The field of systematics is the study of the diversity of organisms and their evolutionary relationships. Systematics includes identifying species, giving them unique names, and classifying them into groups that make sense in terms of evolution. Scientists working in the field of systematics attempt to explain the evolutionary history of living things.

A model of the evolutionary history and relationships of a group of organisms is called a phylogeny. Biologists use different kinds of evidence to create their phylogenetic models: physical characteristics of organisms, genetic and biochemical evidence, the fossil record, and comparisons of the development of various species from embryos to adults. A phylogeny has a lot in common with a family tree. A family tree shows the relationships between ancestors and descendants. It also shows the relationships among the members of the current generation based on their connections to common ancestors.

This family tree shows the youngest generation at the top and ancestors below. Notice that we can identify siblings by their common set of parents, and first cousins by their common set of grandparents. It's possible to establish relationships from direct evidence when individuals are living. To go further back in time, we need to use documents like birth certificates and marriage licenses. Sometimes the evidence is weaker: a mention of relationships in old letters, or stories told. In many cases, small pieces of evidence support each other, and a family tree can be built. A family tree going back many generations is a hypothesis about the relationships among those family members.

Biologists use a tree-like diagram to show the evolutionary relationships among organisms. This kind of diagram is called a phylogenetic tree. It shows the lines of descent of species from their ancestors. Various kinds of scientific evidence must be pieced together to create a phylogenetic tree. A family tree is really a magnified view of a phylogenetic tree.

Let's zoom out from the family tree. As we move back in time, hundreds of thousands, even millions of years, we pass what biologists hypothesize are species different from modern humans, yet they are our direct ancestors. Going back perhaps 5 million years, those distant ancestors gave rise not only to our species, but also to our most closely related species, the chimpanzees and gorillas! Those great apes are very, very distant cousins, but cousins nonetheless.

The first step toward creating a family tree of life is naming organisms, and we'll look at that topic next. Then we'll consider the problem of constructing phylogenetic trees.

We’ll conclude by taking a look at biology's current view of phylogeny in the section on modern taxonomy.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education