Phylogenetic trees

A phylogenetic tree, also known as a cladogram, is a diagram that shows how different living things are related. Phylogenetic trees may show the relationships between species or between larger groups. If you follow the lines connecting any two groups on the tree, you'll get an idea of how closely related they are. The longer the path is, the more distant the relationship.

A Tree of Life is a phylogenetic tree that proposes how all life is related. Generations of scientists have created Tree of Life diagrams by studying and comparing the genetic and physical features of different species or groups of species.

The study of phylogenetic trees changed dramatically in the 1950s and 1960s when the German entomologist Willi Hennig introduced a method called cladistics (from the Greek word clados, referring to the branchlike structure of phylogenetic trees). His key concept was that related groups are characterized by shared derived characteristics.

Many of the statistical approaches used today for DNA sequence comparisons incorporate some of the key concepts that Hennig articulated. All living things have some DNA sequences in common because they evolved from a single ancestral species. Closely related species have more DNA in common than distantly related species do, so they are positioned closer to each other on a tree.

Darwin’s great idea of descent with modification postulated that a grand Tree of Life could illuminate the relationships among all organisms and that the features of organisms would be hierarchically arranged, thus revealing groups within groups. And this is exactly what subsequent generations of biologists have found. Despite many intriguing unanswered questions about the relationships among taxa, the fact that a robust Tree of Life has been built is surely the most compelling evidence we have for evolution.