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The Phylogenetic Tree of Life

Ivica Letunic

Phylogenetics is the study of how species on Earth are related and often results in a graphic representation of the links. Much like a family tree, the phylogeny of a species can be shown using cladograms. The closer the branches of the tree, the more closely the species are related. Phylogeny is a subtopic of systematics, which also includes taxonomy.

History of Phylogenetics

For centuries, scientists such as Carolus Linnaeus have tried to classify organisms into like groups in order to figure out their origins. At first, most of this classification was based on anatomy and similar structures or physical features. As technology increased, this classification system was amended to include newly found biochemical data like similarities in DNA. Scientists than began looking at how closely different species were related to one another and when they seemingly went different evolutionary directions.

Phylogenetics was developed to answer these questions and other questions like them. Scientists hypothesized that all species on Earth have evolved from the same common ancestor. The more closely two species are related, the more recent their last common ancestor.

It was not until the 1950s and 1960s that phylogenetics and cladistics started to be used to explain evolution. It was a very controversial topic. Many scientists rejected this type of grouping based on morphology and biochemical similarities. There is room to be subjective in placement of some species in relation to common ancestors. This leads to arguments and disagreements about the evolutionary path of some species.

Phylogenetics and cladistics really did not catch on until the 1990s. Computers and specialized equipment made obtaining evolutionary data from fossils and current specimen cost effective and time efficient. Also, more was known about DNA and the genomes of many species were starting to be mapped. This biochemical data made it easier to classify the species into correct clades without having disagreements.

Phylogenetics Terminology

All species that did evolve from a single common ancestor are considered to be monophyletic. Groupings of only some of the species that evolved from a common ancestor are then called paraphyletic. A group of species that does not include their most recent common ancestor are polyphyletic. All phylogenetic groupings, including monophyletic, paraphyletic, and polyphyletic groups, are called taxa.

Cladograms, the visual "tree" of the species, also have their own terminology. A clade (pronounced: clayd) is an ancestor and all of its related species. These clades are the "branches" on the tree. If a species is basal to another species, it means it is more primitive and will be found nearer the bottom of the cladogram. Subsequently, clades that are found higher up on the tree are considered to be nested clades in relation to the basal species. The most closely branched speices are called sister clades.

Reading Cladograms

The most basal species on the cladogram is the most recent common ancestor of all other species on the cladogram. A single cladogram will only include the ancestor and its branched species. Unrelated species will not be on the same cladogram. The main "trunk" of the tree has various characteristics listed. Species branch off as the characteristics become more specific and they no longer fit into that category. This is based on the assumption that species become more complex over time.

The Tree of Life

Today, entire genomes of many species have been mapped. This data is available at the touch of a button and is available to everyone. From this data, scientists have put together what is known as the Tree of Life. The Tree of Life is one big cladogram that is almost circular. Versions have been made with only non-extinct species and the even more complicated version that includes every known species that was ever on the Earth at any time. This compilation of clades throughout history gives a relatively clear picture of how life on Earth has evolved since the beginning of time.

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