Phylum is a taxonomic ranking that comes third in the hierarchy of classification, after domain and kingdom. Organisms in a phylum share a set a characteristics that distinguishes them from organisms in another phylum. The qualities that group animals into a phylum have changed throughout scientific history, as better methods have arisen to determine how groups of animals are related. Modern phylogenic systematics, or simply phylogenetics, uses a variety of traits to map the changes between different groups of animals. Different hypotheses of how a phylum is related to other phyla (plural of phylum) create different cladograms, which display the relationships visually. The cladogram that is the most parsimonious, or has the fewest number of changes, is usually accepted as the most likely hypothesis of the relationships between different phyla. Plants, protists, and bacteria have also been categorized into divisions historically. The word was later changed to phylum.
There are approximately 35 animal phyla, 12 plant phyla, and 7 phyla of fungi. The bacteria, including the archaea, are grouped into roughly 34 phyla, although the relationships between these groups are not as well established. The exact numbers of phyla are never known for sure, as new evidence and techniques are discovered. For instance, with the advent of genetic testing many groups that were thought to be monophyletic were found to have very different genetics. Monophyletic is a term that describes a complete group, with all the common ancestors. Polyphyletic groups contain organisms that do not share a recent ancestor, and many more groups would have to be included to make the phylum monophyletic. In these cases, the phylum is split into multiple phyla. Other times, two different phyla are found to be closely related and will be combined into the same phylum.