18.3 Building the Tree of Life Lesson Overview 18.3 Building the Tree of Life

THINK ABOUT IT The process of identifying and naming all known organisms, both living and extinct, is a huge first step toward the goal of systematics. The real challenge, however, is to group everything—from bacteria to dinosaurs to blue whales—in a way that reflects their evolutionary relationships. Over the years, new information and new ways of studying organisms have produced major changes in Linnaeus’s original scheme for organizing living things.

Changing Ideas About Kingdoms During Linnaeus’s time, living things were classified as either animals or as plants. Animals were organisms that moved from place to place and used food for energy. Plants were green organisms that generally did not move and got their energy from the sun. As biologists learned more about the natural world, they realized that Linnaeus’s two kingdoms—Animalia and Plantae—did not reflect the full diversity of life.

Changing Ideas About Kingdoms Classification systems have changed dramatically since Linnaeus’s time, and hypotheses about relationships among organisms are still changing today as new data are gathered.

Changing Ideas About Kingdoms This diagram shows some of the ways in which organisms have been classified into kingdoms since the 1700s.

Three Domains Genetic analysis has revealed that the two main prokaryotic kingdoms are more different from each other, and from eukaryotes, than previously thought. So, biologists established a new taxonomic category—the domain. A domain is a larger, more inclusive category than a kingdom. Under this system, there are three domains—domain Bacteria, domain Archaea, and domain Eukarya.

The Tree of All Life What does the tree of life show? The tree of life shows current hypotheses regarding evolutionary relationships among the taxa within the three domains of life.

The Tree of All Life Modern evolutionary classification is a rapidly changing science with the difficult goal of presenting all life on a single evolutionary tree.

The Tree of All Life