Classification and Biodiversity

Common names cause confusion: one animal can have many local names, and one name can mean different animals in different places. Biologists sort organisms into groups and give each species one scientific name that is used worldwide. The system is named after Carl Linnaeus, a Swedish botanist, zoologist and physician (see OpenStax 20.1).

Each species gets a two-part name, a system called binomial nomenclature: the genus name, capitalized, then the species name, lowercase, both in italics. The gray wolf is Canis lupus; the domestic dog is a subspecies of it (see OpenStax 20.1). For animals, names count from the 10th edition of Linnaeus's Systema Naturae, treated as published on 1 January 1758 (see ICZN Article 3).

Groups are nested inside bigger groups. From broadest to narrowest the ranks are domain, kingdom, phylum, class, order, family, genus and species (see OpenStax 20.1). One way to remember the order: Dear King Philip Came Over For Good Soup. Each step down holds fewer kinds of organisms that share more features. The dog, for example, is in domain Eukarya, kingdom Animalia, phylum Chordata, class Mammalia, order Carnivora, family Canidae and genus Canis (see OpenStax 20.1).

All life falls into three domains: Bacteria, Archaea and Eukarya (see OpenStax 20.1). Bacteria and archaea are prokaryotes with no nucleus. When Carl Woese compared their ribosomal RNA, archaea turned out to be as distinct from bacteria as either is from eukaryotes (see Woese & Fox), and in 1990 the three domains were proposed (see Woese, Kandler & Wheelis).

Eukaryotes, whose cells have a nucleus, include the familiar kingdoms of animals, plants and fungi. The old kingdom Protista, a catch-all for the remaining eukaryotes, has been taken apart: DNA sequences show that it mixed unrelated lineages, and protists that look alike may have evolved similar features separately (see OpenStax 23.3). So “protist” is a useful word but not a natural group.

A phylogenetic tree shows hypothesized evolutionary relationships. Each branch point, or node, stands for a common ancestor that split into two lineages. Two lineages from the same branch point are sister taxa (see OpenStax 20.1). A cladogram groups organisms by shared derived characters, traits that appeared in a common ancestor and were passed to all its descendants, such as the amniotic egg of reptiles, birds and mammals (see OpenStax 20.2).

Common mistakes when reading a tree (see OpenStax 20.1):

• Tips next to each other are not always closest relatives. Relatedness comes from the most recent shared branch point. Rotating the branches at a node changes nothing about the relationships.
• Looking alike is not the test. Lizards look more like frogs, but lizards share a more recent ancestor with rabbits.
• Branch length is not time unless the diagram says so. The tree shows the order of events.

The most used definition is the biological species concept: a species is a group of individuals that interbreed and produce fertile, viable offspring (see OpenStax 18.2). Poodles and cocker spaniels look very different but are one species because they can interbreed (see OpenStax 18.2). The test cannot be run on fossils or on organisms that reproduce without sex, so biologists also define species by shared features or by ancestry.

A dichotomous key identifies an organism through a series of paired choices. Each step, or couplet, offers two statements that cannot both be true. You pick the one that fits and follow it to the next couplet until you reach a name. Good keys use features you can see, such as legs or a shell, not size or color, which vary.

Biodiversity is the variety of life. About 1.5 million species of eukaryotes have been described, less than 20 percent of an estimated 8.7 million (see OpenStax 47.1). That estimate, plus or minus 1.3 million, comes from a 2011 study, which also found that about 86 percent of species on Earth, and 91 percent of those in the ocean, still await description (see Mora and colleagues).

Life is not spread evenly. In 1988 Norman Myers described biodiversity hotspots: areas with high numbers of endemic species, found nowhere else (see OpenStax 47.1).

Species have always gone extinct, at a background rate of about one per million species per year, and Earth has seen five mass extinctions (see OpenStax 47.1). Today about 1 million species are threatened with extinction. A 2019 global assessment ranked the five direct drivers: changes in land and sea use, direct exploitation of organisms, climate change, pollution and invasive alien species (see IPBES).

Why it matters: living things supply food and medicine. Aspirin, codeine and digoxin all come from plant compounds (see OpenStax 47.2). Each species lost also takes with it genes and relationships that cannot be rebuilt.

Classification has changed as tools improved: from careful looking, to evolutionary thinking, to reading DNA.