There are eight taxonomic categories, with domain most inclusive and each subsequent category becoming more specific.

Punctuated equilibrium is one theory within evolutionary biology that seeks to explain the rate of evolution over time. This theory describes most species being in a state of stasis, with little change occurring. When change does occur, however, it is rapid. The theory of punctuated equilibrium stands in opposition to the theory of gradualism and is supported by the fossil record.

A clade is a group of related species all descending from a common ancestor, including that ancestor. They have at least one shared derived trait unique to the clade and differentiating them from all other clades. Clades can be nested within each other: the mammalian clade is part of the vertebrate clade, which is part of the animal clade.

Viruses are not considered to be living organisms because they do not meet many of the criteria for "life", as defined by biologists. Viruses are incapable of energy metabolism, growth, response to stimuli, and independent reproduction. Viruses are simply a core of genetic material surrounded by a coat of protien, and require a living host to multiply.

All other answer options are specific examples of living organisms. Obligate anaerobes are a classification of bacteria that require minimal levels or lack of oxygen to survive. Fungi are a kingdom of organisms in the domain Eukarya. Cyanobacteria are a phylum of bacteria that obtain energy through photosynthesis. Halophiles are organisms that best survive in high salt concentrations.

The characteristic is found among all members of the clade, which makes it shared. It is not present among the ancestors of the clade, but was derived from the last common ancestor and is unique to that clade. These attributes make the trait a shared derived character.

An example of a shared derived character is hair in mammals: all mammals have hair, but no ancestors of mammals (that are not mammals themselves) had hair.

An example of a shared ancestral character is backbones in mammals: all mammals have backbones, and so do other vertebrates. There was a common ancestor between mammals and other vertebrates that had a backbone; however, when comparing all vertebrates to invertebrates, a backbone becomes a shared derived character, as backbones are unique to vertebrates.

Parthenogenesis is a type of asexual reproduction that can be found in plants, fish, birds, reptiles, and several others. This type of reproduction does not require fertilization to form an embryo, rather the embryo develops from an unfertilized egg.

An advantage of this system is that it does not require energy to search for a compatible mate, and allows the species to continue if organisms are isolated from one another. A disadvantage is the lack of genetic diversity, which does not help the species as much in the long term.

Organisms within the kingdom animalia have eukaryotic cells, are multicellular, and are heterotrophic (receives its nutrition through consuming other organisms).

Phylogenetics has several difficulties that complicate the development of accurate phylogenetic trees. Relationships between organisms on a genetic level are incredibly complicated, making it difficult for scientist to create clean and precise phylogenetic trees. These difficulties include hybridization, horizontal gene transfer, and convergent evolution.

Domain is the broadest taxonomic category, and includes bacteria, archaea, and eukarya; all living organisms can be classified into one of these three domains.

A virus is the smallest of the microorganisms, consisting genetic material and a protein coat. The virus takes over the host's cellular functions to reproduce, and carries no organelles of its own.

Bacteria prokaryotic cells, while fungi are eukaryotic cells. Protozoa are a sub-classification within the protista kingdom and are unicellular eukaryotes. Helminth are parasitic worms composed of eukaryotic cells.