and classified into groups called clades. A clade is a taxonomic group that includes an ancestor and all of its descendants. As DNA sequencing (identifying the genetic sequence of an entire organism) has become more common and focus has shifted toward common ancestry, a movement has begun to name and classify groups based solely on shared evolutionary history. The PhyloCode, a set of rules stating that all taxonomic groups should be based on common ancestry, is controversial because it changes historic classifications that have had practical applications. For example, reptiles and birds have traditionally been classified separately based on major structural and physiological differences. However, under the new system, birds are considered reptiles based on shared evolutionary history. Some scientists argue that some traditional classifications should be maintained to avoid confusion, while others argue that the traditional taxonomic system is outdated and gives unreasonable importance to arbitrary sets of traits.