3.3 Organizing Life on Earth

Systematics is the scientific study of the diversity of organisms and their evolutionary relationships. It involves identifying, naming, classifying, and determining the evolutionary connections among species.

Systematics includes two main branches: taxonomy and phylogenetics.

Taxonomy

Taxonomy (which literally means “arrangement law”) is the science of naming and grouping species to construct an internationally shared classification system. The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a Swedish naturalist) uses a hierarchical model. A hierarchical system has levels, and each group at one of the levels includes groups at the next lowest level, so that at the lowest level, each member belongs to a series of nested groups. An analogy is the nested series of directories on the main disk drive of a computer. For example, in the most inclusive grouping, scientists divide organisms into three domains: Bacteria, Archaea, and Eukarya. Within each domain is a second level called a kingdom. Each domain contains several kingdoms. Within kingdoms, the subsequent categories of increasing specificity are: phylum, class, order, family, genus, and species.

As an example, the classification levels for the domestic dog are shown in Figure 3.3.1. The group at each level is called a taxon (plural: taxa). In other words, for the dog, Carnivora is the taxon at the order level, Canidae is the taxon at the family level, and so forth. Organisms also have a common name that people typically use, such as a domestic dog or a wolf. Each taxon name is capitalized except for species, and the genus and species names are italicized. Scientists refer to an organism by its genus and species names together, commonly called a scientific name, or Latin name. This two-name system is called binomial nomenclature. The scientific name of the wolf is therefore Canis lupus. A recent study of the DNA of domestic dogs and wolves suggests that the domestic dog is a subspecies of the wolf, not its own species. Thus, it is given an extra name to indicate its subspecies status, Canis lupus familiaris.

Figure 3.3.1 also shows how taxonomic levels move toward specificity. Notice how within the domain we find the dog grouped with the widest diversity of organisms. These include plants and other organisms not pictured, such as fungi and protists. At each sublevel, the organisms become more similar because they are more closely related. Before Darwin’s theory of evolution was developed, naturalists sometimes classified organisms using arbitrary similarities, but since the theory of evolution was proposed in the 19th century, biologists work to make the classification system reflect evolutionary relationships. This means that all of the members of a taxon should have a common ancestor and be more closely related to each other than to members of other taxa.

Phylogenetics 

Recent genetic analyses have revealed that some traditional taxonomic classifications do not accurately reflect evolutionary relationships. As a result, classifications are continually revised as new discoveries are made. A notable example of this is the reclassification of prokaryotes. Until the 1970s, all prokaryotic organisms were grouped under Bacteria. However, molecular studies uncovered significant genetic differences, leading to the division of prokaryotes into two distinct domains: Bacteria and Archaea, now recognized as two of the three fundamental branches of life.

Understanding these evolutionary relationships is the focus of phylogenetics—the study of the evolutionary history and connections among organisms. Scientists use phylogenetic trees to visually represent these relationships. A phylogenetic tree is a branching diagram that reflects how species or groups of organisms have evolved from common ancestors. As species evolve and diverge over time, they form distinct lineages, which are depicted as branches on the tree.

By analyzing inherited traits and other historical evidence, biologists can reconstruct evolutionary pathways. These trees allow us to trace the lineage of a species back to its common ancestors and identify how closely related different organisms are. The branching patterns illustrate how evolution has shaped the diversity of life through descent with modification.

All life on Earth evolved from a common ancestor, so the phylogenetic tree starts with a single point and then branches into the three domains of Archaea, Bacteria, and Eukarya. Each domain then diverges and branches repeatedly. The small branch that plants and animals (including humans) occupy in this diagram shows how recently these groups had their origin compared with other groups.  

The tree is supported by many lines of evidence, but it is probably not flawless. Scientists constantly re-evaluate hypotheses and compare them to new evidence. As scientists gather even more data, they may revise these particular hypotheses, rearranging some of the branches on the tree. For example, evidence discovered in the last 50 years suggests that birds are dinosaurs, which required an adjustment to several “vertebrate twigs.”

Understanding Phylogenetic Trees

The point where a split occurs in a tree, called a branch point, represents where a single lineage evolved into distinct new ones. Many phylogenetic trees have a single branch point at the base representing a common ancestor of all the branches in the tree. When two lineages stem from the same branch point, they are called sister taxa. A branch point with more than two groups illustrates a situation for which scientists have not definitively determined relationships. It is important to note that sister taxa share an ancestor, which does not mean that one taxon evolved from the other. The branch point, or split, represents a common ancestor that existed in the past but no longer exists.

Click on the Information tab in each box to learn more about phylogenetic trees.

“12.1 Organizing Life on Earth” from Biology and the Citizen by Colleen Jones is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.