This is a field of study that allows biologists to reconstruct a pattern of evolutionary events resulting in the distribution and diversity of present-day life. Achieving this goal requires classifying organisms into groups in a meaningful and universal manner. This classification is based on evolutionary events that occurred long before human civilization appeared on Earth. Taxonomy is the system used to name organisms based on their evolutionary relationships. A taxon is a hierarchical category used in the naming process, and taxa is the plural form of the word. The main taxa, in order of broadest to most specific designation, are: kingdom, phylum, class, order, family, genus, and species. For example, a domestic cat's kingdom is Animalia because it is an animal, its class is Mammalia because it is a mammal, and its genus and species are Felis domesticus. Phylogenetics refers to the study of an organism's evolutionary history: when it first appeared on Earth, what it evolved from, where it lived, and when and why it went extinct (or survived). Systematics, then, refers to naming and organizing these biological taxa into meaningful relationships. For example, if two species of deer that are alive today are both thought to have evolved from a different species that subsequently went extinct, the taxonomic nomenclature (scientific name) of the deer should reflect that relationship.
Cladistics is an important tool for forming hypotheses about the relationships among organisms. Cladistics is a mechanism for providing a testable phylogenetic tree, a diagram representing the relationships of different organisms as a tree, with the oldest ancestors at the trunk of the tree and later descendants at the branch ends. The underlying assumption of cladistic analysis is that members of a single group are more closely related to each other than to members of a different group. When several organisms share a suite of features, they are grouped together because these shared features are likely to have belonged to a common ancestor of all the group members. When common features are thought to have this sort of evolutionary relevance, they are called "synapomorphies."
Conversely, those features that distinguish each member within a group from each other are called "apomorphies". These are derived characters, meaning that they evolved anew in the descendant and did not belong to the ancestor. As an example, both owls and sparrows have feathers and a beak because they share the synapomorphies of being birdlike; however, owls have very large eyes at the front of their head whereas sparrows have small eyes on either side of their head, and these are apomorphies. Cladistic analysis sums up the number of apomorphies and synapomorphies among different organisms and produces possible phylogenetic trees that minimize the apomorphies in particular groups. This is one method by which evolutionary relationships are estimated.
The information that is used in cladistic analysis can be morphological or molecular. Morphological measurements are taken from fossils or from living animals. In fossil evidence, imprints of an organism or the fossilized organism itself provide evidence for the size and connectivity of hard body parts. Extant, or living, organisms make it possible also to measure the organism's soft parts, those that are unlikely to be fossilized. This is the most common type of cladistic study. Molecular evidence comes from comparing the genetic codes of extant species. Because DNA is thought to evolve at a constant rate, the molecular clock can be set at a particular, confidently estimated evolutionary event such as the divergence of placental from marsupial mammals. Then the amount of time since the divergence of two groups of organisms can be estimated based on the number of differences between their genetic codes.
see also Phylogenetic Relationships of Major Groups.
Rebecca M. Steinberg