Phylogenetic Relationships of Major Groups

The evolutionary history of a species or group of related species is called its phylogeny . When the evolutionary history is diagrammed, it is shown in the shape of a tree that traces evolutionary relationships as they have changed over time. The reconstruction of phylogenetic history is part of the field of systematics . The diversity of the phylogenetic tree is a reflection of speciation.

A phylogenetic tree shows not only how closely related two groups are but also how once-related species evolved independently. The further back in time a group branched represents a greater amount of time for divergent evolution to occur. When systematists construct a phylogenetic tree, they consider as much data as possible. Whenever possible, they take the fossil record into account to identify when branching occurred. Scientists can compare ribosomal RNA or mitochondrial DNA of different organisms to pinpoint branches in the evolutionary history. After all the available data are compiled, the relationship can be drawn as a phylogenetic tree. Scientists often revise phylogenetic trees as new techniques or new data further clarify evolutionary relationships.

A phylogenetic tree can be used to show the evolutionary relationships of different groups. The figure (opposite) shows a simple phylogenetic tree of selected bird families. The first branch of the tree, which branched off about fifty million years ago, leads to the modern flamingo family on one branch and the other families off the other branch. The shoebill and the pelican families branched off about forty-five million years ago. It is important to realize that even though flamingos branched off much earlier than pelicans, each family has been influenced by evolutionary change. As environmental pressures such as climate change took place, each family adapted to the changes. This phylogenetic tree does not show any branches that became extinct.

Using a phylogenetic tree to find how closely related animals are is a relatively simple task. The closer together two phyla are on the tree, the more closely related the phyla. On the phylogenetic tree within the frontmatter of this book, only the phyla are shown. Each phyla can be further divided into classes, orders, families, genera, and species. Also note that phyla that branch very close to the edge of the circle are closely related, while those that branch closer to the center of the tree are more distantly related. For example, frogs and salamanders are very closely related because they branch close to the outer edge of the circle. Eubacteria and methanogens are close together, but they branch close to the center of the tree. This means that they are distantly related.

Allan B. Cobb

Bibliography

Barnes-Svarney, ed. The New York Public Library Science Desk Reference. New York: Macmillan, 1995.

Rupert, Edward R., and Robert D. Barns. Invertebrate Zoology. New York: Oxford University Press, 1994.